Anisotropic Shale Research Articles

An approximate method for calculating anisotropy parameters and reflectivity of shales with horizontal fractures

Abstract Shale is a typical medium of transverse isotropy with a vertical axis of symmetry (VTI), and its strong anisotropy is mainly due to the combined effect of intrinsic anisotropy and that induced by horizontal fractures. To calculate the anisotropy parameters of shale, a physical rock model is built based on Hudson's thin-coin fracture model and Schoenberg's linear-sliding model, and an approximate theoretical calculation method for Thomsen's anisotropy parameters of VTI media with horizontal fractures is proposed. These calculation results using the proposed method confirm that this anisotropy contributed to by horizontal fractures cannot be ignored to the overall anisotropy of shale. To simplify Rüger's formula that is an approximate theoretical formula for calculating the anisotropic reflection coefficients of VTI media, a new four-term approximate formula is derived in a standard reflectivity form based on Rüger's and Aki–Richards’ formulas. The simulation results of a VTI theoretical model and logging data of shale reservoirs show that there is only a small difference between the newly derived four-term formula and Rüger's formula for incidence angles <40°, and the new four-term formula can correctly reveal the seismic amplitude-versus-offset (AVO) characteristics of VTI media and fully retain the corresponding anisotropic seismic responses. Compared to Rüger's formula, the proposed new formula only has four terms of unknown parameters and can directly decouple Thomsen's anisotropy parameter ε from them, which helps to alleviate the ill-posed problems of simultaneous inversion of multiple parameters and enhance its application potential in seismic inversion of VTI media as shale.

Mechanical properties and failure characteristics of anisotropic shale with circular hole under combined dynamic and static loading

In deep hard rock mines, the drilling and blasting method is mainly used for mining, which results in a complex environment of high static stress and frequent dynamic disturbance in some underground engineering surrounding rocks for a long time. In addition, a large number of underground engineering are in the geological tectonic zone with poor stability, which often triggers rock disasters when stress waves propagate to the bedding. Therefore, it is significant to study the failure process and mechanism of underground engineering in bedded rock masses under combined dynamic and static loading. In this study, a combined dynamic and static impact experiment was conducted on circular-hole-containing shale specimens with different bedding angles using a modified split Hopkinson pressure bar (SHPB) device, and the damage process was recorded using Digital Image Correlation (DIC) technique. The experimental results show that the stress-strain curves of shale specimens under combined dynamic and static conditions include three stages elasticity, yielding, and failure. With the increase of the bedding angle, the dynamic strength and Young's modulus of the specimens showed a trend of decreasing and then increasing, with obvious anisotropy. However, with the increase of pre-static load, the dynamic strength and Young's modulus of the specimens showed a tendency to increase first and then decrease. In addition, repeated refraction (transmission) and reflection of the stress wave between the beddings and the stress concentration phenomenon of the circular holes resulted in different failure characteristics of the specimens with different bedding angles.

Propagation of multiple hydraulic fractures in a transversely isotropic shale formation

Shale anisotropy can lead to deviated patterns of hydraulic fractures in multi-stage hydraulic fracturing (MSHF) operations, significantly hindering hydrocarbon production. Such deviation usually occurs in formations where the induced fractures interfere with natural bedding planes or where the stress shadow is created. In this work, the elastic behavior of a transversely isotropic shale rock during MSHF is investigated using the extended finite element method (XFEM) in conjunction with the cohesive zone model (CZM) in ABAQUS. In comparison to the linear elastic fracture mechanics approach, the CZM considers the plasticity and softening effects at the fracture tip, and therefore, leads to a more accurate prediction of fracture geometry. The XFEM model is capable of simulating fractures with an arbitrary path without requiring re-meshing. The 2D numerical model is first verified against the Kristianovich-Geertsma-de Klerk (KGD) analytical solutions. Five injection clusters in a single fracturing stage are simulated to analyze the effects of the material isotropy, the dip angle of shale bedding, and the fracture spacing on the geometry of multiple hydraulic fractures. The transversely isotropic poroelasticity model predicts narrower and longer hydraulic fractures than the isotropic poroelasticity model. The bedding dip angle plays a crucial role in the deviation and the shape of multiple hydraulic fractures. Increasing the dip angle from zero to 90° results in an increase in the average fracture width and an increase-peak-decrease trend in the fracture length. A strong correlation is observed between the fracture spacing and the fracture geometry, and a spacing of 75 m is considered optimal because it results in no discernible stress shadow at all dip angle cases.

Poroelastic response of inclined wellbore geometry in anisotropic dual-medium/media

Shale gas formations are typically characterized by anisotropic and fracture properties. Existing shale wellbore stability models have only considered the anisotropy of shale rocks and have ignored the effect of fractures. However, the mechanism of wellbore instability under the combined effects of shale anisotropy and dual-media (DM) remains unknown. Therefore, based on anisotropic poroelasticity, the DM theory, and the assumption of generalized plane strain, this study established a pseudo-3D hydraulic-mechanical coupling finite element model considering the DM and anisotropy of shale in wellbores. The effects of elastic anisotropy, fracture volume fraction, fracture permeability, mass transfer coefficient, and fluid pressure on wellbore stress were analyzed parametrically. The results showed that an anisotropic single-medium predicts that the wellbore will fail when the formation is drilled, whereas an anisotropic DM predicts periodic wellbore instability. The poroelastic effect of the matrix pore pressure is much greater than that of the fracture pore pressure. The matrix elastic anisotropy parameters control the distribution of mechanical parameters throughout the DM, further affecting the poroelastic response of the entire DM. When the drilling direction was mostly parallel to the bedding plane, the anisotropy of the matrix and fracture system of the elastic parameters were larger, and the wellbore was more stable. A larger fracture permeability leads to a larger fracture volume fraction and mass transfer coefficient, the matrix pore pressure and fracture pore pressure increase, and the instability of the wellbore also increases.

Experimental investigation on the anisotropy of mode-I fracture and tensile failure of layered shale

Mode-I fracture toughness (KIC) and tensile strength (σt) are significant parameters in hydraulic fracturing. Owing to the unique mineral composition and sedimentary environment of shale, the mechanical properties of shale exhibit anisotropy. In this study, the notched semi-circular bend (NSCB) and Brazilian disc (BD) tests were combined with acoustic emission (AE) and digital image correlation (DIC) technology to investigate the anisotropic fracture and tensile behaviors of layered shale. The critical plane approach (CPA) criterion is extended and validated, and the universality of the extended CPA criterion is confirmed for both KIC and σt prediction. In addition, the variation of KIC and σt versus loading angle, the relationship between KIC and σt, the typical failure modes of both NSCB and BD specimens, and the fracture initiation angle of layered shales are discussed comprehensively. The following conclusions were drawn: (1) KIC and σt increased with increasing in the loading angle (β), which indicated significant anisotropy of layered shale, and the AE responses for different loading angles also differed significantly. (2) The variation of anisotropic KIC and σt of different shales could be predicted by adjusting the spatial distribution parameter (Ω0), so that the extended CPA criterion could predict KIC and σt of different shales accurately, which confirmed the universality of the extended CPA criterion for both KIC and σt prediction. (3) There exists an approximately linear or power relationship between KIC and σt, and the power relationship can match both experimental and theoretical results much better. (4) The data from DIC and failure modes confirmed that when β was low, the failure modes were primarily affected by the bedding plane. In contrast, when β was great, the failure modes were primarily affected by the matrix. The NSCB specimens exhibited tensile failure, whereas the BD specimens exhibited more complex failure modes, which were a combination of tensile and shear failure. The results of this study can be used for guiding the design and construction of hydraulic fracturing and rock engineering.

Acoustic and mechanical tests of sandstone-shale composites in Songliao Basin and prediction of uniaxial compressive strength

Rock uniaxial compressive strength is a vital rock mechanics parameter, which accurate assessment is critical for high-efficiency unconventional reservoir development. However, its realization for standard cores is quite problematic due to difficulties and high costs related to the collection and preparation of samples. Unconventional fracturing treatment have evolved from single to composite lithology. This study collected shale and sandstone samples at different bedding angles (0°, 45°, 90°)to prepare the sandstone-shale composite samples at different interlayer ratios. It measured the prepared composites’ longitudinal wave and transverse wave velocities with ultrasonic velocity testing. The uniaxial compressive strength of the sandstone-shale composites with various bedding angles were tested via a servo rock triaxial testing machine. The anisotropic characteristics of the prepared sandstone-shale composites with various bedding angles were analyzed, and relation between the uniaxial compressive strength of the anisotropic sandstone-shale composite and P-wave velocity was derived and experimentally validated. The results show that: the wave velocity test results of the sandstone-shale combination show that with the increasing proportion of shale, the wave velocity of the longitudinal wave and the transverse wave both increase; The wave velocity results of different bedding planes in shale are as follows: Vp(0°)> Vp(45°)> Vp(90°). The average P-wave and S-wave velocities at 45° are 0.92, while the average P-wave and S-wave velocities at 90° are 0.85 compared to 0°; the P-wave and S-wave anisotropy of shale are both 0.14, and the shale shows significant anisotropy. In the sandstone-shale composite, the uniaxial compressive strength gradually decreases and the strain increases with the increase of shale proportion. The value of uniaxial compressive strength is UCS(0°) > UCS(90°) > UCS(45°). Based on the P-wave modulus, a fitting relationship formula for the uniaxial compressive strength of sandstone-shale composites with different bedding angles and interlayer ratios was established and verified. The anisotropic strength predictive formula established based on experimental data is simple, precise, and easy to obtain, with strong practicability. The present research results can provide proper technical support for sandstone-shale composite fracturing optimization and reservoir development.

Pore anisotropy in shale and its dependence on thermal maturity and organic carbon content: A scanning SAXS study

Pore orientation in shale governs the fluid transport properties that are key to hydrocarbon production and potential CO2 sequestration. The present work deals with the detailed study of nano-heterogeneities in shales, across the bedding plane, of varying thermal maturity and total organic carbon content using scanning small-angle X-ray scattering (SAXS) experiments. The complementary X-ray Micro-Computed Tomography (μCT) and functional group mapping elucidate the heterogeneity in micrometer resolution. 2D SAXS profiles of the shales show elliptical patterns indicating the nanoscale pore anisotropy in shale. The orientation of pores and their spatial variation is strongly dependent on the content of organic matter. The variance in the anisotropy parameter is validated with chemical mapping and high-resolution 3D imaging.

The investigation of multiple factors on elastic anisotropy of artificial shale based on the orthogonal experiment

Elastic anisotropy of shales is critical to accurate constraints for rock physical models, quantitative interpretation and hydraulic fracturing. However, the causes of elastic anisotropy of shales are very complicated, and the understanding of how multiple influence factors affect the elastic anisotropy of shales is still not clear. Hence, the orthogonal experiment, as an effective multiple factors experimental method, is adopted in this study to analyze the effect of multiple factors for shale elastic anisotropy. Three factors, clay content, organic matter (OM) content and compaction stress are selected as independent variables, the orthogonal test table L16(43) with four levels for each factor is adopted. According to the designed orthogonal table, sixteen artificial shales are constructed based on the cold-pressing method, and all the dry artificial shales are measured by the ultrasonic measurements. The influence of each factor on the elastic anisotropy and the sensitivity orders of three factors are obtained using the range analysis. The orders of sensitivity for selected factors follow the sequence clay content > compaction stress > OM content for velocity anisotropy parameters. The compaction mechanism of artificial shales is also discussed by the compaction factor, which are positively correlated with the velocity anisotropy parameters. The clay platelets orientation distribution function (ODF) of samples is evaluated by a theoretical model, the ODF coefficients are significantly affected by the clay content and compaction stress, and W200 are much more sensitive to these factors than W400. The results can provide a critical rock physics basis for quantitative interpretation and reservoir prediction of the low-maturity or maturity shale reservoir.

The effects of clay mineralogy and crack properties on elastic properties of dry and water-saturated transversely isotropic rocks

The causes of the intrinsic elastic anisotropy of shales are very complicated, and clay mineralogy and crack properties are critical for rock-physics models and geomechanics applications in shales, but they are still poorly understood. Further, it is difficult to directly measure the elastic properties of single-crystal clays due to their small particle size. To reduce the multiparametric influence factors caused by complex components in natural shales, two sets of artificial rocks with different clay minerals are fabricated, and their porosity, permeability, and ultrasonic velocities are measured under different pressure conditions. The results suggest that clay mineralogy can significantly affect physical and elastic properties and their pressure sensitivity, and the permeabilities of the sample composed of kaolinite are much higher than those of the sample composed of smectite due to the larger particle size of kaolinite compared with smectite. Further, the type of clay can cause a difference in “intrinsic” velocity and anisotropy. The microcrack can enhance the velocity anisotropy on the basis of intrinsic anisotropy, and crack properties such as maximum dip angle and normal-to-tangential compliance ratio are obtained by a theoretical model, which is also affected by clay type, causing the difference in the pressure sensitivity of elastic properties. Water has a significant influence on the elastic properties and anisotropy of artificial clay samples, which can reduce the S-wave velocity and increase the P-wave velocity driven by multiple competing factors. Water reduces the velocity anisotropy because the presence of water narrows the difference between the velocity parallel and perpendicular to the axis of symmetry. The normal-to-tangential compliance ratio of cracks is also affected by water as a stiffer fluid can reduce the normal compliance of cracks at the grain scale.

Geochemical and Microstructural Characteristics of Clay Minerals and Their Effects on the Pore Structure of Coal-Measure Shale: A Case Study in Qinshui Basin, China

As the essential component of shale, clay minerals have a vital influence on the pore structure and the gas content of reservoirs. To investigate the compositional characteristics of coal-measure shale and its effects on pore structure, a total of thirteen Taiyuan formation shale samples were collected from the Qinshui Basin and were analyzed using a combination of X-ray diffraction analysis, X-ray fluorescence spectrometry, Fourier transform infrared spectroscopy (FE-SEM), polarized optical microscopy, and field emission scanning electron microscopy. The results show that the principal minerals of the samples are quartz, kaolinite, and illite. Most of the kaolinite was an original terrigenous detrital material with low crystallinity and a low degree of ordering, whereas the illite was mainly composed of 1Md resulting from diagenesis. Clay minerals developed slits, irregularly-shaped or multisized pores during diagenesis, which can be classed into interlayered pores, intergranular pores, and microfractures. Eight micro-morphological forms of clay minerals were summarized based on FE-SEM observations, such as compacted, parallel, bent, tilted, mutually supporting structures, etc., which are mainly formed by the mechanical compaction of clay minerals with different sizes, shapes, and contact relationships. The diversity and complexity of the micro-morphological forms of clay minerals contribute to the strong heterogeneity, low porosity and high permeability anisotropy of shale.

Study on Anisotropic Petrophysical Modeling of Shale: A Case Study of Shale Oil in Qingshankou Formation in Sanzhao Sag, Songliao Basin, China

Seismic petrophysics is an important link between seismic elastic properties and reservoir physical properties. Based on the petrological and microstructure characteristics of shale in the Qingshankou formation of Sanzhao sag in the north of Songliao Basin, this paper presents an anisotropy petrophysical model with complex pore structure suitable for organic shale constructed with the use of the Voigt-Reuss-Hill average model, an anisotropy self-consistent approximation+differential effective medium model, and the layering of clay and kerogen is simulated by using the Voigt-Reuss-Hill average and bond transform to achieve the simulation of shale anisotropy. Based on the proposed model, the effects of the organic volume fraction, porosity, and pore aspect ratio on rock elastic properties are discussed. The result shows that with the increase of matrix porosity, all elastic parameters show a decreasing trend; with the increase of the organic volume fraction, except shear modulus, other elastic parameters show an increasing trend. Through comparative analysis, the elastic parameters (Lamé impedance and Shear impedance) sensitive to the organic volume fraction and porosity are optimized; the seismic petrophysical cross-plot template with core calibration is constructed. The application shows that the predicted S-wave velocity based on the proposed model is in good agreement with the S-wave velocity derived from dipole source logging. Combined with the high-precision prestack elastic parameter inversion, the “sweet spot” characteristics can be well described, and the research could contribute to a better “sweet spot” description and provide a better support for shale exploration in Sanzhao sag.

Seismic prediction of shale reservoir quality parameters: A case study of the Longmaxi–Wufeng formation in the WY area

Shale is a crucial natural gas resource, attracting global exploration and development interest. China has abundant shale gas resources that will drive future oil and gas exploration advances by increasing reserves and production. The WY shale gas field is the most productive and has the greatest potential for exploration and development. This study analyzed high-quality shale logging response characteristics and drilling logging, seismic, and analytical test data in the WY area to establish a rock physical model of seismic attribute parameters and shale reservoir quality parameters. Seismic elastic parameters were converted into indicators that directly reflect shale reservoir quality, such as total organic carbon (TOC), high-quality reservoir thickness, porosity, brittleness index, and crack development strength. Corresponding regression equations were established to predict quality parameters.The results showed that shale reservoir quality parameters have a good correlation with seismic parameters. The TOC distribution ranged from 2% to 5% in the study area and was generally high in the north but low in the south. The high-quality shale reserve had a thickness of over 40 meters, and except for the northwest region, the porosity was nearly over 4%. The overall brittleness of the study area was favorable, and the brittleness index was over 35%, which is suitable for network fractures formation in subsequent fracturing operations. The anisotropy of shale in S1l1I was small, and the overall fractures were underdeveloped in the study area. Drilling verifications showed that the prediction results of the quality parameters of high-quality shale reservoirs were consistent with actual drilling test results with high reliability. This study provides guidance for comprehensive prediction of sweet spots and subsequent fracturing and well location deployment.In summary, this study provides valuable insights into shale gas exploration and development in the WY area by establishing a rock physical model, predicting quality parameters, and offering guidance for fracturing and well location deployment.

A failure criterion for shale considering the anisotropy and hydration based on the shear slide failure model

A failure criterion fully considering the anisotropy and hydration of shale is essential for shale formation stability evaluation. Thus, a novel failure criterion for hydration shale is developed by using Jaeger’s shear failure criterion to describe the anisotropy and using the shear strength reduction caused by clay minerals hydration to evaluate the hydration. This failure criterion is defined with four parameters in Jaeger’s shear failure criterion (S1, S2, α and φ), three hydration parameters (k, ωsh and σs) and two material size parameters (d and l0). The physical meanings and determining procedures of these parameters are described. The accuracy and applicability of this failure criterion are examined using the published experimental data, showing a cohesive agreement between the predicted values and the testing results, R2 = 0.916 and AAREP (average absolute relative error percentage) of 9.260%. The error (|Dp|) is then discussed considering the effects of β (angle between bedding plane versus axial loading), moisture content and confining pressure, presenting that |Dp| increases when β is closer to 30°, and |Dp| decreases with decreasing moisture content and with increasing confining pressure. Moreover, |Dp| is demonstrated as being sensitive to S1 and being steady with decrease in the data set when β is 0°, 30°, 45° and 90°.

Microscopic Study of Shale Anisotropy with SEM In Situ Compression and Three-Point Bending Experiments

The microscopic anisotropy of shale has an important impact on its mechanical properties and crack behavior, so it is essential to understand the microscopic origin of anisotropic growth with a more effective laboratory work scheme. Uniaxial compression test and three-point bending test are considered to be efficient means to study the elastic properties and crack behavior of rocks. In this paper, uniaxial compression experiments and three-point bending experiments were conducted on shale outcrops in the Changqing area using field emission scanning electron microscopy (SEM) and in situ tensile testing, and the microscopic deformation and crack processes were quantitatively characterized by the digital image correlation (DIC) method. For the compression experiments, the observation of the first principal strain fields indicated that the microscopic anisotropy of shale was related to the bedding planes, and the microscopic deformations were mainly concentrated in the clay mineral accumulation area and at the microcracks. Elastic moduli and compressive strengths of specimens with different bedding angles were affected by the strong shear stress effects. The specimens with a bedding angle of 30° showed lower peak loads and compressive strengths, and the specimens with a bedding angle of 60° had lower elastic moduli. Three-point bending experiments were conducted for studying the effects of crack-bedding orientation relationships on cracking processes, and four critical fracturing mechanical properties were calculated. The short transverse-type cases were prone to break and had a lower peak load, tensile strength, fracture toughness and elastic-bending modulus. The divider-type cases were more difficult to break, formed a more tortuous crack and had a higher tensile strength, fracture toughness and elastic-bending modulus. The arrester-type cases had a middle range of mechanical parameters but developed the longest cracks. This study provides a feasible experimental and analysis method for understanding the microscopic anisotropy of shale samples. The small specimen size also makes the requirements of core samples easier to be satisfied considering the field application. Furthermore, the anisotropy of cracking processes can be understood better by building the connections between microstructural characteristics and mechanical performances.

Anisotropy and Heterogeneity Induced by Shale in Aquifer lithology—Influence of Aquifer Shale on the Leaky Model With Tidal Response Analysis

AbstractTidal and barometric water‐level responses in wells have been widely used to calculate the hydraulic properties of aquifer systems. The effect of anisotropy induced by shale content on such responses has not received significant attention. In this study, we examine how the presence of shale (anisotropy, extremely low porosity/permeability; approximately 10−4 to 10−3 mD), which occurs as interlayers in aquifers, affects the tidal responses of the leaky model. Our findings show that the number of wells with shale in the screened section of the study aquifer is limited. Here, we focus on the study of the limited number of wells in the North China Platform to ensure a similar geological background for each well. Calculations indicate that wells, even with a small amount of shale (≥∼5%) in the observation aquifer, may exhibit strong anisotropy and heterogeneity, deviating from the theoretical analytical solutions obtained using isotropic and homogeneous assumption models. Generally, the higher the shale content in aquifer lithology, the greater the phase shifts deviated. Thus, theoretical ideal models with isotropic and homogeneous assumptions may only obtain a rough estimation for wells with shale in aquifer lithology, suggesting that we avoid setting observation aquifers onto shale layers.

3D anisotropy in shear failure of a typical shale

It is inadequate to study the shear failure anisotropy of shale in only 2D space. Aiming at a 3D analysis, a series of direct shear tests was conducted on Longmaxi shale with three typical bedding orientations: arrester, divider and short-transverse orientations. During testing, acoustic emission (AE) and digital image correlation (DIC) techniques were simultaneously employed to monitor failure development, after testing, X-ray computed tomography (CT) scanning was adopted to acquire and reconstruct the fractures inside typical ruptured samples for more detailed analysis. The results indicated that the shear strength parameters exhibited 3D anisotropies and those of the arrester sample did not have equivalent shear strength parameters to the shale matrix. The maximum (minimum) shear strength and cohesion were obtained with the divider (short-transverse) orientation, and the internal friction angle reached its maximum (minimum) with the divider (arrester) orientation. Combining the AE, DIC and CT techniques, four characteristic stress levels that can capture the progressive shear failure process of shale rocks were identified, and the onset and accelerated development of shear damage-induced dilation were observed at the crack initiation and coalesce stress thresholds, respectively. During the crack coalescence stage, the dominated microcracking mechanism transferred from tensile-mode to shear-mode. For the arrester and divider orientations, more tensile-mode AE events were generated due to the microcracking along the vertical beddings. Compared with the divider samples, a more complex fracture network with a larger fracture area and volume was obtained in the arrester samples, whose strengths were smaller.

Numerical analysis of anisotropic stiffness and strength for geomaterials

In numerical modelling, selection of the constitutive model is a critical factor in predicting the actual response of a geomaterial. The use of oversimplified or inadequate models may not be sufficient to reproduce the actual geomaterial behaviour. That selection is especially relevant in the case of anisotropic rocks, and particularly for shales and slates, whose behaviour may be affected, e.g. well stability in geothermal or oil and gas production operations. In this paper, an alternative anisotropic constitutive model has been implemented in the finite element method software CODE_BRIGHT, which is able to account for the anisotropy of shales and slates in terms of both deformability and strength. For this purpose, a transversely isotropic version of the generalised Hooke's law is adopted to represent the stiffness anisotropy, while a nonuniform scaling of the stress tensor is introduced in the plastic model to represent the strength anisotropy. Furthermore, a detailed approach has been proposed to determine the model parameters based on the stress–strain results of laboratory tests. Moreover, numerical analyses are performed to model uniaxial and triaxial tests on Vaca Muerta shale, Bossier shale and slate from the northwest of Spain (NW Spain slate). The experimental data have been recovered from the literature in the case of the shale and, in the case of the slate, performed by the authors in terms of stress-strain curves and strengths. A good agreement can be generally observed between numerical and experimental results, hence showing the potential applicability of the approach to actual case studies. Therefore, the presented constitutive model may be a promising approach for analysing the anisotropic behaviour of rocks and its impact on well stability or other relevant geomechanical problems in anisotropic rocks.

Experimental Study on the Effect of Supercritical CO2 on Mechanical Properties and Fracture Characteristics of Longmaxi Shale

Conventional hydraulic fracturing techniques typically consume large amounts of water when producing shale gas. Fracking fluids may cause environmental pollution. In contrast, supercritical carbon dioxide (scCO2) (above 31.8°C, 7.29 MPa) can displace CH4 in shale reservoirs. Achieve CO2 sequestration while increasing the shale gas production. We studied the mechanical properties and fracture characteristics of a shale under the action of scCO2, nitrogen, helium, and water by comparing the triaxial compression tests of shale samples with seven coring angles. The results show that: (1) scCO2 effectively reduced compressive strength of the shale and weakened the anisotropy of shale; (2) scCO2 caused the content of dolomite, calcite, and illite to decrease by 4.7%∼13.5%, respectively; (3) scCO2 produced micropores and microfractures 10 times larger than the original size in the microstructure. These microstructures can help improve the seepage and gathering of shale gas, leading to enhanced shale gas recovery and CO2 storage.

A multi-fracture balanced extension control method for horizontal wells in anisotropic shale

Shale has anisotropy in elastic parameters and permeability coefficient in parallel and perpendicular directions to the bedding plane. In the study of fracture propagation during hydraulic fracturing, it is necessary to consider the influence of shale anisotropy. Through the development of fluid pipe element, fluid pipe connector element, cohesive damage element, and rock matrix element with pore pressure, a fluid–structure coupling numerical model with comprehensive consideration of dynamic flow distribution, dynamic fracture propagation and extension, and stress interference are established. The anisotropic characteristic parameters of shale reservoirs are given to study the balanced propagation control method of multi-fracture in horizontal shale Wells. The results show that under anisotropic conditions, hydraulic fractures are easy to expand in the direction parallel to the bedding plane but not in the direction perpendicular to the bedding plane. Finally, It forms a long extension in the length direction and a short extension in the height direction. The stress interference along the direction of the minimum horizontal principal stress is gradually intense with the increase of the degree of anisotropy. During multi-fracture propagation in horizontal shale wells, the stress interference generated in the direction of minimum horizontal principal stress can be balanced by effectively controlling the perforation friction so as to promote the balanced extension of multiple fractures. At the same time, with the increase of injection flow and the decrease of the distance between clusters, the stress interference degree gradually becomes serious. Therefore, it is necessary to strictly design how to control the perforation friction in the multi-fracture fracturing process of shale horizontal Wells with large flow rates and small cluster distances. The research conclusions provide a theoretical reference for the efficient development of shale petroleum resources.

Experimental Study on Stress Sensitivity Evaluation and Permeability Anisotropy of Shales in Songliao Basin

Shale oil and gas resources are abundant in Songliao Basin, and shale reservoirs tend to have certain permeability anisotropy with different pore or laminar structures in different directions, and there is no experimental analysis of shale anisotropy and reservoir sensitivity. To solve this problem, this paper tests the shale permeability by pulse decay method, obtains the variation curve of permeability and effective stress, and analyzes the experimental data. The experimental results of permeability show that the permeability of shale in Songliao basin is basically 10-4mD, and the permeability parallel and perpendicular to the laminae shows strong anisotropy, and the permeability perpendicular to the laminae is about 3%-14% The permeability of different rock samples decreases exponentially with the increase of effective stress. It provides basic data support to ensure the development of shale oil in Songliao Basin.