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Matrix acidizing is a widely used stimulation process during which acid is injected into the formation at pressures that are less than the fracturing pressure. The purpose of matrix acidizing is to improve formation permeability or to bypass damaged zones through rock dissolution. Because of precipitation and diagenesis, carbonate rocks might conta...
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... Carbonate reservoirs hold 60% and 40% of the world's total oil and gas reserves, respectively. To ensure their efficient development is vital for global energy security [1,2]. Acidizing fracturing, which constructs highly conductive fractures, can greatly boost oil and gas production [3][4][5][6]. ...
Successful exploitation of carbonate reservoirs relies on the acid-fracturing process, while the thickeners used in this process play a key role. It is a common engineering problem that thickeners usually fail to function when used in high-temperature environments. Until now, no research has ventured into the field of synthesizing thickeners which can be effectively used at ultra-high temperatures up to 180 °C. In our current study, a novel high-temperature-resistant polyacrylamide thickener named SYGT has been developed. The thermal gravimetric analysis (TGA) reveals that SYGT is capable of withstanding temperatures of up to 300 °C. Both our scanning electron microscopy (SEM) and rheological analysis demonstrate that the SYGT exhibits excellent resistance to both temperature and shear. At 180 °C, the viscosity of the SYGT aqueous solution is no lower than 61.7 mPa·s at a 20% H+ concentration or high salt concentration, and the fracture conductivity of the thickened acid reaches 6 D·cm. For the first time, the influence of the polymer spatial network’s structural parameters on the viscosity of polymer solutions has been evaluated quantitatively. It was discovered that the length and surrounding area of the SNS skeleton have a synergistic effect on the viscosity of the polymer solution. Our experiments show that SYGT effectively reduces the acid–rock reaction rate and filtration loss under harsh working conditions such as high temperature, strong shear, high salinity, and a high concentration of acid. The synthesized acid-fracturing thickener (SYGT) has wide application potential in the development of carbonate reservoirs under high-temperature conditions.
... Acidizing is one of the most common enhanced oil recovery (EOR) methods for oil and gas reservoirs. In matrix acidizing [1], the wormhole [2] penetrates the contaminated area near the wellbore, connecting the formation and the wellbore to increase production. There are typically five types of dissolution patterns for acidizing carbonate cores according to laboratory studies [3][4][5][6][7]. ...
Acidizing is a widely adopted approach for stimulating carbonate reservoirs. The two-scale continuum (TSC) model is the most widely used model for simulating the reactive process in a carbonate reservoir during acidizing. In realistic cases, there are overburden pressure and pore pressure at present. When the injected acid reacts with the rock, the dissolution of the rock and the consumption of the acid in the pore will break the mechanical balance of the rock. Many experimental studies show that cores after acidizing have lower strength. However, it is still not clear how the deformation of rocks by the change of ground stress influences the acidizing dynamics. For fractured carbonate reservoirs, fractures play a leading role in the flow of injected acid, which preferentially flows into the fractures and dissolves the fracture walls. The effect of the combined action of rock mechanical balance broken and fracture wall dissolution on the formation of wormholes in fractured carbonate reservoirs remains to be studied. To address the above-mentioned issues, a thermal-hydrologic-mechanical-chemical coupled model is presented based on the TSC model for studying the wormhole propagation in fractured carbonate reservoirs under practical conditions. Linear and radial flow cases are simulated to investigate the influences of fracture distribution, reaction temperature, and effective stress on acidizing dynamics. The simulation results show that more wormhole branches are formed by acidizing if the fractures are perpendicular to the flow direction of acid. Temperature is a key parameter affecting the acidification dissolution patterns, so the influence of temperature cannot be ignored during the acidification design. As the effective stress of the formation increases, the diameter of the wormhole gradually decreases, and the branching decreases. More acid is needed for the same stimulation result under higher effective stress.
... Currently, more than 60% of oil reserves and 40% of natural gas reserves come from carbonate reservoirs [1]. Carbonate oil and gas reservoirs have poor matrix permeability and low porosity [2], but they have well-developed natural fractures and caves, of which limestone caves and fractures are the main reservoir and seepage space [3]. ...
The failure of thickeners at high temperature results in gelled acid acidification fracturing. To solve the problem, 8 kinds of polymers were synthesized by free radical polymerization of aqueous solution using AM, AMPS, NaAMPS, MAPTAC, DTAB and NVP as raw materials. The polymer was characterized by infrared spectroscopy and viscosity-average molecular weight, and the temperature resistance, rheology, salt resistance and shear resistance of the polymer solution were compared, and the mechanism was analyzed. The results show that the viscosity of GTY−2 is 181.52 mPa·s, and the viscosity loss rate is 56.89% at 180 °C and 100 s−1, and its temperature resistance is the best. Meanwhile, the viscosity retention rate of GTY−2 is 84.58% after 160 min shear, showing the strongest shear resistance. The viscosity loss rate of GTY−1 in 20% hydrochloric acid solution is 80.88%, and its acid resistance is stronger than that of GTY−2. Moreover, due to the amphiphilicity of DTAB, the molecular hydration film becomes thicker, and the salt resistance of GTY−2 is lower than that of GTY−1. The experimental results show that GTY−1 and GTY−2 have good temperature resistance, salt resistance, acid resistance and shear resistance, and can be used as thickeners for acid fracturing with thickened acid to improve the effect of acid fracturing under high temperature conditions.
... To investigate if similar phenomena occur within potash seams, heterogeneous rock distributions need to be examined. In addition, mineralogical heterogeneity and the associated variations in dissolution rate need to be taken into account, because these can significantly influence dissolution structure, especially at high Péclet numbers Wei et al. 2019). ...
Salt deposits offer a variety of usage types. These include the mining of rock salt and potash salt as important raw materials, the storage of energy in man-made underground caverns, and the disposal of hazardous substances in former mines. The most serious risk with any of these usage types comes from the contact with groundwater or surface water. It causes an uncontrolled dissolution of salt rock, which in the worst case can result in the flooding or collapse of underground facilities. Especially along potash seams, cavernous structures can spread quickly, because potash salts show a much higher solubility than rock salt. However, as their chemical behavior is quite complex, previous models do not account for these highly soluble interlayers. Therefore, the objective of the present thesis is to describe the evolution of cavernous structures along potash seams in space and time in order to improve hazard mitigation during the utilization of salt deposits. The formation of cavernous structures represents an interplay of chemical and hydraulic processes. Hence, the first step is to systematically investigate the dissolution and precipitation reactions that occur when water and potash salt come into contact. For this purpose, a geochemical reaction model is used. The results show that the minerals are only partially dissolved, resulting in a porous sponge like structure. With the saturation of the solution increasing, various secondary minerals are formed, whose number and type depend on the original rock composition. Field data confirm a correlation between the degree of saturation and the distance from the center of the cavern, where solution is entering. Subsequently, the reaction model is coupled with a flow and transport code and supplemented by a novel approach called ‘interchange’. The latter enables the exchange of solution and rock between areas of different porosity and mineralogy, and thus ultimately the growth of the cavernous structure. By means of several scenario analyses, cavern shape, growth rate and mineralogy are systematically investigated, taking also heterogeneous potash seams into account. The results show that basically four different cases can be distinguished, with mixed forms being a frequent occurrence in nature. The classification scheme is based on the dimensionless numbers Péclet and Damköhler, and allows for a first assessment of the hazard potential. In future, the model can be applied to any field case, using measurement data for calibration. The presented research work provides a reactive transport model that is able to spatially and temporally characterize the propagation of cavernous structures along potash seams for the first time. Furthermore, it allows to determine thickness and composition of transition zones between cavern center and unaffected salt rock. The latter is particularly important in potash mining, so that natural cavernous structures can be located at an early stage and the risk of mine flooding can thus be reduced. The models may also contribute to an improved hazard prevention in the construction of storage caverns and the disposal of hazardous waste in salt deposits. Predictions regarding the characteristics and evolution of cavernous structures enable a better assessment of potential hazards, such as integrity or stability loss, as well as of suitable mitigation measures.
... Nevertheless, carbonate rocks are more complicated identified with sandstones because of complicated geological functions, which result in some carbonate reservoirs less efficiently developed due to low permeability matrix (Ghommem et al., 2020;Yuan et al., 2019). Acidizing is one effective stimulation technique for these carbonate reservoirs, in which acid solution is used to dissolve the solid matrix for reducing the fluid flow resistance and ultimately improving the recovery (Wei et al., 2019;Zhang et al. 2019Zhang et al. , 2020. ...
... Moreover, both terms are important in Ali and Nasr-El-Din models (Ali and Nasr-El-Din, 2019). Some studies still think that the transport of acid solution is still dominated by the retardation effect of the porous structure, and the interaction between fluids can be ignored (Liu et al., 2021;Wei et al., 2019). This work is mainly to conduct a comprehensive study on fluid phase momentum transfer in carbonate acidizing. ...
This work mainly studies the effect of fluid phase momentum transfer mechanisms on the acidizing results, including the retardation effect of the porous structure and the interaction between the fluid phase, such as viscous dissipation and inertial effect. The results show that the acid fluid momentum transfer is influenced by the complex porous structure and fluid viscous dissipation. Eventually, the Stokes-Darcy equation is recommended to be adopted to describe the fluid phase momentum transfer in the following numerical simulation studies on the carbonate acidizing process. Based on this model, a parametric research is carried out to investigate the impact of acid on rock physical characteristics on the stimulation process. Increasing the acid concentration appears to minimize the quantity of acid consumed for the breakthrough. The acid surface reaction rate has a considerable impact on the pore volume to breakthrough and the optimum acid injection rate. The influence of permeability on the acidizing results basically shows a negative correlation with the injection rate. The difference between the acidizing curves of different permeability gradually becomes insignificant with the decrease of injection rate. The existence of isolated fracture and vug significantly reduces acid consumption for the breakthrough.
... A two-scale continuum model offers relatively less computational cost compared with a network model and it is more compatible than the empirical model. It is further extended to 2D radial flow (Kalia and Balakotaiah 2007), 3D linear flow (Maheshwari et al. 2013), and to consider the influence of acid types (Maheshwari et al. 2016b;Hosseinzadeh et al. 2017), oil saturation (Wei et al. 2017(Wei et al. , 2019, fracture (Liu et al. 2017), and temperature Liu et al. 2019). Mahrous et al. (2017) further extended the two-scale continuum model to consider different chemical reaction mechanisms involved in the acidizing process, which greatly improves the simulation accuracy of the specific chemical reactions involved in the acidizing process. ...
... We continue to study the effect of the fluid-phase-momentum-conservation equation on the acidizing results. In the classical twoscale continuum model, the Darcy's-law equation is widely used to determine the fluid superficial velocity (Panga et al. 2005;Kalia and Balakotaiah 2007;Maheshwari et al. 2013Maheshwari et al. , 2016bHosseinzadeh et al. 2017;Mahrous et al. 2017;Wei et al. 2019). In this work, a more fundamental equation, the Navier-Stokes-Darcy equation, is used to describe fluid flow, which is gained through volume averaging the Navier-Stokes equation at pore scale according to the representative elementary volume (Whitaker 2013). ...
... Several models are developed from both experimental studies (Ullah et al. 1992;Kirchner and Eigenberger 1996) and analytical studies (Turitto 1975;Tronconi and Forzatti 1992;Balakotaiah and West 2002). Eventually, the model developed by Balakotaiah and West (2002) is selected and used in nearly all two-scale continuum models (Panga et al. 2005;Maheshwari et al. 2013;Wei et al. 2019). ...
Matrix acidizing is a common technique for carbonate reservoir stimulation. In this work, a new two-scale continuum model is developed to study the 2D acidizing process. The Navier-Stokes-Darcy equation is used instead of the Darcy’s-law equation to describe fluid flow. The continuity equation is also modified to consider the mass-exchange term between fluid and solid phases. The comparison results show that neglecting the solid-matrix-dissolution source term results in overestimation of pore volume (PV) to breakthrough (PVBT). The Darcy’s-law equation does not well-capture physical behaviors of fluid phase with low acid-injection velocity compared with the Navier-Stokes-Darcy equation. On the basis of this model, we discuss different processes influencing matrix acidizing, including convection, diffusion, and reaction, and different models, including classical and new two-scale continuum models. Besides, a comprehensive parametric study is also conducted to study the effect of parameters with respect to acid and rock physical parameters on the matrix-acidizing process. The typical dissolution patterns and optimal acid-injection rate presented in experimental studies can be well-observed by the new two-scale continuum model. Increasing the acid-injection concentration has a limited effect on the amount of acid mass but substantially reduces the amount of solute required. The acidizing curve is very sensitive to the dispersity coefficient, acid-surface-reaction rate, and porosity/permeability relationship.
... To investigate if similar phenomena occur within potash seams, heterogeneous rock distributions need to be examined. In addition, mineralogical heterogeneity and the associated variations in dissolution rate need to be taken into account, because these can significantly influence dissolution structure, especially at high Péclet numbers [11,12]. ...
Potash seams are a valuable resource containing several economically interesting, but also highly soluble minerals. In the presence of water, uncontrolled leaching can occur, endangering subsurface mining operations. In the present study, the influence of insoluble inclusions and intersecting layers on leaching zone evolution was examined by means of a reactive transport model. For that purpose, a scenario analysis was carried out, considering different rock distributions within a carnallite-bearing potash seam. The results show that reaction-dominated systems are not affected by heterogeneities at all, whereas transport-dominated systems exhibit a faster advance in homogeneous rock compositions. In return, the ratio of permeated rock in vertical direction is higher in heterogeneous systems. Literature data indicate that most natural potash systems are transport-dominated. Accordingly, insoluble inclusions and intersecting layers can usually be seen as beneficial with regard to reducing hazard potential as long as the mechanical stability of leaching zones is maintained. Thereby, the distribution of insoluble areas is of minor impact unless an inclined, intersecting layer occurs that accelerates leaching zone growth in one direction. Moreover, it is found that the saturation dependency of dissolution rates increases the growth rate in the long term, and therefore must be considered in risk assessments.
... Compared with sandstone sedimentary rocks, carbonate reservoirs have strong heterogeneity due to complex geological processes, which result in more formation energy consumption in some carbonate reservoirs with low permeability (Zhao et al., 2020;Liu et al., 2021). Carbonate acidizing is a widely used stimulation technology to effectively improve the oil recovery of carbonate reservoirs using the acid solution to dissolve the low permeability matrix (Wei et al., 2019). ...
Carbonate reservoirs are one of the most important fossil fuel sources, and the acidizing stimulation is a practical technique for improving the recovery of carbonate reservoirs. In this study, the improved two-scale continuum model, including the representative elementary volume (REV) scale model and the upscaling model, is used to study the acidizing process with an isolated fracture. Based on this model, a comprehensive discussion is presented to study the effect of the physical parameters of the isolated fracture on the acidizing results and dissolution images, including the isolated fracture geometry, location, and morphology. Results show that the isolated fracture system is still the target system for the acidizing stimulation. The isolated fracture provides a limited contribution to the core porosity. The permeability of the core sample with fracture can be obviously increased only when the fracture penetrates through the whole sample. The existence of the isolated fracture reduces the consumption of acid solution to achieve a breakthrough. The acidizing curve is sensitive to the change of the length, aperture, and position of the isolated fracture. The acidizing curve difference corresponding to different rotation angles has not changed significantly for clockwise rotation and anticlockwise rotation groups.
... They occur during solvent imbibition for curation work (Léang et al. 2019), waterwicking in sportswear (Lei et al. 2020), and particle transport across filtering devices (Sanaei and Cummings 2019). For the oil and gas industry, in particular, understanding how fluids move through reservoir rock is relevant to characterize flow units (Oliveira et al. 2016), design well patterns , and acidize the porous matrix (Wei et al. 2019;Ferreira et al. 2020). Porosity, permeability, specific surface area, grain sphericity, pore tortuosity, pore radius, and shape factor are a few examples of grain and pore properties complicated to characterize through experiments only. ...
Porous media characterization is crucial to engineering projects where the pore shape has impact on performance gains. Membrane filters, sportswear fabrics, and tertiary oil recovery are a few examples. Kozeny–Carman (K–C) models are one of the most frequently used to understand, for instance, the relation between porosity, permeability, and other small-scale parameters. However, they have limitations, such as the inability to capture the correct dependence of permeability on porosity, the imperfect handling of the linear and nonlinear effects yielded by its fundamental quantities, and the insufficiency of geometrical parameters to predict the permeability correctly. In this paper, we cope with the problem of determining shape factors for generic geometries that represent sundry porous media configurations. Specifically, we propose a method that embeds the Poiseuille number into the classical K–C equation and returns a substitute shape factor term for its original counterpart. To the best of our knowledge, the existing formulations are unable to obtain shape factors for pores whose geometry is beyond the regular ones. We apply a Galerkin-based integral (GBI) method that determines shape factors for generic cross sections of pore channels. The approach is tested on straight capillaries with arbitrary cross sections subject to steady single-phase flow under the laminar regime. We show that shape factors for basic geometries known from experimental results are replicable exactly. Besides, we provide shape factors with precision up to 4 digits for a class of geometries of interest. As a way to demonstrate the applicability of the GBI approach, we report a case study that determines shape factors for 19 generic individual pore sections of a laboratory experiment involving flow rate measurements in an industrial arrangement of a water-agar packed bed. Porosity, flow behavior, and velocity distributions determined numerically achieve a narrow agreement with experimental values. The findings of this study provide parameters that can help to design new devices or mechanisms that depend on arbitrary pore shapes, as well as to characterize fluid flows in heterogeneous porous media.
... Reservoir simulation is a hot topic in recent decades (Thibodeaux et al., 2019) (Bao et al., 2017) (Bao et al., 2018) (Wei et al., 2019) (Wei et al., 2017). One category of reservoir simulation is coupled geomechanics and reservoir simulation, which captures the interaction between fluid flow, displacements, and stresses (Ren et al., 2016) (Ren et al., 2017) (Zheng et al., 2019a(Zheng et al., , 2019b. ...
The traditional way of analyzing wellbore stability of infill wells relies on the analytical solution of the stress path, which assumes uniaxial strain, homogenous depletion, constant overburden stress, and linear elastic condition. The objective of this study is to re-evaluate all the above four assumptions and analyze the plasticity, stress arching, lateral deformation and inhomogeneous depletion of the reservoir. Coupled geomechanics and fluid flow analysis is conducted to get the time and location dependent stress field and pressure field, which serve as inputs for wellbore stability analysis for infill wells. Considering four cases, the ranking according to the width of the safe drilling mud weight windows is 1) plasticity, 2) elasticity, 3) plasticity with stress arching, and 4) elasticity with stress arching. This coupled geomechanics and fluid flow can provide dynamic information concerning where to drill, when to drill, and how to drill for infill wells.
