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Shale gas is a crucial unconventional natural gas; carbon dioxide is a serious greenhouse gas. Therefore, using supercritical carbon dioxide as a fracturing fluid can fracture shale reservoirs and exploit shale gas as well as store carbon dioxide to slow the greenhouse effect. In this study, supercritical carbon dioxide fracturing experiments were...
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... verify the effect of the borehole excavating damage zone on the fracturing failure pressure, numerical experiments of the borehole drilling were conducted by introducing the fast Lagrangian analysis of the continua (FLAC). The numerical model can be simplified as Figure 6. The plastic zone is one of the factors for assessing the failure mode, propagation of the fracturing crack, and effect zone of the pore pressure (Yin 2014). ...
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... results illustrate that effective crack networks can be generated by supercritical CO 2 fracturing. The three-dimensional crack morphology can be reconstructed based on CT scanning images, as shown in Figure 16. The fissure planes at the three drilling depths were highlighted by transparentizing the shale matrix. ...
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... box-dimension, crack length, and crack area of the sample at a drilling depth of 120 mm change slightly despite ignoring the effect of the borehole, illustrating that the crack morphology of each cutting plane is similar, i.e., the fissure plane is relatively flat. This result is in accord with the result shown in Figure 16. Moreover, the crack length and crack area at the sample drilling depth of 130 mm 140 mm increase with the sample length increase at the drilling section, reach maximum at the bottom of the borehole, and then decrease dramatically at the ending of the drilling borehole. ...
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... the crack length and crack area at the sample drilling depth of 130 mm 140 mm increase with the sample length increase at the drilling section, reach maximum at the bottom of the borehole, and then decrease dramatically at the ending of the drilling borehole. These results exhibit that the crack morphology of the sample at a drilling depth of 130 mm 140 mm is more complex at the bottom of the borehole, which matches the exhibition shown in Figure 16. Therefore, it can be assumed that the fracturing crack initial failure occurs at the borehole bottom under these two conditions. ...
Citations
... In experimental works [99,100], the authors have demonstrated that hydraulic fracturing using supercritical CO 2 can create three-dimensional sinuous fractures with a large number of secondary branches. A model of thermal effects during hydraulic fracturing using supercritical carbon dioxide was developed in [14]. ...
... According to the developed methodology, the authors concluded that supercritical CO 2 had up to 4.4 times greater fracture capacity, and could create fractures up to 2.6 times more complex and 23.4% more tortuous than water-based hydraulic fracturing (Figure 7). The authors of the article [99] conducted experimental studies of hydraulic fracturin of a shale formation with supercritical carbon dioxide, depending on the drilling depth o a well. In addition, the morphology of cracks was analyzed using computed tomography As a result, it was found that the rupture pressure decreases with the depth of drilling. ...
... The authors of the article [99] conducted experimental studies of hydraulic fracturing of a shale formation with supercritical carbon dioxide, depending on the drilling depth of a well. In addition, the morphology of cracks was analyzed using computed tomography. ...
The unique properties of supercritical fluid technology have found wide application in various industry sectors. Supercritical fluids allow for the obtainment of new types of products with special characteristics, or development and design of technological processes that are cost-effective and friendly to the environment. One of the promising areas where supercritical fluids, especially carbon dioxide, can be used is the oil industry. In this regard, the present review article summarizes the results of theoretical and experimental studies of the use of supercritical fluids in the oil and gas industry for supercritical extraction in the course of oil refining, increasing oil recovery in the production of heavy oil, hydraulic fracturing, as well as processing and disposal of oil sludge and asphaltenes. At the end of the present review, the issue of the impact of supercritical fluid on the corrosion of oil and gas equipment is considered. It is found that supercritical fluid technologies are very promising for the oil industry, but supercritical fluids also have disadvantages, such as expansion or incompatibility with materials (for example, rubber).
... Cipolla et al. [15,16] established their respective mathematical models for determining the generation of complex fractures to understand the mechanism of fracture propagation. Yang et al. [17] carried out different tests under uniaxial and biaxial compression conditions; the crack propagation processes under different loading conditions were analyzed in the data. Li et al. [18] and Huang et al. [19] observed the great influence of coal cleats and nature fractures on the propagation paths of hydraulic fractures. ...
Hydraulic fracturing can increase the fracture of coal seams, improve the permeability in the coal seam, and reduce the risk of coal and gas outburst. Most of the existing experimental specimens are homogeneous, and the influence of the roof and floor on hydraulic fracture expansion is not considered. Therefore, the hydraulic fracturing test of the simulated combination of the coal seam and the roof and floor under different stress conditions was carried out using the self-developed true triaxial coal mine dynamic disaster large-scale simulation test rig. The results show that (1) under the condition of triaxial unequal pressure, the hydraulic fractures are vertical in the coal seam, and the extension direction of hydraulic fractures in the coal seam will be deflected, with the increase of the ratio of the horizontal maximum principal stress to the horizontal minimum principal stress. The angle between the extension direction of the hydraulic fracture and the horizontal maximum principal stress decreases. (2) Under the condition of triaxial equal confining pressure, the extension of hydraulic fractures in the coal seam are random, and the hydraulic fracture will expand along the dominant fracture surface and form a unilateral expansion fracture when a crack is formed. (3) When the pressure in one direction is unloaded under the condition of the triaxial unequal pressure, the hydraulic fractures in the coal seam will reorientate, and the cracks will expand in the direction of the decreased confining pressure, forming almost mutually perpendicular turning cracks.
... These advantages are related to the strong adsorption capacity of CO 2 relative to methane gas (CH 4 ), as CO 2 is adsorbed easily in shale pores, which leads to releasing the natural pre-adsorbed CH 4 [18][19][20][21], Thus providing both economic and environmental benefits for CCS in shales [3]. The recoverable reserves for shale gas exceed 200 × 10 12 m 3 globally [22]. China and the US hold together more than 30% of the global recoverable reserves, which gives a great commercial value for their shale gas production [23]. ...
The influence of Supercritical CO2 (SCCO2) on geochemical interaction is considered a key factor affecting CO2 storage capacity in shales. To address this issue, samples from Eagle Ford and Mancos shales were treated with SCCO2 for 30 days at 70 °C and 18 MPa. Analytical methods including X-ray diffraction (XRD), optical microscope, and Fourier Transform Infrared spectroscopy (FTIR) were used. The alteration in shale/water contact angles was evaluated based on Sessile drop method. The results show that SCCO2 treatment can alter the mineral composition of shales. Quartz content generally increased, while clay and carbonate minerals’ contents decreased. Evaluating the dissolution of carbonate minerals, in particular, is beneficial to form an effective mineral carbonation trapping for long-term CO2 storage. The changes in surface morphology suggest that clay-rich shales are more affected by SCCO2 treatment compared to sandy/quartz-rich shales. The aromatic hydrocarbons showed minor changes after SCCO2 treatment compared to the aliphatic hydrocarbons. The increase in oxygen-containing groups after SCCO2 treatment proves the high adsorption capacity of CO2 in shales. However, hydroxyl functional groups showed various trends after SCCO2 treatment, depending on the clay content. Eagle Ford shales displayed a possible turn to CO2-wet behavior, while the surface of Mancos shales remained strongly hydrophilic. In conclusion, quartz-rich shales could be favorable for CO2 adsorption and providing more storage capacity.
The microstructure, mineral composition, total organic carbon content, etc., of gas shale are crucial parameters for shale reservoirs, which can directly/indirectly affect shale brittleness, fracturing effect, adsorption ability and production efficiency. The study proposed a workflow to characterize the physical and mechanical parameters of Lower Silurian Longmaxi shale outcrop samples extracted from the favorable block in Changning, Sichuan, southwest China. This study elaborated on the influence of these physical and mechanical characteristics and proposed a corresponding brittleness index on shale extraction. In addition, it put forward corresponding suggestions for development and risk control. For a better understanding the mechanisms of shale gas storage and production, XRD, XRF, SEM, low temperature Nitrogen adsorption method, nuclear magnetic resonance and other measurements were employed to analyze and study the mineral composition, microstructure, and adsorption performance of shale. The results demonstrated that the pores of shale are mainly slit pores; there are diverse pore types in shale, mainly including intergranular pores, mineral particle dissolution pores, and internal pores of organic matter; The samples with relatively low porosity also noticeably exhibit ultra-low permeability, and the nanopore structure is remarkably significant, with distribution primarily in range of 5–237 nm. Finally, a brittleness index considering the influence of water content and the mechanical properties was proposed, and the coupling interaction of various minerals components and mechanical properties on the brittleness index can more objectively reflect the brittleness characteristics of deep shale formation.
As a waterless fracturing fluids for gas shale stimulation with low viscosity and strong diffusibility, supercritical CO2 is promising than the water by avoiding the clay hydration expansion and reducing reservoir damage. The permeability evolution influenced by the changes of the temperature and stress is the key to gas extraction in deep buried shale reservoirs. Thus, the study focuses on the coupling influence of effective stress, temperature, and CO2 adsorption expansion effects on the seepage characteristics of Silurian Longmaxi shale fractured by supercritical CO2. The results show that when the gas pressure is 1–3 MPa, the permeability decreases significantly with the increase in gas pressure, and the Klinkenberg effects plays a predominant role at this stage. When the gas pressure is 3–5 MPa, the permeability increases with the increase in gas pressure, and the influence of effective stress on permeability is dominant. The permeability decreases exponentially with the increase in effective stress. The permeability of shale after the adsorption of CO2 gas is significantly lower than that of before adsorption; the permeability decreases with the increase in temperature at 305.15 K–321.15 K, and with the increase in temperature, the permeability sensitivity to the temperature decreases. The permeability is closely related to supercritical CO2 injection pressure and volume stress; when the injection pressure of supercritical CO2 is constant, the permeability decreases with the increase in volume stress. The results can be used for the dynamic prediction of reservoir permeability and gas extraction in CO2-enhanced shale gas development.
The injection of Carbon Dioxide (CO2) into shale gas formations is a promising approach that not only reduces the impact of greenhouse gas on climate change but also enhances the gas recovery due to geochemical interactions between CO2 and clay minerals. However, these geochemical interactions have not been fully explored and changes in the petrophysical properties of shales have been reported over time. This paper evaluates the geochemical reactions and physical changes caused by the injection of supercritical carbon dioxide (ScCO2) on the properties of clay-rich shales. Samples from the Eagle Ford formation were collected and exposed to ScCO2 at different pressures ranging from 10 to 24 MPa at 70 °C. Analytical methods such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thin-section microscopy were used to characterize the ScCO2-treated samples. The results showed dissolution of clays and precipitation of quartz with increasing ScCO2 treatment pressure. The content of carbonate minerals was also reduced at high pressure, which can be attributed to the reactions of the dolomite with H⁺ to form magnesium carbonate. The percentage of absorption of aromatic hydrocarbons and oxygenated groups gradually increased with increasing pressure, which can be attributed to the increase in CO2 adsorption. On the other hand, the absorption of aliphatic and hydroxyl groups decreased after treatment. ScCO2 treatment pressure is an important factor to evaluate the CO2 adsorption capacity of clay-rich shales. The presented results enrich the understanding of the interactions between CO2 and shale under different pressures, which may be helpful to determine the feasibility of long-term injection and storage in shale.
This paper examines the fracture propagation problems of supercritical carbon fracturing in low permeability shale. Acoustic emission monitoring and computerized tomography (CT) scanning methods were used to study the influence of initial stress ratios on crack initiation and propagation crack in fracturing experiments. The results show that crack initiation pressure and crack morphology are very different under different stress conditions. Under the condition of constant confining pressure, when the initial stress ratio λ = 1, cracks are mainly in a horizontal direction; while for an initial stress ratio of λ < 1, cracks are mainly in a vertical direction. With the decrease of λ, crack initiation pressure, reopening pressure, and fracturing liquid volume also decrease, and crack propagation is not as obvious. According to CT scanning results, the crack propagation direction is the same as the maximum principal stress, and fewer cracks are initiated with a smaller initial stress ratio. Based on the acoustic emission characteristics, the fracturing process (including crack initiation, propagation, and closure), can be divided into three stages: 1) the pressure accumulation in the wellbore, 2) Pump Closure; and 3) crack reopening. This study provides the basis for a reasonable selection of shale gas fracturing formation and geo-sequestration of greenhouse gas CO2.
X-ray CT imaging provides a 3D view of a sample and is a powerful tool for investigating the internal features of porous rock. Reliable phase segmentation in these images is highly necessary but, like any other digital rock imaging technique, is time-consuming, labor-intensive, and subjective. Combining 3D X-ray CT imaging with machine learning methods that can simultaneously consider several extracted features in addition to color attenuation, is a promising and powerful method for reliable phase segmentation. Machine learning-based phase segmentation of X-ray CT images enables faster data collection and interpretation than traditional methods. This study investigates the performance of several filtering techniques with three machine learning methods and a deep learning method to assess the potential for reliable feature extraction and pixel-level phase segmentation of X-ray CT images. Features were first extracted from images using well-known filters and from the second convolutional layer of the pre-trained VGG16 architecture. Then, K-means clustering, Random Forest, and Feed Forward Artificial Neural Network methods, as well as the modified U-Net model, were applied to the extracted input features. The models’ performances were then compared and contrasted to determine the influence of the machine learning method and input features on reliable phase segmentation. The results showed considering more dimensionality has promising results and all classification algorithms result in high accuracy ranging from 0.87 to 0.94. Feature-based Random Forest demonstrated the best performance among the machine learning models, with an accuracy of 0.88 for Mancos and 0.94 for Marcellus. The U-Net model with the linear combination of focal and dice loss also performed well with an accuracy of 0.91 and 0.93 for Mancos and Marcellus, respectively. In general, considering more features provided promising and reliable segmentation results that are valuable for analyzing the composition of dense samples, such as shales, which are significant unconventional reservoirs in oil recovery.
As unconventional oil and gas have increasingly gained considerable attention, the application of water-free fracturing technology represented by carbon dioxide (CO2) fracturing technology in unconventional oil and gas resources development has broad prospects. The CO2 fracturing technology has several advantages, such as low damage and easy flowback. It is especially suitable for complex rock strata exhibiting low pressure, low permeability, and strong water sensitivity. Moreover, this technology positively affects the reservoir with low water content and experiencing serious pollution. In this paper, the related literature of the different CO2 fracturing technologies from the aspects of technical principle and technological process, technical characteristics, fracturing fluid system, and field application is analyzed, the fracture initiation and propagation are summarized as well. The results indicate that CO2 fracturing can stimulate unconventional reservoirs more effectively than the conventional hydraulic fracturing, exhibits high technical feasibility, and can be applied in the reservoir stimulation and reconstruction in the future. In this paper, how to further improve the viscosity of fracturing fluid, enhance the sand-carrying capacity of fracturing fluid, and reduce the pipeline friction is determined.
