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Seepage properties of crushed coal particles
Abstract
Gas release from the crushed coal formation during coal mining poses a threat to the normal storage and gas transport within coal seam. This sort of release is largely related with the evolution of gas seepage properties of crushed coal, namely, the evolution of gas seepage properties plays an important role in the gas extraction and the outburst prevention of coal seam. Meanwhile, the evolution is directly related to the volume and geometry of fractures within the crushed coal and strongly influenced by particle size and porosity (axial displacement). In order to investigate the effect, a gas flow apparatus connecting with the MTS815.02 system was designed. The gas seepage properties of crushed coal specimens with five particle sizes of a (2.5-5 mm), b (5-10 mm), c (10-15 mm), d (15-20 mm) and e (mixed sizes) under the variable axial displacement of 10, 15, 20, 25, 30, 35, and 40 mm, respectively are measured by the revised MTS815.02 system. The results of test show that: The porosity decreases with the increase of axial displacement and the weight loss of larger particle size, respectively. Particle crushing during compaction is a main reason to increase small-size and emerge size 0-2.5 mm. Gas flow seepage properties of crushed coal are found to be strongly influenced by particle size and axial displacement. The permeability decreases and non-Darcy coefficient increases with the increase of axial displacement (decreases of porosity) and the decrease of larger particle size, respectively. The permeability and non-Darcy coefficient are controlled by the fracture among the coal particles. The seepage properties of crushed coal are not only related to particle size, loading levels (axial displacement), but also the compound mode. Based on the regression analysis, the relationship between the permeability and non-Darcy coefficient and porosity shows that the exponential function can fit the experimental data well.
... For crushed coal particles, many experimental, theory and numerical methods have been applied to investigate the effect of particle size and density on fixed characteristics . Ma et al. (2008) studied the seepage characteristics of fractured coal with five particle sizes and found that the gas flow seepage properties of crushed coal are strongly influenced by particle size and axial displacement. The permeability decreases and non-Darcy coefficient increases with the increase of axial displacement (decreases of porosity) and the decrease of larger particle size, respectively. ...
... In the process of compaction, the larger the particle size and the particle size ranges of the coal samples, the higher the change rate of the porosity and the permeability. This finding is consistent with the conclusion drawn by Ma et al. (2008). Ma et al. (2008) only considered that the non-Darcy gas flow seepage properties are strongly influenced by the initial particle size and axial displacement during compaction; in general, during the compaction (axial compression), the permeability decreases, while the Darcy coefficient increases with the decreasing porosity. ...
... This finding is consistent with the conclusion drawn by Ma et al. (2008). Ma et al. (2008) only considered that the non-Darcy gas flow seepage properties are strongly influenced by the initial particle size and axial displacement during compaction; in general, during the compaction (axial compression), the permeability decreases, while the Darcy coefficient increases with the decreasing porosity. However, the relationship between porosity and permeability was not taken into consideration. ...
The permeability of broken coal is not only mainly controlled by coal structure, but also unavoidably influenced by variations of stress, particle size, moisture content, and temperature. In this paper, a self-designed experimental device was utilized to investigate the permeability evolution of seven coal samples with different particle size ratios during the loading processes under the condition of temperature, water content, and pore pressure. The results show that both the permeability and the porosity are negative exponential functions of axial stress. Under the same stress, the porosity and the permeability decrease with the increase of the particle size and the particle size range of the coal samples; in the process of compaction, the larger the particle size and the particle size range of the coal samples, the higher the change rate of the porosity and the permeability. Under the same stress, any increase of temperature, water content, or pore pressure would cause the decrease of the permeability of the coal samples, i.e., they are negatively correlated. With the establishment of the permeability calculation model of the coal samples under different temperatures, water contents, and pore pressures during the loading processes, the permeability of different locations in different environments can be speculated. Based on the estimated data, the air leakage in different areas can be obtained to determine the spontaneous combustion zone in goaf, which is crucial for fire prevention and control in goaf.
... The gas-flow apparatus using nitrogen as fluid was developed by Ma and Chu separately, and the high-speed non-Darcy gas-flow properties of crushed coal were investigated by their studies. [31][32][33] The studies mentioned above provide a large number of references for the estimation of mining gob gas migration during CMM drainage. However, these experiments were mainly conducted using water or nitrogen as fluid and the resulting laws are under relatively large Re numbers. ...
... Large intact coal blocks were drilled from the working face and then transported to the laboratory directly. The crushing procedure followed the procedure reported by Ma et al. 32 Sirotyuk 38 performed field observations on the preparatory mine workings in 13 types of rock with a wide range of properties, and found that about 15.42ß86.04% of crushed rocks had a size below 0-0.05 m after blasting. Hou 39 tested the dynamic fragmentation of three brittle rocks by the split Hopkinson pressure bar system under laboratory conditions, and found that the particles with the size below 0-2 mm and below 0-0.5 mm accounted for 1.28ß17% and 1ß12.83% of the total, respectively. ...
... In the initial stage of CMM drainage, relatively larger gas pressure and flow rate can induce a larger Reynolds number, and Forchheimer equations are widely used to illustrate the high-speed non-Darcy flow properties of crushed coal and rocks. 16,[27][28][29][30][31][32][33] For smaller Reynolds numbers corresponding to the latter stage of CMM drainage, the methane flow velocity is directly proportional to gas pressure gradients as shown in Fig. 7. The larger slope corresponds to the larger porosity of crushed coal, which means that crushed coal with larger porosity is more sensitive to the change of pressure gradients. ...
... Step [36]. Vertical displacement at the bottom of the model is fixed to be zero, and the horizontal displacements of the four vertical plains are limited by the normal direction. ...
... The rock material has the Poisson ratio of 0.25 and the Young's modulus of 190 MPa [39]. [36]. Vertical displacement at the bottom of the model is fixed to be zero, and the horizontal displacements of the four vertical plains are limited by the normal direction. ...
Groundwater inrush is a typical hydrologic natural hazard in mining engineering. Since 2000 to 2012, there have been 1110 types of mine groundwater inrush hazards with 4444 miners died or missing. As a general geological structure in the northern China coalfields, the karst collapse pillar (KCP) contains a significant amount of granular rocks, which can be easily migrated under high hydraulic pressure. Therefore, the KCP zone acts as an important groundwater inrush pathway in underground mining. Grouting the KCP zone can mitigate the risk of groundwater inrush hazard. However, the fracture or instability of the coal pillar near KCP can cause the instability of surrounding rock and even groundwater inrush hazard. To evaluate the risk of groundwater inrush from the aquifer that is caused by coal pillars instability within grouted KCP in a gob, an in-situ investigation on the deformation of the surrounding strata was conducted. Besides, a mechanical model for the continuous effect on the coal pillar with the floor-pillar-roof system was established; then, a numerical model was built to evaluate the continuous instability and groundwater inrush risk. The collective energy and stiffness in the floor-pillar-roof system are the two criterions for judging the stability of the system. As a basic factor to keep the stability of floor-pillar-roof system, the collective energy in coal pillar is larger than that in floor-roof system. Moreover, if the stiffness of floor-roof or coal pillar meets a negative value, the system will lose stability; thus, the groundwater inrush pathway will be produced. However, if there is a negative value occurring in floor-pillar-roof system meets, it indicates that the system structure is situated in a damage state; a narrower coal pillar will enlarge the risk of continuous instability in the system, leading to a groundwater inrush pathway easily. Continuous coal pillars show a lower probability of instability. Conversely, the fractured coal pillars have a greater probability of failure. The plastic zone and deformation of the roadway roof in the fractured coal pillar are larger than that of continuous coal pillar, indicating that the continuous coal pillars mitigate the risk of groundwater inrush hazard effectively.
... When the first stage of compaction treatment displacement loading (10 mm) was finished and kept stable, change the plunger pump to the 4th flow velocity V. After the water flow, start the next stage of compaction treatment displacement loading (15,20,25,30,35, and 40 mm) and water flow again. The flow velocity of plunger pump in the last two stages of compaction treatment can be reduced appropriately to avoid the rapid increase of pore pressure. ...
... In previous literature [17,30,31], the Forchheimer equation (a non-Darcy flow equation) can be used to model the relationship between pore-pressure gradient and velocity of water flow in granular rocks, which can be expressed as ...
A test system for water flow in granular gangue mineral was designed to study the flow characteristics by compaction treatment. With the increase of the compaction displacement, the porosity decreases and void in granular gangue becomes less. The main reason causing initial porosity decrease is that the void of larger size is filled with small particles. Permeability tends to decrease and non-Darcy flow factor increases under the compaction treatment. The change trend of flow characteristics shows twists and turns, which indicate that flow characteristics of granular gangue mineral are related to compaction level, grain size distribution, crushing, and fracture structure. During compaction, larger particles are crushed, which in turn causes the weight of smaller particles to increase, and water flow induces fine particles to migrate (weight loss); meanwhile, a sample with more weight of size (0–2.5 mm) has a higher amount of weight loss. Water seepage will cause the decrease of some chemical components, where SiO 2 decreased the highest in these components; the components decreased are more likely locked at fragments rather than the defect of the minerals. The variation of the chemical components has an opposite trend when compared with permeability.
... In the test, according to the relationship between axial displacement and porosity, the immediate porosity at every moment in the permeation process is [19]: ...
In order to study the penetration characteristics in areas with different water content and different stress distributions in the radial direction of the hole after hydraulicization measures, an improved LFTD1812 triaxial permeability meter was used to conduct a test to measure the polar permeability characteristics of coal with different water content combinations were measured by permeability instrument, and the porosity, permeability, pressure gradient and seepage velocity of different samples were analyzed. The relationship between sample porosity, permeability, pressure gradient and seepage velocity was discussed, the influence of moisture content on permeability was discussed, and the directionality and the directivity and polarization effect of permeability were found.. Result shows that The relationship between permeability and porosity shows two trends of exponential type and logarithmic type, and the porosity-permeability({\phi}-k) plane is divided into three influence regions: super index (I), index (II) and logarithm (III).
... 34 On this basis, it was obtained that the relationship between permeability, non-Darcy coefficient, and porosity of crushed sample can be fitted using an exponential function. 35 Gas permeability experiments with different stress carried out on crushed coal and rock were used to construct a stress-permeability fitting model by varying the stress conditions and the number of cycles of loading and unloading. 36,37 Under constant total stress, permeability to sorption gas increases with a decrease of pressure due to crushed coal and rock swelling 38,39 and decreases with increasing pressure due to matrix shrinkage. ...
The mining goaf is enriched in coalbed methane (CBM) resources, and it is imperative to realize its efficient extraction. The gas permeability properties of the crushed coal and rock in the caved zone of mining goaf are the basis for the study of its internal CBM migration and enrichment law. In this study, the compaction deformation and gas permeability properties of crushed limestone with different particle sizes were revealed. The results show that (1) the deformation resistance capacity of the crushed limestone increased with increasing stress. The decreasing trend of porosity of samples with different particle sizes in the early and later compression periods is significantly different. Particle RR of the lower layer is smaller than that of the other layers. (2) The permeability of the sample decreases with decreasing porosity and nitrogen pressure, and it is between 10–12 and 10–10 m². Nitrogen migration within the crushed limestone requires the pseudo-threshold pressure gradient, which ranges from 64.86 to 311.42 Pa/m. (3) The average permeability growth amplitude of the sample shows a logarithmic decreasing trend with the decrease of porosity. The average permeability growth amplitude of the 5–10 mm sample at the same porosity was 15.9–22.3 times that of the 0.315–0.63 mm sample. (4) The permeability of crushed limestone on both sides of the lower layer in the caved zone is much larger than that of other locations. The results are of great practical significance for accurately predicting the CBM enrichment area of mining goaf and then selecting the final position of the extraction drilling hole.
... Later literatures were investigated from the air leakage seepage in the mined-out area. In fact, the mined-out area consists of caved rocks and residual coals, where the permeability at each point is generally considered to be related to the particle size and porosity of the crushed rock-coal at that point [17][18][19][20], so the air flow in the mined-out area can be inferred by investigating the air seepage behavior in the broken rocks [21]. However, the air seepage process in the broken rocks is subject to the resistance that consists of two parts, one is the energy loss due to the viscous force of the fluid, and the other is the energy loss due to the inertial force. ...
Seepage behaviors of air leakage directly affect the coal spontaneous combustion (CSC) in longwall gobs, which is usually regarded as Darcy flow, but in fact, non-linear seepage should better characterize the properties of air flow in the gobs. To address this issue, steady-state percolation tests were conducted on broken rocks with five particle sizes, and the non-Darcy permeability parameters versus porosity and particle size were obtained by binary fitting the experimental data, and then applied to an actual longwall gob. The multi-physics coupling model of CSC in longwall gobs was modified accordingly, which was then validated by comparing with on-site observation data. Numerical solving procedures based on Darcy seepage and non-Darcy seepage were developed separately to quantify the effects of these two types of seepage on the CSC. The results show that (i) the seepage tests confirm that the seepage behavior in the broken rocks is more consistent with the non-Darcy seepage; (ii) the distributions of oxygen concentration and solid temperature under the two seepage flows are relatively similar, but the high-temperature zone in the gob under non-Darcy seepage is larger and hotter; (iii) the non-Darcy permeability coefficient has a positive effect on the coal temperature rise in the gob, while the non-Darcy flow factor has almost no effect, so taking plugging measures at the working face can effectively prevent the spontaneous combustion in gobs. This work contributes to the understanding of the air seepage behavior in longwall gobs and its impact on the CSC.
... increases by degrees, and the peak value also increases significantly. In other words, with increasing porosity, the larger the pore size, the higher the permeability [42,43] . The larger the bore diameter, the more obvious the effect of the pore on the fluid migration in the uranium-bearing sandstone. ...
As an important nuclear fuel, uranium in sandstone uranium deposits is mainly extracted by in situ leaching. The porosity of sandstone is one of the important indexes determining in situ leaching efficiency. Moreover, the microscopic pore size distribution (PSD) of the uranium-bearing layer has an important effect on porosity. It is necessary to feature the pore structure by various techniques because of the different pore types and sizes in the uranium layer. In this paper, combined with nitrogen gas adsorption, nuclear magnetic resonance techniques and scanning electron microscopy, the full-scale PSD features of uranium-bearing sandstone in the northwest of Xinjiang are effectively characterized. The results show that pores structure of uranium-bearing sandstone include dissolution pores (d <= 50 nm), intergranular pores (50 nm < d 200 mu m) and microfractures. Intergranular pores of 60 nm and 1 mu m are the significant contributors to pore volume. The effects of the pore volume of two pore types (dissolution pores and intergranular pores) on the porosity of uranium-bearing sandstone are analysed. The results show that intergranular pores have the greater influence on the porosity and are positively correlated to the porosity. Dissolution pores have little effect on the porosity, but it is one of the key factors for improving uranium recovery. Moreover, the greater the difference of PSD between sandstones, the stronger the interlayer heterogeneity of uranium-bearing sandstone. This kind of interlayer heterogeneity leads to the change of permeability in the horizontal direction of strata. It provides a basis for a reasonable setting of well type and well spacing parameters
... In recent years, researches focused on the water inrush mechanisms and prediction and early warning of geological hazards, and disaster prevention and control measures of practical engineering have been extensively performed [1,4,9,18,[20][21][22][23][24][25]. A lot of interesting findings have been identified. ...
This study analyzes the spatiotemporal evolution characteristics of seepage through a large-scale rock mass containing a filling joint. Firstly, a conceptual model was established to characterize the geomechanical occurrence of a typical water-resistant slab adjacent to a water-bearing structure. Then, a special apparatus was developed to conduct a hydromechanical test of a 3D large-scale rock mass. For a certain boundary stress and inlet water pressure, the pore water pressure in the joint first experiences a dramatic increase before approaching a constant value, and the steady pore water pressure presents a linear decrease along the joint length. A water inrush phenomenon happens as a result of connected flowing channels induced by migration of fillings. Using the finite element of COMSOL multiphysics, the influences of filling joint permeability, matrix permeability, and joint thickness as well as the inlet water pressure on seepage evolution in the jointed rock mass were, respectively, investigated. The pore water pressure increases with all these factors, and the stable pressure values increase with the inlet water pressure but decrease along the joint length. The flow velocity undergoes an increase with both the joint permeability and inlet water pressure but presents constant values independent on the matrix permeability or joint thickness. The water pressure contour planes distributed along the flowing path generally transfer from a “long funnel” shape to a “short funnel” shape before reaching a series of parallel pressure planes perpendicular to the joint direction. By using the genetic algorithm, the coupling influences of these factors on the pore water pressure and flow velocity were investigated, and the decision parameters were optimized. The calculated values show a good agreement with the numerical results, indicating a good prediction of the seepage evolution through the filling joint.
... For a large number of applications where crushed rock aggregate is used (for example, aggregate bases and geotechnical filters), it is important to quantify the hydraulic conductivity and drainage characteristics of such materials and, mostly for preliminary evaluations, to estimate hydraulic conductivity using available predictive equations. Toward this end, it is of interest to note that laboratory tests have been conducted in the past on various crushed rocks such as limestone, sandstone, mudstone, coal, and hard rock mine tailings (for example, Aubertin et al. 1996;Liu and Wang 2012;Ghabchi et al. 2013;Haider et al. 2014;Akbulut 2016a, 2016b;Ma et al. 2016aMa et al. , 2016bMa et al. , 2016cYang et al. 2019;Hatipoglu et al. 2020). However, and notwithstanding testing errors as described by Chapuis (2012), testing conditions frequently involved non-Darcian flow, fines (D < 0.074 mm) and/or gravel-size grains (D > 4.75 mm) were often a significant component of the gradation, and hydraulic conductivity predictions or comparison of predictions to measurements were attempted in a very small number of cases. ...
The hydraulic conductivity of 54 sand-sized crushed limestone materials was measured by conducting constant head tests in a rigid-wall permeameter and was estimated using six predictive equations requiring easily obtainable parameters. The gradations tested had effective grain size, D 10 , from 0.079 to 2.15 mm; uniformity coefficient, C u , from 1.19 to 15.79; and void ratio, e, from 0.42 to 0.76. The measured hydraulic conductivity had a range of about three orders of magnitude (3.4*10 −3 to 3.3 cm/s). Four of the predictive equations, based on the square of the effective grain size, D 10 2 , yielded closely grouped results differing by not more than a factor of 2. Long existing equations by Terzaghi (1925) and by Hazen (1892), adjusted for void ratio according to Taylor (1948), were found to have a high predictive efficiency with a ratio of predicted to measured values between 1/2 and 2 for 70% of the materials tested. The Kenney et al. (1984) equation, based on D 5 2 , was also efficient but underestimated measured values for 63% of all cases. The Kozeny-Carman equation (Taylor 1948; Chapuis 2012), based on specific surface, overestimated measurements for 90% of the tested materials by a factor of up to 3.
... In addition, relevant studies have shown that the changes in the geometry of mineral particles and the volume of fractures between particles have a great influence on the porosity and seepage characteristics of the caprock. 68 This may pose a threat to the effectiveness of CO 2 sequestration in caprock. ...
Storing CO2 in geological formations can reduce the amount of greenhouse gases in the atmosphere. In order to explore the fracture behavior of caprock during CO2 geological storage, three caprock adsorption experiments for supercritical, liquid, and gaseous CO2 were designed. The fracture toughness of mode I, mode II, and mixed‐mode I/II of sandstone before and after the experiment was tested, and X‐ray diffraction (XRD), X‐ray fluorescence (XRF), and scanning electron microscopy (SEM) were used to examine the fracture mechanical characteristics of sandstone. Results showed that the pure mode I fracture toughness of sandstone immersed in supercritical CO2 (ScCO2), liquid CO2, and gaseous CO2 for 30 days decreased by 27.89%, 11.01%, and 17.43%, respectively, compared to nonimmersed sandstone. Pure mode I fracture toughness was more sensitive than mixed‐mode I/II and pure mode II fracture toughness to the various CO2 phase states. Furthermore, the ability of sandstone to resist fracture and failure was significantly reduced by the adsorption of CO2 in different phases. The effects of the different phases were in the following order: ScCO2 > gaseous CO2 > liquid CO2. The decreased ability of sandstone to resist fracture was primarily due to the geophysical and chemical reactions between CO2 and minerals, the alteration of minerals, or the formation of new substances. SEM observations showed that liquid CO2 adsorption caused the sandstone to undergo intergranular fracture, and the adsorption of gaseous CO2 promoted the occurrence of transgranular fractures. In particular, due to the ScCO2 adsorption, various fracture forms such as intergranular fractures, transgranular fractures, and mutual coupling fractures existed simultaneously. Sandstone exhibited numerous fractures and pores, and fracture resistance was weakened. Results of this study have important significance for evaluating the stability and safety of CO2 geological storage.
... The fractal characteristics of the seepage pores of the coal in China are determined by the variation in micro-pore content and aromaticity during coalification [19]. Permeability decreases with the decrease of the porosity and the increase of particle size [20]. Although the previous studies have made great progress, large-scale application of those approaches is often expensive and may cause irreparable damage to coal. ...
In order to improve the permeability of coal seams, Microemulsions (MEs) are used as seepage reagents and their effect is compared with those of distilled water and Sodium Dodecyl Sulfate (SDS) solution. For comparison, the seepage time and permeability of different seepage reagents are measured by a seepage apparatus. Furthermore, the scanning electron microscopy, X-ray diffraction analysis, Fourier transform infrared spectroscopy and contact angle measurement approach are adopted to explore the seepage characteristics of MEs. The results show that MEs take less time than the SDS solution and distilled water for the seepage flow to reach 1 ml, and that MEs have a higher permeability in coal than the SDS solution and distilled water. The reason is that the MEs treated coal has better pore connectivity than the SDS solution treated, water treated and raw coal because its effective seepage pores are enlarged and increased. Moreover, MEs can reduce the content of montmorillonite, and thus reduce the blocking volume in seepage pores. In addition, MEs can increase the oxygen-containing functional groups, and their contract angle with the coal surface is smaller than that of SDS solution and distilled water. Therefore, MEs have better wettability than the SDS solution and distilled water since they can be easily spread on the coal surface and in coal seam pores. As a consequence, they can wet the coal pore wall more easily during seepage, thus reducing the difficulty of the seepage and migration of MEs and enhancing the seepage effect. Therefore, this study on MEs is of great significance to coal seam water injection in deep mining.
... The gas transmission law in the coal seam basically conforms to Darcy's law [28,29], and it can be expressed as ...
Determining the width of the stress relief zone on roadway surrounding rocks is the premise to optimize drilling borehole effect and increase gas extraction efficiency. In this study, a new width measurement method of the stress relief zone on the roadway surrounding rocks was proposed, which determined the width according to gas pressure attenuation speeds in roadway boreholes at different depths. Then, the variation curve of the gas pressure in boreholes at different depths with the time was gained through a field test. On this basis, laws of the gas pressure attenuation and the gas transmission and loss in boreholes at different depths were explored through a numerical simulation based on COMSOL Multiphysics, thus concluding the stress on roadway surrounding rocks, the distribution of plastic zones, and the stress-permeability relation. The scientificity of the proposed method was illustrated theoretically. Finally, the proposed method was verified by the field test data and numerical simulation results of the gas extraction at different sealing depths. Research results demonstrate that the pressure in boreholes attenuates in the logarithmic function pattern. The attenuation speed decreases with the increase of the drilling depth. The width of the stress relief zone on roadway surrounding rocks in the studied area was determined to be about 11 m according to the proposed method. Both the numerical simulation and the field test of the gas extraction efficiency prove the feasibility and validity of the proposed method in determining the sealing depth of the borehole for the gas extraction. Research conclusions are of important significance to enrich width measurement methods of the stress relief zone on roadway surrounding rocks and to optimize sealing parameters of underground boreholes for gas extraction.
... Kudasik [54] proved that the porosity of coal is a parameter that has a deciding impact on permeability as described by Klinkenberg. The absolute permeability of coal increases and the slippage factor decreases as a result of increasing porosity [65]. As it was done in the previous section, only the briquette samples were used to describe the influence of the porosity on the Klinkenberg permeability parameters b and k . ...
Temperature is one of the main factors influencing the parameters describing the phenomena of sorption, diffusion and filtration of gases occurring in the pore space of coal. While the influence of temperature on sorption capacity and the diffusion coefficient of coal is known, changes in the coal permeability coefficient due to temperature changes are undetermined. The main objective of this work was to study and explain the influence of temperature on the permeability of raw and briquette coals with various porosities. In order to describe coal permeability, the Klinkenberg equation was used, as it takes into consideration the slippage effect, typical of porous media characterized by low permeability. The experiments on coal permeability to helium and nitrogen were carried out using an original measurement stand. They were performed on five samples prepared from the same bituminous coal material (two raws and three briquettes samples) from Budryk coal mine, located in the Upper Silesian Coal Basin in Poland. On the basis of the obtained results, it was established that as the temperature rises, the value of absolute permeability of raw coal to nitrogen and helium drops significantly. The influence of temperature changes on briquette coal permeability changes could be neglected, and it could be compensated by the changes in the value of the dynamic viscosity factor, which is present in the Darcy permeability equation. The results show that in the case of raw and briquette coals, the influence of temperature on permeability is ambiguous.
... Effective stress, in this particular case, would only have an indirect effect on a change in coal's permeability. Such an effect was suggested by Ma et al. [60], which investigated the impact of changes in the porosity of a granular coal sample on the permeability of that sample. Here, the change in the porosity was generated solely by reduction of the free pore space between grains, and not by applying stress. ...
Among the numerous factors that have an impact on coal permeability, coal porosity is one of the main parameters. A change in the mechanical stress applied to coal results in a change of porosity. The main objective of the conducted research was to answer the following question: is a decline in coal permeability a direct effect of a decrease in coal porosity, and does mechanical stress result solely in a porosity change? A study of coal porosity under mechanical stress conditions was conducted using a uniquely constructed measurement stand. The coal samples used were briquettes prepared from a granular coal material (middle-rank coal of type B—meta bituminous, upper carboniferous formation) from the “Zofiówka” coal mine, in Poland. In order to describe coal permeability, the Klinkenberg equation was used, as it takes into consideration the slippage effect, typical of porous media characterized by low permeability. On the basis of the obtained results, it was established that the values of the Klinkenberg permeability coefficient decrease as the mechanical stress and the corresponding reduction in porosity become greater. As the briquette porosity increased, the Klinkenberg slippage effect: (i) disappeared in the case of nitrogen, (ii) and was minor for methane. The briquettes used were characterized by various porosities and showed that mechanical stress results mainly in a change in coal porosity, which, in turn, reduces coal permeability.
... To further establish whether equation (2) suitably describes real geologic porous media representing different processes inducing pore geometry variation, we focused on the results from groups of previous laboratory experiments where geologic samples were deformed through compression (Dan et al., 2016;Gostick et al., 2006;Jones, 1987;Ma et al., 2015;Ma et al., 2016;Schrauf & Evans, 1986;Zhou et al., 2015), torsion (Okumura et al., 2009), shearing (Javadi et al., 2014;Wang, 2017), or decompression (Lindoo et al., 2016;Takeuchi et al., 2009), and from modeling experiments where pore networks with ducts of different shapes were systematically varied (Veyskarami et al., 2016), like we did with our CFD simulations discussed below. The complete details on the data set including the type of geologic porous media, the geometry-varying processes represented, and the fitted parameters are provided in Table S4 in the supporting information. ...
Plain Language Summary
Fluid flow through geologic porous media is dictated by permeability which is the resistance imparted by the medium. Flows in porous media are described by either Darcy's law or its extension for high flow rates, the Forchheimer equation. In both models, permeability represents the dissipation of mechanical energy by inertial losses and by fluid viscosity. Thus, permeability depends on both fluid properties and the configuration of pores. Decades of research has made the permeability in Darcy's law predictable from medium properties such as porosity and grain size, but the additional permeability in the Forchheimer equation has remained almost impossible to predict. This has hindered the application of the Forchheimer equation for many settings where it is potentially more appropriate. Through a broad synthesis of published data and through computational simulations, we were able to relate the permeability in Darcy's law to the permeability in the Forchheimer equation for the diversity of geologic porous media representing varied pore geometries and configurations. Thus, both kinds of permeability are now predictable and linked. This knowledge will help in many geophysical and engineering applications where it is necessary to consider flows at high rates.
... On this basis, the parameters of pressure relief gas extraction were optimized. Ma et al. [16,17] investigated the influence of stress rate and grain size on gas seepage characteristics in crushed coal particles. ...
In order to enhance gas extraction from unloaded coal seam by drilling borehole in the floor roadway, the mechanism of stress relief improving permeability by protective coal seam mining was analyzed. Based on the multiphysics field theory, the hydraulic-mechanical coupling model of gas extraction in the unloading coal seam was established, and the gas extraction process by drilling borehole in the floor roadway in the overburden of Panyi Coal Mine 1551 (1) panel was simulated. The influence of different drilling arrangements on the gas extraction effect was analyzed. The results show that the permeability of protected coal seams is characterized by zoning and can be divided into the permeability-enhanced zone, the permeability-reduced zone, and the original permeability zone according to the stress state of coal seam. Under the condition of uniform borehole distribution, the gas pressure decreased slowly in the permeability-reduced zone and is still greater than 0.74 MPa after 180 days of extraction, and there is a large extraction blind area in the protected panel. Under the condition of nonuniform borehole distribution arrangement according to the characteristics of permeability zoning, the effective extraction area can almost cover the protected panel, and the blind extraction area is reduced by 91.22% when compared to uniform borehole distribution. These can provide a reference for unloading gas extraction under similar conditions.
... Former studies often focused on the mining effect on water-inrush in the fractured geological structure [4][5][6][7][8][9][10][11]; on the contrary, latter studies preferred to pay more attention to the permeate properties of the fractured rock. e research team in China University of Mining and Technology had carried out a series of related theoretical and experimental research studies on the permeability of fractured rock based on the MTS 815 rock mechanics servo-controlled testing system [12][13][14][15][16][17][18][19][20][21][22][23]. ey considered that water flow in the fractured rock disobeyed Darcy's law, seemed the seepage system of fractured rock as a nonlinear dynamic system, and analyzed the stability and bifurcation of the system by Lyapunov's first method to reveal water-inrush mechanism in coal mines with fractured geological structure [12,13], which is named "seepage instability theory". ...
In order to study the water-inrush mechanism in fractured geological structure, seepage instability theory is picked up, which considers that water-inrush is the embodiment of seepage instability. Seepage tests are the basis for studying seepage instability in the fractured rock system, while the experimental system and equipment are the foundation of seepage tests. In this paper, we introduce a new experimental system for the seepage test on fractured rock accompanying with mass migration and loss and discuss its development and application. It presents the designed and manufactured experimental system, the subsystems, components and their functions, and the experimental scheme and the experimental process. Compared to the previous experimental system, more functions are satisfied, among which, the most important improvements are realizing a long-term permeate and keeping water pressure stable at a high level. These improvements are verified by a series of tests, and the results show that our experimental system has higher accuracy, stability, and reliability. The distribution of the permeate times and lost mass of different Talbol power exponents are obtained, and the time-varying rules of water pressure, water flow, the lost mass, and porosity are also revealed through the results. Although the experimental system also has some limitations, for instance, the measuring accuracy of pressure transducers and flow transducers, the provided maximum pressure of the quantitative displacement piston pump, the fine particles collection subsystem, etc., we will continue to improve it in our further research.
... With the increase of the displacement load, the specimen is first in the linear elastic stage (AB); when the cracks are first found at the stage and the number of cracks is increasing slowly, then the specimen is in the plastic deformation stage (BC); when the number of cracks is increasing dramatically and the peak stress appears at point C, next the specimen comes to the strain-softening stage (CE), and it should be noted that the most active AE events is found in this postpeak stage at point D, which also could be observed in the uniaxial compressive laboratory experiments; and finally, the specimen comes to the residual stage (EF), and the AE activities are maintained at a lower level in this stage. e whole process of the uniaxial compression is in good agreement with the observation of laboratory experiments [42,43] and numerical simulations [24,44]. e damage evolution of the specimen is shown in Figure 4. ...
It is of vital importance to understand the failure processes of the heterogeneous rock material with different kinds of preexisting fractures in underground engineering. A damage model was introduced to describe the initiation and propagation behaviors of the fractures in rock. Reduced parameters were applied in this work because the microcracks in the rock were neglected. Then, the numerical model was validated through comparing the simulation results with the laboratory observations. Finally, a number of numerical uniaxial compressive tests were performed on heterogeneous rock specimens with preexisting fracture, and the influence of the heterogeneity of the rock and the angle and length of the preexisting fractures was fully discussed. The results showed that the brittleness of the rock increased with the increase of the homogeneity index, and tensile failure was the main failure form for relatively heterogeneous rock, whilst shear failure was the main failure form for relatively homogeneous rock. The uniaxial compressive strengths of the specimens with the angles of 0, 30, 45, and 60 of the preexisting fracture dropped 62.7%, 54.7%, 46.6%, and 38.2% compared with that of the intact specimen; the tensile cracks were more difficult to form, and the required load was increasing with the increase of the angle of the preexisting fracture; besides, antiwing cracks were difficult to form than wing cracks because the tensile stress in wing cracks’ area was greater than that in antiwing cracks’ area. The uniaxial compressive strengths of the specimens with the lengths of 20 mm, 25 mm, 30 mm, and 35 mm of preexisting fracture dropped 38.6%, 46.6%, 53.4%, and 56.6% compared with that of the intact specimen, and the damage conditions of the samples with different lengths of preexisting fracture were similar.
... A study about the permeability evolution law in the fissure process of rock strata is presented by Yang et al. [18][19][20][21], according to the coupling equation of seepage and stress in the process of rock damage evolution. Some scholars [22][23][24][25][26][27][28][29][30][31][32][33] used similar simulation, numerical simulation, indoor test, and other methods to study the different areas, such as permeability characteristics and development characteristics of the pore structure of the roof and floor strata. In the rare hydrogeological condition where the coal seam is a pore-fissure confined aquifer, there are relatively few research on the connection between the permeability and development characteristics of the pore structure of the roof, floor strata, and confined water-rich coal seams. ...
During the construction and exploitation process in the Xiaojihan coal mine, which is located in Yulin of northern Shaanxi, we find a special hydrogeological phenomenon that coal seam is acted as a confined fractured aquifer. The water-rich coal seam has natural fissures which are confined with water storage. However, the water comes from the static and original reserves in coal seams, which have a weak link to other aquifers in the roof and floor strata. It indicates that the roof and floor strata provide a natural waterproof barrier for the fissure water in the coal seam, resulting in a relatively closed storage space of confined water. In order to further investigate the critical function that mechanical properties of permeability play in the confined fractured aquifer, the rock complete stress-strain permeability test and pore development structure test are carried out with rock samples of the roof and floor strata in this field. Results are as follows: (1) coal seams of Xiaojihan coal mine are confined fractured aquifers, the fissure confined water in coal seams has a strong relationship with total stress-strain permeability and development characteristics of the pore structure of the roof and floor strata. (2) The permeability of the roof and floor strata is extremely low, when the strata is less than 30 meters away from the coal seam with the magnitude order remaining less than 10 ⁻¹² m/s. If they are closer to the coal seam, the watertightness and plasticity of the strata will be stronger, and the antidestructive capability of the strata during the distortion process will be also increased, resulting in the larger strain for the formation of macroscopic water-conducting fissures. The roof and floor strata effectively cut off the hydraulic connection between the fissure water in the coal seam and other aquifers, which ensure the coal seam acts as water storage space of confined fractured aquifers. (3) For undeveloped fissures, the effective porosity is less than 2% of the roof and floor strata which are less than 30 meters away from the coal seam, and particularly, this index is less than 1% of the strata within 20 meters away from the coal seam, indicating that the strata have good water resistance. When the distance between the roof strata and the coal seam is greater than 40 meters, the effective porosity has a large fluctuation, and the effective porosity of the partial strata is greater than 10%, reflecting that the rock strata fissure has been well developed. It should be attached great importance to prevent water-conducting fissures from getting hydraulic connection with the fractured water-rich coal seams and other aquifers of the roof and floor, so as to reduce the risk of mining water hazards.
... To explore the applicable conditions for Darcy's law, a typical parameter, Reynolds number (Re), which presents the ratio of the inertial force and viscous force in different fluid flow situations (Zou et al. 2017b;Ma et al. 2016b), was applied in this paper. Re has been widely used in evaluating the influence of the inertial force in the flow field and in scaling of similar flow situations. ...
Filtration behaviour of cement particles, especially under the high grouting pressure with a rapid grout flow velocity, has a significant effect on the grout injection. However, there have been few studies on this field where the governing equation of this behaviour remains unclear. In the present study, a novel experimental procedure for grout injection was adopted to acquire the spatial and temporal variations in porosity and viscosity of high-speed grout flow in coarse sand. Experimental observations showed that there were dramatic variations in viscosity and porosity during the grout penetration within the first 50 s, suggesting that the high velocity had a significant influence on the distribution of the filtration coefficient. A model based on the Stokes–Brinkman (S–B) equation and advection–filtration equations was established to describe the filtration of grout flow in porous media. Meanwhile, numerical solutions from both the proposed model and traditional Darcy’s law were compared with experimental results. The comparative results showed that the proposed approach can match the laboratory tests well; the analysis indicated that Darcy’s law was unable to properly describe high-speed grout flow in porous media due to the lack of a shear force and the inertial term. Nonuniform filtration behaviour of cement grout flowing in porous media was revealed. Due to the nonuniform distribution of the pore velocity isoline caused by Poiseuille flow, it led to a heterogenous distribution of porosity as well. Parametric studies on the applicability of Darcy’s law and S–B equation for grout flow were discussed, in which an error of less than 10% was calculated when the Reynolds number was less than 2.5.
... Chen et al. [24][25][26] through their experimental studies found that the variation of the postpiping sand outflow over time satisfies Boltzmann's nonlinear relationship. Ma et al. [27] through the gas seepage properties of crushed coal specimens measurement found that (1) particle crushing during compaction was the main reason to increase small-size, indicating that gas seepage properties were strongly influenced by the particle size and axial displacement, and (2) the porosity decreased with the weight loss of larger particle size. ...
When the collapse column and its adjacent rocks in complex geological structures are disturbed by mining, concomitant fine particle migration, mass loss, and porous structure variation during the water seepage process in broken rocks are the inherent causes for collapse column activation and water inrush. Studying the time-varying characteristics of the mass-loss rate in the dynamic seepage system of the broken rocks is of theoretical importance for the prevention of water inrush from the collapse columns. In this study, the seepage tests of the broken mudstone were carried out using the patented pumping station seepage method, and the time-varying function of the mass-loss rate was generalized. Then, the optimal coefficients in the function of mass-loss rate were computed using the genetic algorithm. At last, the mass-loss rate in the dynamic seepage system of the broken rocks with consideration of the acceleration factor was calculated using Lagrange discrete element method. The results showed that (1) the mass-loss rate was expressed as a time-dependent, exponential function with its coefficient related to the porosity, and its time-varying characteristics were affected by gradation; (2) the time-varying curves with Talbol power exponents less than 0.6 had a rapid change stage and a slow change stage, while the time-varying curves with Talbol power exponents greater than 0.6 had an initial gradual change stage, a rapid change stage and a slow change stage; (3) at the early seepage stage, the mass-loss rate decreased with Talbol power exponent increasing; and (4) after long time seepage, the mass-loss rate was close to zero and unrelated to Talbol power exponent, and the porous structure in broken rocks remained stable with its porosity close to a certain stable value.
... Since there are always broken rock layers and granular rock mass around the goaf, the permeability of this area plays an important role in controlling the water and sand movement. In recent years, a great deal of effort has been put to investigate the permeability of the granular rock mass [12][13][14][15]. The main outcome is that the permeability of granular rock mass is sensitive to the stress condition and the granular composition, and its value varies due to the loss of small particles during seepage. ...
... Since there are always broken rock layers and granular rock mass around the goaf, the permeability of this area plays an important role in controlling the water and sand movement. In recent years, a great deal of effort has been put to investigate the permeability of the granular rock mass [12][13][14][15]. The main outcome is that the permeability of granular rock mass is sensitive to the stress condition and the granular composition, and its value varies due to the loss of small particles during seepage. ...
Water and sand inrush is one of the most serious threats in some shallow coal mines in China. In order to understand the process of sand inrush, experiments were performed to obtain the criterion for sand inrush. First, seepage tests were carried out to study the hydraulic properties of granular sandstone. The results indicate that seepage velocity has a linear relation with the porosity and particle-size distribution parameter. Then, sand inrush tests were conducted to investigate the critical conditions for sand inrush occurrence. It is determined that the sand inrush zone can be clearly distinguished based on the values of porosity and particle-size distribution parameter. Additionally, sand inrush tended to happen in the conditions of high porosity, high seepage velocity, and large particle-size distribution parameter. Further, general principles for preventing the water and sand inrush were proposed, such as reducing the porosity, improving the pore structure, and decreasing the seepage velocity. The proposed principles have been successfully used in situ to control the water and sand inrush.
... However, numerous engineering practices have proved its poor effect on squeezing failure (Wang and Feng 2005). Moreover, in ultrabasic rock, where water invading rocks result in strength deterioration and expansion of volume (Ma et al 2016(Ma et al , 2017Zhou et al. 2016Zhou et al. , 2017, the application of grouting support is not recommended (Wu et al. 2003;Fu et al. 2011). Therefore, it is very necessary to study the failure mechanisms and support technology of surrounding rock masses around floors and lower ribs. ...
Under high horizontal ground stress, the squeezing failure has been a common failure pattern for deep roadway. And the large deformation of surrounding rock mass around the roadway has also been a major challenge for deep mining practices. This paper describes a case study of the failure mechanisms and stability control technology of deep roadway under high horizontal ground stress in Jinchuan mine in Gansu Province, China. And this paper also aims to propose a kind of support strategy to prevent the plastic slippage around the floor of roadway. This study started with an in situ broken zone measurement programme to determine the depth of broken zone at each part of the roadway. Combined with long bolt and floor beam, an improved support strategy has been proposed. The field test results show that the improved support strategy can successfully solve the problems of side wall collapse and floor heaven. It can significantly restricts rock mass large deformation and avoid frequent repair. Compare to the original support, the improved support strategy can greatly save investment of mines and also has good application value.
... The soft coal blocks were smashed and sieved to particles with size fraction smaller than 1.0 mm (Figure 1d). The coal particles were sufficiently mixed to ensure that the distribution of particle was uniform [21,22]. Then, the raw particles were mixed with a certain weight percentage of water according to Equation (1). ...
Injecting water into a coal seam to enhance the cohesive strength of coal and thus minimize and reduce the coal wall spalling risk must be considered in underground coal mining systems. In general, coal with low cohesive strength contains clay minerals which may affect the stability of coal by interacting with water. Therefore, the coupled effects of moisture content and inherent clay minerals on the physical properties (i.e., cohesive strength and internal friction angle) of coal samples should be addressed. In this paper, direct shear tests were conducted by remodelling the Yiluo coal with various moisture contents ranging from 6.6% to 20.7%. According to Mohr–Coulomb failure criterion, cohesive strength and internal friction angle of coal were obtained. Afterwards, effects of moisture content and clay minerals (i.e., Kaolinite, Smectite and Illite) on the cohesive strength of coal were analysed using X-ray diffraction (XRD) method. The results show that cohesive strength increases when the moisture content rises from 6.6% to 17.6%, after which it decreases with increasing moisture content. This trend can be well illustrated by the relationship between typical water retention curve (WRC) and suction stress of soil. Therefore, a moisture content of 17.6% would be an optimal value to enhance the stability of the Yiluo coal seam.
... Well-designed experiments may yield important insights into behaviors that are not available from numerical simulations. The geo-mechanical model test is one of the most important means of physical model testing to investigate the stability of large-scale geotechnical engineering structures [15][16][17][18][19]. Once the similarity principles are satisfied, the geo-mechanical model test is expected to characterize the prototype quantitatively. ...
Cave collapses emerge during the process of oil reservoir development, seriously affecting oil production. To reveal the collapse and failure mechanism of the carbonate cavern with a buried depth of 5600 m in Tahe Oil Field, using a self-developed ultra-high pressure model test system with the intelligent numerical control function, the model simulation material of carbonate rocks developed to carry out the 3D geo-mechanical model test. The model test and numerical results indicate that: (1) collapse and failure mechanism of the deep-buried caves mainly involve the failure mode of tensile shear. The rupture plane on the side wall is approximately parallel to the direction of maximum principal compressive stress. The V-type tension and split rupture plane then emerges. (2) In the process of forming holes in the model caverns, micro cracks are generated at the foot of the left and right side walls of the caverns, and the roof panels are constantly moving downward. The shorter the distance to the cave wall, the severer the destructiveness of the surrounding rocks will be. (3) The displacement of the top of the model cavern is relatively large and uniform, indicating that the cave roof moves downward as a whole. The area of the cavity suffering damage is 2.3 times as large as the cave span. The research results in this paper lay a solid test basis for revealing the cave collapse and failure mechanism in super depth.
... Gas permeability is also influenced by fracture geometry [6,22], fracture geometry and water-content [23], and the presence of water [24]. The permeability evolution of granular coal was tested and the particle size [25] and temperature [26] were proven as affecting factors. According to the experimental studies of different rocks [27,28], altered permeability strongly relates to the original particle size distribution. ...
Coal seam gas, held within the inner pores of unmineable coal, is an important energy resource. Gas release largely depends on the gas seepage characteristics and their evolution within granular coal. To monitor this evolution, a series of experiments were conducted to study the effects of applied compressive stress and original grain size distribution (GSD) on the variations in the gas seepage characteristics of granular coal samples. Grain crushing under higher stress rates was observed to be more intense. Isolated fractures in the larger diameter fractions transformed from self–extending to inter-connecting pathways at a critical compressive stress. Grain crushing was mainly caused by compression and high-speed impact. Based on the test results of the original GSD effect, the overall process of porosity and permeability evolution during compression can be divided into three different phases: (1) rapid reduction in the void ratio; (2) continued reduction in the void ratio and large particle crushing; and (3) continued crushing of large particles. Void size reduction and particle crushing were mainly attributed to the porosity and permeability decreases that occurred. The performance of an empirical model, for porosity and permeability evolution, was also investigated. The predictive results indicate that grain crushing caused permeability increases during compression, and that this appeared to be the main cause for the predictive values being lower than those obtained from the experimental tests. The predictive accuracy would be the same for samples under different stress rates and the lowest for the sample with the highest proportion of large grain diameters.
... For the flow to be of Darcy type, β should be almost equal to zero. Equation (2) could be applied to match the water seepage in coal samples as described below [41,51,52]. In this case, the Reynolds number (Re) can be expressed as: ...
In order to better understand groundwater influx and protection in coal mining extraction works, an in-house water flow apparatus coupled with an industrial rock testing system, known as MTS 815.02, were used to study the effects of grain size mixtures on the compaction and flow properties of disintegrated, or non-cemented, coal samples. From the Reynolds number evaluation of the samples with different grain mixtures, and the relationship between the water flow velocity and pore pressure gradient differences, it was found that seepage through the mixtures are of non-Darcy flow type. The porosity of coal specimens was found to be highly affected by compaction, and the variations of the porosity were also influenced by the samples’ grain size distribution. It was found that the sample porosity decreases with increasing compaction and decreasing grain sizes. Grain crushing during compaction was observed to be the main cause of the appearance of fine grains, and the washing away of fine grains was consequently the main contributing factor for the weight loss due to water seepage. It was observed that during the tests and with the progression of compaction, permeability k decreases and non-Darcy factor β increases with decreasing porosity φ. The k-φ and β-φ plots show that as the sizes of disintegrated coal samples are getting smaller, there are more fluctuations between the porosity values with their corresponding values of k and β. The permeability value of the sample with smallest grains was observed to be considerably lower than that of the sample with largest grains. Non-Darcy behavior could reduce the hydraulic conductivity. It was found that the porosity, grain breakage and hydraulic properties of coal samples are related to grain sizes and compaction levels, as well as to the arrangement of the grains. At high compaction levels, the porosity of disintegrated coal samples decreased strongly, resulting in a significant decrease of the permeability at its full compression state; Non-Darcy flow behavior has the slightest effect in uniform samples, therefore, indicating that disintegrated coal in uniform grain size mixtures could be treated as an aquicluding (water-resisting) stratum.
... Stress-dependent permeability has been extensively studied in fractured rocks (Zimmerman and Bodvarsson 1996;Zimmerman 2000;Min et al. 2004;Liu and Rutqvist 2010). A previous study (Li et al. 2005;Ma et al. 2014Ma et al. , 2016 investigated the permeability measurement of the water flow in crushed rocks. The flow properties of a non-Darcy flow in crushed rocks were obtained, and it showed that the water seepage in crushed rocks no longer obeys the Darcy law but obeys the Forchheimer equation. ...
Given the importance of airflow seepage properties to coal self-oxidation in gob, this paper develops a method and self-designed apparatus to assess seepage properties of compacted broken coal. This study mainly focuses on the strain, porosity and permeability evolution under the different conditions of particle size, vertical stress and temperature. The studied results show: (1) The strain, porosity and permeability were enlarged when the particle size increased under the same loading stress. The porosity and permeability reduced when the vertical stress increased. (2) The non-Darcy coefficient was negative in all tests, but the absolute value of the non-Darcy coefficient generally increased when the vertical stress increased. (3) The experiment results indicated that the larger the particle was, the easier to be compacted. The larger the grain diameter was, the lower the porosity and permeability were, which shown that the void volume in broken coal with larger grain diameters could be easily compacted. (4) The permeability was reduced when the temperature increased, which indicated the permeability of the compacted broken coal decreased during low-temperature oxidation in gob. (5) By the effects of stress and the particle size diameter on the porosity and permeability, the vertical stress recovery and generally increase are advantageous to reduce the porosity and permeability in gob. It is favorable to reduce the porosity and permeability and prevent coal self-heating by reducing the degree of fragmentation and percentage of small particles or consolidate the small particles.
Gob is one of the most serious areas of coal spontaneous combustion. With the continuous increase of mining depth, the residual coal in gob bears increased axial stress, the rise of ground temperature, and the degree of fragmentation of mining coal and rock continues to increase, the external water in the mining process changes the water content of broken residual coal, under the influence of multiple factors, the prevention and control situation of coal spontaneous combustion in gob is becoming more serious. Based on the actual accumulation environment of residual coal in gob, this paper carried out the oxidation and temperature rise experiment of broken coal under the condition of water bearing and pressure. The variation laws of oxidation characteristic parameters such as outlet oxygen concentration, oxidation derived gas, oxygen consumption rate and heat release intensity were obtained. It is found that with the increase of water content of coal sample, the water content first promotes and then inhibits the oxidation process of coal. It is found that after water molecules form a relatively stable hydrogen bond with oxygen-containing functional groups, the total energy of the two-phase materials decreases, and the reason for the decrease of wetting heat release is analyzed. The influence mechanism of axial stress loading on coal spontaneous combustion oxidation process is studied. It is found that the increase of axial stress first promotes and then inhibits the process of coal spontaneous combustion.
Historically coal mining wastes have been viewed as heterogenous and hazard-prone geomaterials. Given that failures of colliery tips and tailing dams are reported on a regular basis, reclamation of coal mining wastes from storage facilities is increasingly being considered. There is a resistance to the use of coal mining waste in construction industry despite scattered but growing reports of successful applications. As the construction industry around the globe seeks to reduce its carbon emissions by looking for supplements for cement, the voluminous amount of coal mining wastes currently stored in spoil heaps and impoundment facilities present a potential source of raw materials. This article reviews the literature on the geochemical, geotechnical and structural engineering properties of coal mining waste geomaterials to assess their suitability as replacement for both aggregates and binders in concrete and cementitious composites (as opposed to reviewing the properties of those products themselves). It is found that coal mining wastes are indeed good candidates (as raw materials) for the uptake and process into higher level construction purposes. Geochemically, the key to a successful upcycling operation is the knowledge of their mineral contents (which is typically diverse and varies from one mine to another) and the processes they undergo while being transformed into constituents of new materials. The few studies on concretes made with coal mining wastes indicate that the mineralogical and mechanical characterisation of the wastes to obtain a mix featuring strength and durability performance that meets specification is important to a successful utilisation. In the geotechnical literature, coal mining wastes are known to be highly heterogeneous and may host expandable minerals with potential durability problems. However, this review also found that simple geotechnical index tests can be conducted to yield useful information for the initial screening of coal mining wastes into a construction product. The state-dependent properties of coal mining wastes (e.g., water retention, hydraulic conductivity, shear strength) are found to be governed by complex factors such as coal content, particle size and shape, pore size and shape, and the presence and interaction of pore air and pore water in the void space, some of these are well-studied but much of these are to be further researched.
Backfill mining is a lucrative method for extracting coal buried under buildings, and water bodies, which can substantially increase the resource usage efficiency by mitigating the strata movement and surface subsidence. Its effectiveness depends on the mechanical properties of granular backfill materials. A permeability test was performed on gangue and fly ash samples under different stress levels using an original seepage test system. The variation patterns of the broken rock’s internal pressure and permeability were determined. The test results indicate the weakening of the seepage effect on granular materials and a gradual reduction of washed away fly ash. The permeability values fall into the range of 3.2 × 10⁻¹⁵ ~ 3.2 × 10⁻¹³m⁻², and non-Darcy factor is between 3.2 × 10¹⁰ and 3.2 × 10¹² m⁻¹. This phenomenon was more pronounced in samples with smaller particle sizes. As the axial stress increased, the backfill material showed a decline in permeability and an increase in the non-Darcy flow coefficient. As the content of fly ash increased, the mass loss grew sharply, which occurred mainly at the early seepage stage. The results are considered instrumental in the characterization of water and sand inrush.
As mines continue to extend deeper, the stress and ground temperature of the residual coal in the goaf are increased, and the degree of coal rock fragmentation also increases, making the spontaneous combustion of coal more prominent. In this paper, a low-temperature oxidation experiment of coal under axial stress loading conditions was conducted. By analysing the outlet gas flow rate, oxidizing gas derivatives, oxygen consumption rate, heat release intensity, apparent activation energy and other parameters, we studied the effect of axial stress variations on the spontaneous combustion of coal. The results showed that, in an experiment of heating oxidation with the same axial stress, the flow rate of gas generated by the oxidation in the coal sample room gradually decreased with an increasing temperature; at the same temperature, the flow rate of the outlet gas decreased as the axial stress increased. At a low temperature, as the axial stress increased, the rate of oxygen consumption and the amount of derived gas in the reaction system both increased, and the activation energy decreased. When the temperature reached the critical temperature and continuously increased, with an increase in the axial stress, the oxygen consumption rate, heat release intensity and the amount of derived gas in the reaction system first increased and then decreased. Moreover, the temperature point of ethylene gas produced by oxidation was advanced. The effect of the axial stress on the spontaneous combustion of coal was obtained, and the increase in the axial stress first promoted and then suppressed the spontaneous combustion of broken coal.
The internal stress levels of the crushed rock masses will change the movement states of the rock masses in the gob, and then affect the void distributions of crushed rock mass, and finally change the flow of gas in the gob. In this study, under the Lagrange framework, based on a discrete element method and a Hertz-Mindlin contact model, self-programming was used to simulate the compaction processes of a rock mass in mining gobs. The compression amounts were set as 0%, 5%, 10%, 15%, 20%, and 25%, respectively. The effects on the void distributions of the crushed rock masses of the internal stress levels of the crushed rock masses under the external load conditions were examined by changing the motion state of the rock blocks within in the gobs. The results showed that during the compression processes, the external loads could not be uniformly transmitted from the top of the crushed rock masses to the lower sections. Therefore the stress levels in the upper sections of the crashed rock masses were significantly higher than those of the lower sections. In addition, due to the rock blocks in the upper section of the crushed rock masses experiencing higher degrees of interference from the external forces, the sliding speeds of the upper rock blocks were also larger. At the same time, the rock blocks in the upper sections of the crushed rock masses tended to slide downward down longer. The migration of the rock blocks caused the upper rock blocks to become densely packed, and the coordination numbers of the rock blocks were large. However, the lower rock blocks were loosely distributed, and the coordination numbers of the rock blocks were small. This resulted in the void ratios in the lower sections of the crushed rock masses being higher than the void ratios in other layers of the crushed rock masses, with the void ratios of the lower sections of the rock masses determined to be 1.1–1.4 times that of the upper sections.
As the depth of coal mining continues to increase, the weight of the overlying strata and the ground stress on the residual coal in goaves also increase. At the same time, the temperature of the coal rocks continues to rise, and the high ground temperature environment of the mining and heading faces becomes more severe. As a result, coal oxidation and spontaneous combustion disasters become more prominent. In this work, an anaerobic heating experiment of coal under the thermal-mechanical coupling effect was performed. The effects of thermal-mechanical coupling on the thermal stability of coal were studied by analyzing parameters such as the heat flow peak temperature, endothermic peak value, heat absorption, and activation energy. The results showed that the heat flow peak temperature exhibited hysteresis as the initial temperature and the initial stress increased. At the same time, the endothermic heat flow peak value and the heat absorption decreased. Under the same initial stress, the activation energy gradually decreased as the initial temperature of the coal sample increased; the higher the initial stress, the lower the decrease in the activation energy. At the same initial temperature, the activation energy first increased and then decreased as the initial stress increased; the higher the initial temperature, the more sensitive this change in the activation energy was to stress. In this study, the influence of thermal-mechanical coupling on the thermal stability of coal was obtained. This work provides on-site guidance for the prevention and control of spontaneous coal combustion in deep mining.
A novel particle fluctuating motion induced by chemical reaction is investigated. Fluctuation characteristics of petcoke particles at different reaction temperatures are analyzed. The fluctuating velocity of particles increased with the combustion temperature. It is revealed that the irregular micro-structure of particles and nonuniform distribution of active sites on particle surface account for the particle fluctuations. The various release velocity of gaseous product in different reaction micro-areas induce unbalanced bearing force of particles, which directly result to the particle fluctuating motions. Besides, the micro-structure characteristics of single particle evolved with the reaction proceeding. In this case, the random fluctuation of petcoke particles occurred during the reaction process. The average fluctuating velocity of petcoke particles follows the Arrhenius equation. This study is a supplement to the fluid mechanics of gas-solid phase flows existing chemical reaction.
The transfer and evolution of stress among rock blocks directly change the void ratios of crushed rock masses and affect the flow of methane in coal mine gobs. In this study, a Lagrange framework and a discrete element method, along with the soft-sphere model and EDEM numerical software, were used. The compaction processes of rock blocks with diameters of 0.6, 0.8, and 1.0 m were simulated with the degrees of compression set at 0%, 5%, 10%, 15%, 20%, and 25%. This study examines the influence of stress on void ratios of compacted crushed rock masses in coal mine gobs. The results showed that stress was mainly transmitted downward through strong force chains. As the degree of compression increased, the strong force chains extended downward, which resulted in the stress at the upper rock mass to become significantly higher than that at the lower rock mass. It was determined that under different degrees of compression, the rock mass of coal mine gobs could be divided, from the bottom to the top, into a lower insufficient compression zone (ICZ) and an upper sufficient compression zone (SCZ). From bottom to top, the void ratios in the ICZ sharply decreased and those in the SCZ slowly decreased. Void ratios in the ICZ were 1.2–1.7 times higher than those in the SCZ.
Moisture is one of the most important factors that influences coal seepage and coal-bed methane (CBM) extraction. To obtain the water occurrence state and dynamic processes of water change in coal, a series of microscopic observation experiments of Wei Jiagou coal by using field-emission environmental scanning-electron microscopy (ESEM) was conducted under the condition of a fixed point. Afterwards, a mathematical model to explain the influence of water on porosity and permeability was proposed based on the ESEM observations. It was found that there were three main types of water occurrence state: a crescent shape, a full filled shape and an annulus shape, which can provide powerful evidence to explain the influence of water on porosity and starting pressure gradient. As well as this, the box counting reached a minimum at a chamber pressure of 520 Pa and the box counting reduced after water wetting. Based on the mathematical model analysis, the water-occupied area of crescent shapes would reach a peak value with an increase of the contact angle, which has a critical impact on the effective porosity. The influence model that we built matched well with experimental data, which in turn demonstrated the validity of the mathematical model. The prominent combined effect of strain and water saturation appeared on the ridge of the permeability contour, while strains have little influence on permeability at a large initial porosity. Furthermore, a model for contact angle and wetting height was proposed and discussed, and contact angles with different improving fluids were tested. It also can be shown that using better wettability improving-fluid can save the cost of volume and have a good performance on the results of hydraulic technology based on model and experimental tests.
Karst collapse pillar (KCP) is widespread in North China coalfields, where coal extraction above the Ordovician limestone aquifer is threatened by the abundant supply of water and a very high hydraulic pressure. KCP is composed of rock skeleton and fine fillings, which can be transferred under the effect of water flow, thus KCP usually functions as a channel for groundwater inrush. In order to investigate the mechanism of mining-induced groundwater inrush of KCP which was caused by fine fillings transfer, a self-designed particle transfer permeability testing system was used to test the crushed mudstones’ flow properties, which included fine filling particle transfer rate, porosity increase rate and permeability under the conditions of varying pore pressure, particle size mixture and compaction level (initial porosity). The tests indicate fine fillings transfer is the essential reason for mining-induced groundwater inrush (flow instability) of KCP. The flow properties changeability during fine particle transfer could be divided into four stages, i.e., initial flow stage, flow inrush stage, continued particle flow stage and stable flow stage, and flow inrush stage which is the key point to cause water inrush. Due to the crushing of edges and corners and the adjustment of the structure, the fluctuation of permeability–time relationship mainly distributed in the first two stages, which make a change to flow channel. Moreover, with the increasing of pore pressure, particle size mixture and initial porosity, the water inrush time and stable seepage time decreased more rapidly; the fine fillings, porosity and permeability increased gradually. The efficiency criteria analysis between measurement and calculation permeability shows an empirical equation can fit the relationship between porosity and permeability well, and not all of the pore structures were in flow; this means there was a part of pores that were invalid, but the effective porosity in flow can be treated as calculation value approximately.
As mining activity rapidly develops to deeper and deeper ground, there have been an increasing number of groundwater inrush events occurring in China. The Ordovician limestone karst collapse pillar (KCP), which contains a lot of crushed rocks, usually functions as a channel for groundwater inrush. The factors, such as the geological conditions, groundwater flow and extraction percentage of coal seam in underground mines, are the main causes of recompaction and permeability change of crushed rock particles in KCP. The high permeability of rock will cause large volumes of groundwater flow, thus posing a great threat of loss of lives in the mines. It is important to take into account the compaction and seepage behaviors of the crushed rocks in Ordovician limestone KCP. The MTS815.02 system and a self-designed water flow apparatus were used to investigate the effect of particle size distribution on compaction and seepage behavior of crushed limestone particle mixture. The Reynolds number calculation of particle mixture shows that the seepage has been influenced by non-Darcy flow. Testing results indicate that the effective porosity of crushed limestone sample is strongly influenced by compaction and particle size distribution. The effective porosity decreases with the increase in compaction and decrease in larger particle size, respectively. Particle crushing during compaction is a main cause of size 0–2.5 mm materials, whereas some fine particles are washed away due to the effect of water seepage, which is a main cause of weight loss. Non-Darcy seepage properties of the crushed limestone are strongly influenced by compaction and particle size distribution. In general, during the compaction, the permeability decreases while the non-Darcy coefficient increases with the decreasing of effective porosity. The effective porosity, particle crushing and seepage properties of crushed limestone are not only related to compaction levels and mixture sizes but also to style of arrangement and initial pore structure.
The amount of grout to inject into a given volume of fractured foundation rock depends on the fracture porosity and on the spacing and sizes of fracture openings. These properties cannot be measured directly but can be calculated from water-pressure tests if several simplifying assumptions are made. The method is valid for fractured rock masses whose intergranular permeability is very small compared to the fracture permeability, and if solution cavities or pervious interbeds are absent. Studies of 35 dam sites indicate that maximum fracture porosities are about 0.05% near the surface, decreasing to about 0.005% at the 200-ft depth. The volume of grout required for impregnation is correspondingly small. Fracture openings decrease from about 100 microns to 50 microns in the same depth interval. Cement grout penetration is accordingly limited to a small proportion, only the largest, of fractures. The minimum spacing of open fractures increases from 4 ft to 14 ft. All rock types appear to be similar in fracture properties.
Permeability tests are performed at different hydraulic gradients by using a constant head permeameter. It is clarified experimentally that the critical value of the hydraulic gradient (or Reynolds Number) at which the flow of water through crushed rock change from laminar to turbulent flow varies with the grain size of crushed rock. Empirical formulas for the critical values of the hydraulic gradient and Reynolds number are proposed as a function of the effective grain size of crushed rock. Further, empirical formulas for the average discharge velocity under the laminar and turbulent flow conditions are proposed as a function of the effective grain size of crushed rock and the hydraulic gradient.
A hybrid composite of Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes (LMNCO NTs) wrapped with reduced graphene oxide (RGO) nanosheets ([email protected]/* */) was prepared as cathode for lithium-ion batteries. The discharge capacity of the [email protected]/* */ composite is only reducing 3.5% after 100 cycles at 1 C. Such composite which simultaneously combines a high surface area of LMNCO NTs with shorten ionic diffusion pathway and high conductivity of 3D graphene hierarchical architectures as well as structural protection layers, displaying a good cycling stability.
It is seen from the literature that practically no study has been carried out to analyse the converging flow behaviour through coarse granular media. An experimental programme to study the behaviour of large size crushed rock and marbles on a converging permeameter was undertaken. The applicability of the well known Forchheimer's, Missbach's and Wilkins' equations to analyse the flow behaviour in a converging permeameter was examined. Nasser's experimental data on converging flow through crushed rock were re-examined and the results are compared with that of the present study. The relative role played by Darcy parameter a and the non-Darcy parameter b on the flow behaviour with converging permeameter are brought out.
The geostatic stress, coal mine gas and disturbances produced by external loading can undermine the stability of a borehole and cause the borehole to collapse in a soft coal seam, which results in a poor efficiency of drilling long boreholes and draining gas in a soft coal seam. To improve the efficiency of drilling boreholes and draining gas in a soft coal seam, we propose a drilling technology of protecting the borehole by spraying foam concrete slurry onto the surface of borehole wall during drilling in a soft coal seam. To realize this process, we propose spraying foam concrete onto the borehole wall during drilling with a foldable drill bit and double drill pipes. Based on this new technology, the equipment and the material required for drilling long boreholes in a soft coal seam were studied. Finally, this new technology was tested in the Zhaozhuang mines in the Shanxi Province of China. The test results indicate the following: the process of spraying foam concrete on the borehole wall can relieve borehole collapse; the plant-based foam concrete has good air permeability, and has no effect on the permeability of the borehole wall; and the novel process of drilling boreholes and spraying foam concrete is an effective drilling process, which can relieve borehole collapse, improve the drilling efficiency and improve the draining efficiency.
Aiming to resolve the technical problems of lower gas concentrations and a reduced effective drainage period caused by gas-drainage borehole fracture development, a flexible gel (FG) gas-drainage borehole sealing material was developed that adapts to borehole deformation. In this study, based on orthogonal tests, the effect of the ratio of material to water, stirring time and stirring speed on the viscosity, filtration property, water retention and pumpability of the FG were studied. The results indicate that the stirring speed, ratio of material to water and stirring time in turn increased the viscosity and the ratio of material to water, stirring time and stirring speed enhanced the filtration property and water retention. The FG pumps smoothly and achieves the optimal state of high water retention, low fluid loss and low viscosity when the ratio of material to water is 1:10, the stirring speed is 800 r/min, and the stirring time is 12 min. The field test results indicate that, after using the FG, the average drainage gas concentration increases by 25.9% and 27.6% and the average negative pressure of extraction increases by 2.7 kPa and 3.5 kPa compared with expansive cement and polyurethane, respectively. © 2015 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
In order to solve the problems of top-coal inadequate destruction and large amounts of gas emission in mining extra thick and hard coal seam, this study investigated the pre-splitting for deep borehole blasting and gas pre-draining technologies on top coal. The mechanism of the technologies was systematically expounded based on hard top-coal cracks development obtained by numerical simulation and theoretical analysis. The results show that explosive blasting in the hard rock results in a large number of cracks and large displacement in the rock mass due to the effect of explosion stress. Meanwhile, the thick top-coal caves, and desorbing gas flows along the cracks improve gas extraction. Finally, the pre-splitting for deep borehole blasting and gas pre-draining technologies was applied in No. 3802 working face of Shui Liandong Coal Mine, which increases monthly output in the face to 67.34 kt and the drained gas concentration to 86.2%. The drained gas average concentration from each borehole reaches 40%, and the effect is remarkable. © 2015 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
Based on the principle of effective stress in porous media, a nonlinear dynamic model of the fluid-structure coupling for non-Darcy seepage in the over-broken rock mass was established, and for its decoupled model of one dimensional seepage, the time series of the pore pressure and the seepage velocity convergent to steady-state were obtained by non-dimensional transformation and the iteration method of successive lower relaxation. The dynamic responses of the system to variant parameters were analyzed, and also their trajectory patterns were given. The results indicate that the curve of the time series of the seepage velocity exists a track with two periods. When parameter of the nonlinear term increases or the system is far from steady state, the system will lose stability gradually, thus result in dynamic disaster of the water seepage.
We now have all the elements needed in order to formulate the complete statement of a problem of forecasting the distribution of state variables within a porous medium domain at the macroscopic level. The statement includes two parts: a conceptual model and a mathematical one.
A thorough study of the sorption behavior of coals to methane and carbon dioxide (CO2) is critical for carbon sequestration in coal seams and enhanced coalbed methane recovery. This paper discusses the results of an ad/de-sorption study of methane and CO2, in single gas environment, on a set of coal samples taken from the San Juan and Illinois Basins. The results indicate that, under similar temperature and pressure conditions, coals exhibit higher affinity to CO2 as compared to methane and that the preferential sorption ratio varies between 2:1 and 4:1. Furthermore, the experimental data were modeled using Langmuir, BET, and Dubinin-Polanyi equations. The accuracy of the models in quantifying coal-gas sorption was compared using an error analysis technique. The Dubinin-Radushkevich equation failed to model the coal-gas sorption behavior satisfactorily. For methane, Langmuir, BET, and Dubinin-Astakhov (D-A) equations all performed satisfactorily within comparable accuracy. However, for CO2, the performance of the D-A equation was found to be significantly better than the other two. Overall, the D-A equation fitted the experimental sorption data the best, followed by the Langmuir and BET equations. Since the D-A equation is capable of deriving isotherms for any temperature using a single isotherm, thus providing added flexibility to model the temperature variation due to injection/depletion, this is the recommended model to use.
In coal permeability models, it is normally assumed that coal matrix pressure is equalized with the fracture pressure (local equilibrium). In this assumption, coal swells uniformly under constant confining (total) stress conditions commonly used in laboratory measurements. Under these conditions, a uniform swelling will not change the fracture aperture for a matchstick model where only two sets of vertical fractures cut through the whole matrix blocks. However, a uniform swelling changes both the fracture aperture and the spacing (the coal bridge swelling increases the fracture aperture while the matrix swelling changes the spacing only) for a fractured coal model, where fractures do not create a full separation between adjacent matrix blocks but where solid coal bridges are present, is used. Therefore, coal permeability remains unchanged for a matchstick model or increases slightly due to the coal bridge swelling under common laboratory conditions. These conclusions are directly contradictory with most laboratory observations in the literature. This direct contradiction suggests that the local equilibrium condition has not been achieved under common laboratory conditions. If this was the case, the current local equilibrium assumption based approach would be inappropriate for the analysis of laboratory measurements.
Coal porosity and permeability are key factors influencing coal-bed methane well production. In order to investigate the permeability behavior during anthracite coal seam methane production, the porosity and permeability of anthracite coal sample from No.3 coal seam in Southern Qinshui Basin of China in net confining stress were measured in laboratory. The correlations between porosity, permeability and effective stress were analyzed. Permeability damage rate, stress sensitivity coefficient and pore compressibility factor were proposed to evaluate the effective stress-dependent sensitivity characteristics of anthracite coal. It turns out that, both porosity and permeability of coal sample decrease exponentially with the increase of effective stress. In the effective stress is less than 5MPa or 6MPa, stress sensitivity coefficient of coal reservoir changed greatly, and the stress sensitivity coefficient decreases rapidly with effective stress increased; The permeability damage rate increases rapidly with increasing effective stress, the stress sensitivity of coal reservoir enhanced; while in the effective stress is greater than 5MPa or 6MPa, the stress sensitivity coefficient of the coal reservoir decreases as effective stress increases slowly, and there is fluctuation, the stress sensitivity of coal reservoir is reduced; while permeability damage rate with the increase of effective stress increased more slowly. With the increase of moisture content and temperature, the permeability damage rate of coal reservoir and stress sensitive coefficient increase, and the stress sensitivity of coal reservoir enhanced.
Crushed rocks, especially mudstones, whose seepage properties are strongly influenced by particle mixture during compaction due to muds will be washed away after water flow. The present study focuses on the porosity evolution, particle crushing and non-Darcy seepage properties, during the compaction, of a crushed mudstone particle size mixture. An experiment based on a self-designed water flow apparatus, the MTS 815.02 system and a non-Darcy testing method were performed to investigate the effect of particle size mixture on seepage properties and compaction behavior of crushed mudstones. In particular, the Reynolds number calculation of particle mixture shows that water flow has involved the influence of non-Darcy flow. Testing results indicate that: (1) The porosity of crushed mudstones is strongly influenced by compaction (axial displacement) and particle mixture. The porosity decreases with the increase in axial displacement and decrease in bigger particle size, respectively. (2) During the compaction, some larger particles were crushed which is a main reason to cause size 0-2.5 mm. Muds washed away are the main reason for weight lost in mudstone samples due to the effect of water seepage. (3) Non-Darcy seepage properties of crushed mudstones were strongly influenced by mixture sizes and compaction, and in general, during the axial compression, the permeability decreases while the non-Darcy coefficient increases with the decrease in porosity . (4) The fluctuations of - and - curves show that the lager the particle size, the more the fluctuation displayed in the curves. The permeability of minimum size shows one order of magnitude less than that for largest one. (5) The porosity, particle crushing and seepage properties of crushed mudstones are not only related to compaction levels, mixture sizes, but also related to the style of arrangement.
In order to reveal the law of raw coal seepage at different gas pressures, the gravity constant load seepage experimental system was developed and used. The law of raw coal seepage at different gas pressures with He, N2 and CO2 was investigated. The results show that, in a given state of stress during the experiment, with the increase of gas pressure, the permeability of raw coal sample prone to outburst exhibits a significantly decrease, and then exhibits an increasing trend when reaching the extreme point. The law of Klingberg coefficient related to the stress state and the gas adsorption properties was also obtained. Under the same experimental conditions, the Klingberg coefficient of He is greater than that of N2; and the Klingberg coefficient of CO2 has minimum value; so the stronger the gas adsorption is, the smaller the Klingberg coefficient of gas goes. Klinkenberg coefficient decreases with the increase of effective stress. Under the same conditions, the permeability of He is greater than that of N2; the permeability of CO2 has minimum value; so the stronger the gas adsorption is, the lower the permeability of the coal sample goes. The results have important significance in revealing the mechanism of gas seepage, predicting coal mine gas disaster, and gas drainage and safety production.
Due to the high permeability of water flow in crushed rocks, flow catastrophes and water inrush accidents are apt to take place in the broken zones of aquifers in coal mining engineering. The pore, crack and fracture geometries needed for water transport are strongly influenced by grains diameter size and axial displacement conditions. In order to inspect and quantify the influence, we designed and manufactured a water flow apparatus that can be connected to the electro-hydraulic servo-controlled test system MTS815.02 which provides loading pressure in the experiment. Using the apparatus and MTS system, we tested crushed mudstone, limestone and sandstone specimens and obtained the relationship between permeability and variable grain diameter of a (2.5-5 mm), b (5-10 mm), c (10-15 mm), d (15-20 mm) and e (mixed sizes) under variable axial displacement (10, 15, 20, 25, 30, 35 and 40 mm). In particular, the permeability calculation based on collection of water flow velocity and pore pressure gradient difference has involved the influence of non-Darcy flow. The results show that (1) The porosity decreases with the increase of axial displacement and decrease of bigger particle size, respectively. Particle crushing during compaction is a main cause of size 0-2.5 mm appearing, some fine particles be washed away is a main cause of weight loss because of the effect of water seepage. (2) Water flow properties of crushed rocks are found to be strongly influenced by axial displacement and grain diameter size; in general, the permeability decreases with the increase of axial displacement and the decrease of grain diameter, respectively. (3) The fluctuations of permeability-axial displacement are especially intense for mudstone and sandstone than that for limestone. The permeability of crushed rocks is not only related to loading levels but also to grain diameters, style of arrangement. (4) To each grain diameter sizes, the permeability change of sandstone has a greater value than that of mudstone and limestone. The permeability of crushed mudstone shows much less than that of limestone and sandstone.
Groundwater inrush has an impartible relationship with geological structures such as karst collapse pillars (KCPs), which are widely distributed in North China. In order to study the mechanism of groundwater inrush from coal seam floor, the variable mass dynamics and nonlinear dynamics were introduced. A mechanical model-plug model is established to describe the behavior of water seepage flow in coal-seam-floor containing KCP. The study shows that: (1) If the mass of the KCP keeps steady, the water seepage velocity in the KCP and the surrounding rocks will reach a constant value soon; (2) if the mass of the KCP and the surrounding rocks increases by grouting, etc, the seepage velocity in the KCP and the surrounding rocks will reach its minimum value gradually, and (3) if the mass of the KCP and the surrounding rocks decreases by scouring, the flow velocity in the KCP and the surrounding rocks shows a monotone increase, the water flow may change into pipe flow, especially when a large number of the mass of the KCP and the surrounding rocks enters into goaf road, which may lead to instable flow and cause groundwater inrush.
Free radicals play an important part in coal utilization, such as carbonization, gasification, liquefaction and pyrolysis processes. The diagenesis of organic sediment, pyrolytic reactions during metamorphosis and radiolysis are the three possible geneses of stable free radicals in coal. The influences of different origins on the nature of free radicals and their subsequent behaviors during the coal utilization are of great interest. In this paper, three experimental studies of super-fine comminution, fixed-bed pyrolysis and in situ ultraviolet irradiation were adopted to investigate the behaviors of the paramagnetic centers in coal/char. The nature of different radical species in coal/char during the influential processes was focused through the deconvolution study of the electron paramagnetic resonance (EPR) spectra. Final results indicate that the comminution, pyrolysis and ultraviolet (UV) irradiation are all effective ways to initiate the free radicals. The super-fine comminution has noticeable influence on the aromatic hydrocarbon radicals. The UV irradiation can promote the formation of the σ-type oxygen-containing radicals in coal. Furthermore, the simple aromatic clusters and the σ-type oxygen-containing radicals are the most active types during thermal degradation of pulverized coal particles.
Gas contents are highly variable in coalbed methane (CBM) reservoirs of the Yanchuannan (YCN) area of the southeast (SE) Ordos Basin, China. We used diverse geologic data derived from more than five years of exploration to provide insight into the origin of this variability and the consequences of gas content on reservoir performance. Major factors affecting gas content variability include gas generation, migration, trapping and preservation. Gas generation affects gas content variability on the scale of the total resource, whereas gas migration influences the inhomogeneous redistribution of gas content on a regional or local scale. Gas trapping and preservation affect the “as-observed” content. The potential for high gas content is controlled directly by the composite result of gas generation, migration, trapping and preservation. CBM in the YCN area is produced from the relatively thick seam (~2.09 m and 8.05 m, with an average of 5.97 m) that is distributed through 450 to 1200 m of the stratigraphic section. Gas content tends to be structurally and hydrodynamically controlled in the order of simple structure (folds and small faults)>complex structure (large regional faults) and groundwater stagnant zones>runoff zones. Coal samples in the YCN area typically have Langmuir volumes between 31.86 and 46.51 cm3/g, which correlates with coal rank. Reservoir heterogeneity including coal composition, pore structure and matrix moisture content may contribute to the heterogeneous gas content. Gas content is generally high where hydrodynamic trapping of gases occurs and may be anomalously low in areas of active recharge with downward flow potential and/or convergent flow where there is no mechanism for entrapment. In the YCN area, the most favorable area for CBM exploration and development is in the center block (block B), where great coal thickness, moderate burial depth, favorable hydrodynamics and an anticlinal trap coincide to yield high gas contents.
The permeability of Westerly granite was measured as a function of effective pressure to 4 kb. A transient method was used, in which the decay of a small incremental change of pressure was observed; decay characteristics, when combined with dimensions of the sample and compressibility and viscosity of the fluid (water or argon) yielded permeability, k. k of the granite ranged from 350 nd (nanodarcy = 10−17 cm2) at 100-bar pressure to 4 nd at 4000 bars. Based on linear decay characteristics, Darcy's law apparently held even at this lowest value. Both k and electrical resistivity, ρs, of Westerly granite vary markedly with pressure, and the two are closely related by k = Cρs−1.5±0.1, where C is a constant. With this relationship, an extrapolated value of k at 10-kb pressure would be about 0.5 nd. This value is roughly equivalent to flow rates involved in solute diffusion but is still a great deal more rapid than volume diffusion. Measured permeability and porosity enable hydraulic radius and, hence, the shape of pore spaces in the granite to be estimated. The shapes (flat slits at low pressure, equidimensional pores at high pressure) are consistent with those deduced from elastic characteristics of the rock. From the strong dependence of k on effective pressure, rocks subject to high pore pressure will probably be relatively permeable.
The research activities in China Institute of Atomic Energy (CIAE) are mainly performed in Beijing National Tandem Accelerator Laboratory (BNTAL), a national user facility in the Department of Nuclear Physics (DNP). During recent years, the main machine is HI-13 tandem accelerator, which is delivering the stable heavy-ion beam with energy of up to 15 MeV/q. It annually provides beam time for about 50 experiments with about 4,000 hours. The beam time sharing by research area is as follows: nuclear reaction 24%, nuclear structure 23.5%, nuclear data 18%, and application 34.5%. In the above research, the annually delivered beam time for outside users are 1,200 hours. The users range from application institutions, universities, to research institutes. The total number of the annual publication of SCI papers are more than 50. About 100 foreign visitors visited our Lab, and more than 50 staff members visited abroad each year.
Force characteristic of the large coal particle was investigated when it moved in a dense medium gas-solid fluidized bed with the 100-300 mu m Geldart B magnetite powder as fluidized medium. We selected two groups of coal particles with the equivalent diameters of 13 and 25 mm as the studying objects. The theoretical force calculation model of the large coal particle was established by force analysis. The approaches of experimental measurement and numerical simulation were combined to determine the force characteristic of the large coal particle. The results indicate that the forces vary within certain suitable ranges and present uniform fluctuations when the dense medium gas-solid fluidized bed maintains a stable fluidization state. The spectral analysis method was introduced to compare the force results of measurement and simulation. Finally, we obtain the conclusion that the theoretical force calculation results provide good agreements with the measurement and simulation results, which also validates the accuracy of the measurement results and the reliability of the simulation results for the force characteristic of the large coal particle.
Based on the steady-state seepage method, we used the Mechanical Testing and Simulation 815.02 System and a self-designed seepage instrument for over-broken stone to measure seepage properties of water flows in three types of crushed rock samples. Three methods of confidence interval in describing permeability coefficients are presented: the secure interval, the calculated interval and the systemic interval. The lower bound of the secure interval can be applied to water-inrush and the upper bound can solve the problem of connectivity. For the calculated interval, as the axial pressure increases, the length of confidence interval is shortened and the upper and lower bounds are reduced. For the systemic interval, the length of its confidence interval, as well as the upper and lower bounds, clearly vary under low axial pressure but are fairly similar under high axial pressure. These three methods provide useful information and references for analyzing the permeability coefficient of over-broken rock.
The effectiveness of transmitting underground water in rock fractures is strongly influenced by the widths of the fractures and their interconnections. However, the geometries needed for water flow in fractured rock are also heavily controlled by the confining pressure conditions. This paper is intended to study the seepage properties of fractured rocks under different confining pressures. In order to do this, we designed and manufactured a water flow apparatus that can be connected to the electro-hydraulic servo-controlled test system MTS815.02, which provides loading and exhibits external pressures in the test. Using this apparatus, we tested fractured mudstone, limestone and sandstone specimens and obtained the relationship between seepage properties and variations in confining pressure. The calculation of the seepage properties based on the collection of water flow and confining pressure differences is specifically influenced by non-Darcy flow. The results show that: (1) The seepage properties of fractured rocks are related to confining pressure, i.e. with the increase of confining pressure, the permeability
$ k $
decreases and the absolute value of non-Darcy flow coefficient
$ \beta $
increases. (2) The sandstone coefficients
$ k $
and
$ \beta $
range from
$ 1.03 \times 10^{ - 18} $
to
$ 1.53 \times 10^{ - 17} $
m2 and
$ - 1.13 \times 10^{17} $
to
$ - 2.35 \times 10^{18} $
m−1, respectively, and exhibit a greater change compared to coefficients of mudstone and limestone. (3) From the regression analysis of experimental data, it is concluded that the polynomial function is a better fit than the power and logarithmic functions. The results obtained can provide an important reference for understanding the stability of rock surrounding roadways toward prevention of underground water gushing-out, and for developing underground resources (e.g. coal).
The commercial extraction of methane from coal beds is now well established in a number of countries throughout the world, including the USA, Australia, China, India and Canada. Because coal is almost pure carbon, its reservoir character is fundamentally different to conventional gas plays. Coalbed methane (CBM) forms as either biogenically- or thermogenically-derived gas. The former occurs in ‘under mature’ (< 0.5% vitrinite reflectance) coals and is the result of bacterial conversion of coal into CO2 or acetate, which is then transformed by archaea into CH4. Thermogenic gas is formed as part of the coalification process and is purely a chemical devolatilization that releases CH4. Methane is primarily stored in coal through adsorption onto the coal surface; thus it is pore surface area that determines the maximum gas holding potential of a reservoir (as opposed to pore volume in a conventional reservoir). Although macro-, meso-, and micropores are present in the coal matrix, it is thought that the micropores are where most methane adsorption occurs. In many of the micropores, the methane molecule may actually stretch, minutely, the pore and thus with de-gassing of the reservoir, could result in matrix shrinkage, allowing opening of the fracture (cleat) system in the coal and thus enhancing permeability. The organic composition of the coal is paramount in determining porosity and permeability character and thus maximum gas holding capacity. In general, the higher the vitrinite content the higher the gas holding potential (and ultimately the amount of desorbed gas) and permeability (all other factors being the same). There are other organic component/gas property relationships but these seem to be specific to individual basins, or even seams.
The liquid–solid fluidized bed has been widely applied in the separation process of coarse coal particles. The coal slurry should be classified before feeding into the liquid–solid fluidized bed. The most of fine coal particles, with the particle size less than 0.2 mm or 0.3 mm, should be removed from the coal slurry in the feeding. However, there are still amounts of fine coal particles which cannot be removed due to low classification efficiency. The separation process may be affected by these fine coal particles which always enter in a part of the clean coal without effective separation process. This investigation was carried out to determine the effect of fine coal particles on the separation process of liquid–solid fluidized bed. A liquid–solid fluidized bed was designed and eight sampling points were fixed on the column axis. The changes in size, density and ash content of coal particles were observed on the column axis and the partition coefficients of + 0.25 mm and − 0.25 mm fractions were obtained.
To predict the erosion of tubes and surfaces of heat exchangers in boilers caused by coal ash particles, particle-laden flows past a 10 × 11 staggered tube bank in a duct are investigated using a high-resolution numerical technique. The flow field is obtained through direct numerical simulation (DNS), the coupling between tubes and underlying flow is done through the immersed boundary method, and the particles are tracked by the Lagrangian approach. Classic particle–wall impact and erosion models are incorporated to investigate the collision and wear on the side walls and the tubes near the bottom side walls of the duct. It is found that particles at different Stokes numbers have different dispersion patterns in the flow field, and lead to different characteristics of collision and erosion on the near-wall tubes as well as the side walls. The larger Stokes number results in higher global erosion on the first tube near the side wall, but the particles with St = 1.0 cause the largest erosion to other downstream tubes because of the preferential concentration effect. The distributions of collision and erosion on each tube are not uniform, but bias toward the side facing the wall, and the more downstream tubes have the higher erosion. For side wall erosion, particles having higher local collision frequency may lead to lower erosion. The reverse trend is also observed for the particles with the intermediate Stokes number of 1. Periodical oscillation of particle–wall collision and erosion happens because of the staggered arrangement of the tubes.
Transport velocities of narrow cut sizes of coarse particles of sand and coal were determined at room temperature and atmospheric pressure. These velocities were obtained by four different methods previously utilized by other authors with fine particles. The four methods tested gave good predictions of the transport velocities. The method based on the measurement of the time required for all the solids to leave the bed without feeding in any fresh solid is specially interesting because of its rapidity and simplicity. The determined transport velocities were strongly dependent on the solid particle size and density. The experimental values were fitted to an equation which fitted both the experimental results obtained in this work and other published results obtained with fine particles.
A thorough study of the sorption behavior of coals to methane and carbon dioxide (COâ) is critical for carbon sequestration in coal seams and enhanced coalbed methane recovery. This paper discusses the results of an ad/de-sorption study of methane and COâ, in single gas environment, on a set of coal samples taken from the San Juan and Illinois Basins. The results indicate that, under similar temperature and pressure conditions, coals exhibit higher affinity to COâ as compared to methane and that the preferential sorption ratio varies between 2:1 and 4:1. Furthermore, the experimental data were modeled using Langmuir, BET, and Dubinin-Polanyi equations. The accuracy of the models in quantifying coal-gas sorption was compared using an error analysis technique. The Dubinin-Radushkevich equation failed to model the coal-gas sorption behavior satisfactorily. For methane, Langmuir, BET, and Dubinin-Astakhov (D-A) equations all performed satisfactorily within comparable accuracy. However, for COâ, the performance of the D-A equation was found to be significantly better than the other two. Overall, the D-A equation fitted the experimental sorption data the best, followed by the Langmuir and BET equations. Since the D-A equation is capable of deriving isotherms for any temperature using a single isotherm, thus providing added flexibility to model the temperature variation due to injection/depletion, this is the recommended model to use. 49 refs., 9 figs., 5 tabs.
Coal is not an inert reservoir rock and reacts to gas des-orbed from its surface. Coal matrix shrinks as gas is desorbed, increasing cleat width and, therefore, permeability. Very few coal matrix shrinkage data have been reported in the literature so a series of experiments was undertaken to measure such data at reservoir pressures, temperatures, and 100% relative humidity. Strain gages were affixed to the coal sample in the face and butt cleat directions as well as the vertical direction. This work reports measured deformations of a sample of high volatile C bituminous coal from the San Jan Basin during sorption and desorption of first methane then CO2. A pressure cycle was also run with helium, a nonsorbing gas, to determine mechanical compliance of the sample. Observed strain gage behaviors are discussed and shrinkage coefficients for both gases reported. Matrix shrinkage was found to correlate with gas content rather than pressure, confirming the work of a previous investigator. Shrinkage coefficients varied more among replicate gages aligned in the same direction than between gages in different directions. Anisotropic shrinkage effects are discussed. Using a matchstick geometry model, equations are derived for permeability change due to matrix shrinkage. Coefficients reported here are used in example calculations of absolute permeability and porosity increases during coalbed depletion.
Laboratory observations have shown that coal permeability under the influence of gas adsorption can change instantaneously from reduction to enhancement. It is commonly believed that this instantaneous switching of permeability is due to the fact that the matrix swelling ultimately ceases at higher pressures and the influence of effective stresses take over. In this study, our previously-developed poroelastic model is used to uncover the true reason why coal permeability switches from reduction to enhancement. This goal is achieved through explicit simulations of the dynamic interactions between coal matrix swelling/shrinking and fracture aperture alteration, and translations of these interactions to perrmeability evolution under unconstrained swellings. Our results of this study have revealed the transition of coal matrix swelling from local swelling to macro-swelling as a novel mechanism for this switching. Our specific findings include: (1) at the initial stage of CO2 injection, matrix swelling is localized within the vicinity of the fracture compartment. As the injection continues, the swelling zone is extending further into the matrix and becomes macro-swelling. Matrix properties control the swelling transition from local swelling to macro swelling; (2) matrix swelling processes control the evolution of coal permeability. When the swelling is localized, coal permeability is controlled by the internal fracture boundary condition and behaves volumetrically; when the swelling becomes macro-swelling, coal permeability is controlled by the external boundary condition and behaves non-volumetrically; and (3) matrix properties control the switch from local swelling to macro swelling and the associated switch in permeability behavior from reduction to recovery. Based on these findings, a permeability switching model has been proposed to represent the evolution of coal permeability under variable stress conditions. This model is verified against our experimental data. It is found that the model predictions are consistent with typical laboratory and in-situ observations available in lietratures.
We report laboratory experiments that investigate the permeability evolution of an anthracite coal as a function of applied stress and pore pressure at room temperature as an analog to other coal types. Experiments are conducted on 2.5cm diameter, 2.5–5cm long cylindrical samples at confining stresses of 6 to 12MPa. Permeability and sorption characteristics are measured by pulse transient methods, together with axial and volumetric strains for both inert (helium (He)) and strongly adsorbing (methane (CH4) and carbon dioxide (CO2)) gases. To explore the interaction of swelling and fracture geometry we measure the evolution of mechanical and transport characteristics for three separate geometries — sample A containing multiple small embedded fractures, sample B containing a single longitudinal through-going fracture and sample C containing a single radial through-going fracture. Experiments are conducted at constant total stress and with varied pore pressure — increases in pore pressure represent concomitant (but not necessarily equivalent) decreases in effective stress. For the samples with embedded fractures (A and C) the permeability first decreases with an increase in pressure (due to swelling and fracture constraint) and then increases near-linearly (due to the over-riding influence of effective stresses). Conversely, this turnaround in permeability from decreasing to increasing with increasing pore pressure is absent in the discretely fractured sample (B) — the influence of the constraint of the connecting fracture bridges in limiting fracture deformation is importantly absent as supported by theoretical considerations. Under water saturated conditions, the initial permeabilities to all gases are nearly two orders of magnitude lower than for dry coal and permeabilities increase with increasing pore pressure for all samples and at all gas pressures. We also find that the sorption capacities and swelling strains are significantly reduced for water saturated samples — maybe identifying the lack of swelling as the primary reason for the lack of permeability decrease. Finally, we report the weakening effects of gas sorption on the strength of coal samples by loading the cores to failure. Results surprisingly show that the strength of the intact coal (sample A) is smaller than that of the axially fractured coal (sample B) due to the extended duration of exposure to CH4 and CO2. Average post-failure particle size for the weakest intact sample (A) is found to be three times larger than that of the sample B, based on the sieve analyses from the samples after failure. We observe that fracture network geometry and saturation state exert important influences on the permeability evolution and strength of coal under in situ conditions.
In coal mining the water flow in broken rock is a very common phenomenon. Study of seepage properties of broken rock is one
of the basic subjects required in order to understand the stability of rock surrounding roadways, preventing disasters such
as water inrush and gas outbursts and developing underground resources. So far, quantitative studies on the nonlinear seepage
properties of broken sandstone under different porosities are not extensive in the research literature. In this article, by
means of an electro-hydraulic servo-controlled test system (MTS815.02) and a patent seepage device, the seepage properties
under different conditions of porosity were tested on broken sandstone of five different grain sizes. Based on the loading
method of controlling the axial compression displacement and steady permeating method, we obtained curves of the relation
of pore pressure with time, as well as the relation curves between the pore pressure gradient for steady seepage and velocity.
Furthermore, we calculated the permeability k and non-Darcy coefficient β corresponding to different porosities by fitting these curves with the binomial expression. This study indicates that: (1)
the seepage properties of broken sandstone are closely related to grain size, load levels, and porosity structure; (2) the
permeability k decreases, while the coefficient β increases with a decrease in porosity φ, but both the k − φ and the β − φ curves show some local fluctuations; (3) the permeability k of the broken sandstone has a magnitude of 10−14–10−12 m2, while the coefficient β ranges from 1010 to 1012 m−1. The results obtained provide some information for further study of the nonlinear seepage behavior of broken rock theoretically.
A phytase gene (phyA), isolated from Aspergillus ficuum (AF537344), was introduced into cotton (Gossypium hirsutum L.) by Agrobacterium-mediated transformation to increase the phosphorus (P) acquisition efficiency of cotton. Southern and Northern blot analyses
showed that the phyA was successfully incorporated into the cotton genome and expressed in transgenic lines. After growing for 45 days with phytate
(Po) as the only P source, the shoot and root dry weights of the transgenic plants all increased by nearly 2.0-fold relative
to those of wild-type plants, but were similar to those of transgenic plants supplied with inorganic phosphorus. The phytase
activities of root extracts prepared from transgenic plants were 2.4- to 3.6-fold higher than those from wild-type plants,
and the extracellular phytase activities of transgenic plants were also 4.2- to 6.3-fold higher. Furthermore, the expressed
phytase was secreted into the rhizospheres as demonstrated by enzyme activity staining. The transgenic plants accumulated
much higher contents of total P (up to 2.1-fold after 30 days of growth) than the wild-type plants when supplied with Po.
These findings clearly showed that cotton plant transformed with a fungal phytase gene was able to secret the enzyme from
the root, which markedly improved the plant’s ability to utilize P from phytate. This may serve as a promising step toward
the development of new cotton cultivars with improved phosphorus acquisition.
A series of laboratory coupled shear-flow tests for fracture replicas under normal stresses was performed with visualization of fluid flow using a newly developed coupled shear-flow-tracer testing equipment and these laboratory tests were simulated by using FEM, considering evolutions of aperture and transmissivity with large shear displacements. The distributions of fracture aperture and its evolution during shearing and the flow rate were calculated from the initial aperture and shear dilations and compared with results measured in the laboratory coupled shear-flow-tracer tests using transparent fracture replicas and a CCD camera that provides continuous images of contact area and flow path evolution in real-time of the shearing tests. Numerical simulation results such as aperture and contact distributions agree well with the images obtained from the coupled shear-flow tests of fracture specimens with visualization of the fluid flow. The numerical models captured complex behavior of fluid flow in fracture samples and agree well with the experimental flow rate results. J. Ross Publishing, Inc.
For deep coal seams, significant reservoir pressure drawdown is required to promote gas desorption because of the Langmuir-type isotherm that typifies coals. Hence, a large permeability decline may occur because of pressure drawdown and the resulting increase in effective stress, depending on coal properties and the stress field during production. However, the permeability decline can potentially be offset by the permeability enhancement caused by the matrix shrinkage associated with methane desorption. The predictability of varying permeability is critical for coalbed gas exploration and production-well management. We have investigated quantitatively the effects of reservoir pressure and sorption-induced volumetric strain on coal-seam permeability with constraints from the adsorption isotherm and associated volumetric strain measured on a Cretaceous Mesaverde Group coal (Piceance basin) and derived a stress-dependent permeability model. Our results suggest that the favorable coal properties that can result in less permeability reduction during earlier production and an earlier strong permeability rebound (increase in permeability caused by coal shrinkage) with methane desorption include (1) large bulk or Young's modulus; (2) large adsorption or Langmuir volume; (3) high Langmuir pressure; (4) high initial permeability and dense cleat spacing; and (5) low initial reservoir pressure and high in-situ gas content. Permeability variation with gas production is further dependent on the orientation of the coal seam, the reservoir stress field, and the cleat structure. Well completion with injection of N2 and displacement of CHâ only results in short-term enhancement of permeability and does not promote the overall gas production for the coal studied.
A series of air permeability tests were conducted on four hand-packed samples of alluvial sands and glass beads using a newly developed air permeameter. The permeameter was tested and found capable of precisely controlling soil-water matric potential (in the range 0 to 1 bar) while simultaneously facilitating the direct measurement of air permeability in porous media. Permeameter results indicate that air permeability increases with a corresponding decrease in water content over a monotonic drainage cycle. It was observed that the rate of change in air permeability with respect to changes in water content is highest at high water content and lowest at low water content. In several soil samples, the air permeability approached a constant value at low water content. Air permeability variations with water content were observed to differ among soils of different texture. For example, the intrinsic permeability of water was 11 to 86% of the maximum air permeability. The new permeameter allowed rapid and accurate measurements of air permeability in fine-textured materials over a wide range of matric potentials and water content.
In order to study the effect of time lag and stress loading rates on rock deformation, the conventional stepped stress loading mode was changed into a continuous mode to investigate the effect of effective pressure on permeability and porosity. The time lag effect of rock deformation illustrating the relationship between changes in permeability and steady time was studied. Permeability reduction ratios were measured under different stress loading rates which were achieved by different pump rate settings. The results show that permeability and porosity gradually decrease with increases in effective pressure. Permeability at high effective pressure attains stability quickly. Steady times at low effective pressure are very long. Reduction in permeability at lower stress loading rates is small, while, in contrast, it is large at high stress loading rates.
Coal permeability is highly sensitive to the stress. Meanwhile, coal swells with gas adsorption, and shrinks with gas desorption. Under reservoir conditions these strain changes affect the cleat porosity and thus permeability. Coal permeability models, such as the Palmer and Mansoori and Shi and Durucan models, relate the stress and swelling/shrinkage effect to permeability using an approximate geomechanical approach. Thus in order to apply these models, stress–permeability behaviour, swelling/shrinkage behaviour and the geomechanical properties of the coal must be estimated. This paper presents a methodology for the laboratory characterization of the Palmer and Mansoori and Shi and Durucan permeability models for reservoir simulation of ECBM and CO2 sequestration in coal. In this work a triaxial cell was used to measure gas permeability, adsorption, swelling and geomechanical properties of coal cores at a series of pore pressures and for CH4, CO2 and helium with pore pressures up to 13MPa and confining pressures up to 20MPa. Properties for the permeability models such as cleat compressibility, Young's modulus, Poisson's ratio and adsorption-induced swelling are calculated from the experimental measurements. Measurements on an Australian coal are presented. The results show that permeability decreases significantly with confining pressure and pore pressure. The permeability decline with pore pressure is a direct result of adsorption-induced coal swelling. Coal geomechanical properties show some variation with gas pressure and gas species, but there is no direct evidence of coal softening at high CO2 pressures for the coal sample studied. The experimental results also show that cleat compressibility changes with gas species and pressure. Then the measured properties were applied in the Shi and Durucan model to investigate the permeability behaviour during CO2 sequestration in coal.