Bedding Plane Angle Research Articles

Influence of inherent anisotropy on the mechanical properties of normally consolidated clays with a wide range of plasticity indices

Inherent (fabric) anisotropy is one of the most important properties of earthen materials which has a significant influence on their strength and stiffness characteristics. In this study, a comprehensive series of unconfined and constrained compression tests is performed on normally consolidated (NC) clay samples with different plasticity indices to examine the effect of inherent anisotropy on their mechanical characteristics. Accordingly, several cylindrical clay samples with different proportions of kaolinite and bentonite are reconstituted at a wide range of deposition angles and then subjected to both unconfined and constrained compressive loadings. According to the experimental results, for a clay sample with a particular plasticity index, the highest and lowest values of the unconfined compressive strength (UCS), the secant modulus (E50), and the constrained Young's modulus (Eoed) are observed to be associated with the deposition angles of 0o and 90o, respectively. The results also show that at a certain bedding plane angle, the sample containing 30% bentonite (PI = 110%) exhibits the highest UCS, E50, and Eoed values. Several practical empirical correlations are developed to estimate the strength and stiffness properties of NC clays using their plasticity indices and bedding plane directions. Scanning Electron Microscopy (SEM) analysis is also conducted to explore the microstructure of samples containing varying percentages of kaolinite and bentonite.

Study on the mechanical and damage properties of laminated limestone under acid mine drainage dissolution

To explore the chemical and mechanical effects of acid mine drainage on water and rock, acid mine drainage (AMD) dissolution tests, triaxial compression tests, and acoustic emission tests were performed on limestone rock samples with different bedding dip angles. Combined with scanning electron microscopy and nuclear magnetic resonance analyses, the changes in the internal pores and surface morphologies of the rock samples before and after dissolution were analyzed. The results were as follows. (1) AMD dissolution mainly occurred in the shallow surfaces and bedding planes of the limestone samples. During dissolution, the shape of the matrix crystal disappeared to form small pores, and residual substances appeared during the dissolution of the bedding plane. These small pores were prone to the creation of large honeycomb-like dissolved pores. (2) With increasing bedding plane angle, the compressive strengths and elastic moduli of the limestone samples exhibited V-shaped distributions. Additional branch cracks were derived from the limestone samples after dissolution, and dissolution reduced the mechanical strength of the limestone by decreasing the crack initiation stress and damage stress. (3) With increasing bedding dip angle, the uniaxial failure modes of the rock samples changed from matrix tensile failure and shear failure along the bedding plane to plane tensile failure. After dissolution, the limestone matrix was prone to cracking and spalling along the surface of the sample. (4) There were differences in the triaxial compression failure modes between the dissolved limestone and the undissolved limestone. When α = 0° or 90°, the limestone samples formed additional branch fissures after dissolution. When α = 45°, the formation of penetrating cracks along the bedding plane was obviously controlled by the bedding plane. (5) A chemical–mechanical damage model was established and modified by the compression coefficient K, which could effectively reflect the deformation of the dissolved rock sample during loading.

Investigation on the fracture mechanics characteristics and crack initiation of deep dense shale

The mechanical behavior of shale fractures significantly affects the structure and stability of hydraulic fracturing networks in shale gas reservoirs. Currently, there is insufficient research on the mechanical characteristics of fractures, crack initiation, and the prediction of fracture trajectories in deep dense shale. This study involved conducting direct tension, shear, and three-point bending fracture mechanical experiments on shale samples, complemented by numerical calculations and mechanistic analyses. The results show a significant influence of the bedding plane angle (γ) on the initiation properties and locations of shale fractures. With γ ranging from 0° to 90°, shale fractures transition from alignment with the bedding plane to penetration through it. Fracture initiation locations show a significant degree of randomness when fractures propagate along the bedding plane. The horizontal tensile stress changes with increasing γ, initially increasing and then decreasing as it moves away from the center. The evolution of the horizontal tensile stress field primarily causes alterations in shale fracture paths, reaching its peak value at γ = 45°. Shale initiation fractures can be classified into four types: tensile fractures through the bedding plane, tensile fractures along the bedding plane, shear fractures across the bedding plane, and shear fractures along the bedding plane. Established a criterion called the “maximum stress strength ratio” to determine fracture initiation, which can predict the initiation properties, locations, and paths of shale fractures. When γ is small, fractures mainly propagate along the bedding plane, with a higher likelihood of initiation occurring at the center of the specimen. Conversely, when γ is large, fractures mainly propagate through the bedding plane. The spacing of the bedding planes minimally affects the predicted fracture paths of shale. This study is of significant theoretical importance for reservoir stimulation and stability assessment in deep dense shale gas reservoirs.

Mechanical response and energy evolution of interbedded shales subjected to multilevel constant/increasing-amplitude cyclic loading

This study aims to investigate the effects of fatigue loading paths and interbed structure on the mechanical properties and energy evolution characteristics of shales. In the experiments, shale specimens with five types bedded angles (0°, 30°, 45°, 60°, 90°) were prepared, and multilevel constant-amplitude fatigue loading test (MCFT) and multilevel variable-amplitude fatigue loading test (MVFT) were performed. Results indicated that shale's fatigue behaviors were highly affected by the bedding plane angles and the cyclic loading paths. Specifically, with the increasing of bedding angle, the peak strength, total dissipated energy, and total input energy of the shales showed a "U" trend, the ultimate macro-damage mode changed from mixed tension-shear failure to failure along the bedding planes, and the damping ratio firstly increased and then decreased. Test schemes of stepwise increase of the lower stress limit or keeping it constant were the differences between MCFT and MVFT, which significantly influenced the evolution of irreversible deformation of shales. Compared to MVFT, the shale in MCFT displayed reduction in peak strength in the range of 4.3% to 23.9%, greater and more drastic variations in elastic modulus and damping ratios, dissipative energy and elastic strain energy were relatively low by an order of magnitude.

Investigation on the anisotropy of mechanical properties and brittleness characteristics of deep laminated sandstones

Rock brittleness is a crucial mechanical property and essential for fracability evaluation and fracturing scheme design in unconventional reservoirs. However, the influence of inherent anisotropy on deep laminated sandstone’s mechanical properties and brittleness characteristics is rarely investigated. The energy transformation and damage evolution reflected by complete stress–strain curves are analyzed during the entire process of rock rupture under compressions. A new brittleness index is established based on energy evolution during sandstone failure. Its advantages involve comprehensively considering the energy transformation characteristics at both pre-peak and post-peak stages and the capability to characterize the effect of confining pressure and bedding plane (BP) geometry on sandstone brittleness. The triaxial compression tests on sandstones are conducted to validate the reliability and accuracy of the new brittleness index. Numerical simulations are then performed to further investigate the manner in which BP angle, BP density, and confining pressure control the brittleness anisotropy of deep laminated sandstones based on the finite element method. Then the acoustic emission (AE) characteristics of anisotropic sandstone and correlations between AE mode and brittleness index are discussed. The results indicated that the anisotropy of mechanical properties and brittleness of deep laminated sandstones were significantly affected by BP angle, BP density, and confining pressure. With the increase of BP angle, the brittleness index of deep laminated sandstone decreases first and then increases, showing a U-shape variation law, whose maximum and minimum values are obtained at φ = 0° and φ = 45°, respectively. The AE characteristics were closely related to rock brittleness, which was jointly controlled by BP geometry and confining pressure. The results provide a basis for the brittleness and fracability evaluation and optimum hydraulic fracturing design in deep laminated sandstones.

Experimental Research on Supercritical Carbon Dioxide Fracturing of Sedimentary Rock: A Critical Review

AbstractSupercritical carbon dioxide (ScCO2) fracturing has great advantages and prospects in both shale gas exploitation and CO2 storage. This paper reviews current laboratory experimental methods and results for sedimentary rocks fractured by ScCO2. The breakdown pressure, fracture parameters, mineral composition, bedding plane angle and permeability are discussed. We also compare the differences between sedimentary rock and granite fractured by ScCO2, ultimately noting problems and suggesting solutions and strategies for the future. The analysis found that the breakdown pressure of ScCO2 was reduced 6.52%–52.31% compared with that of using water. ScCO2 tends to produce a complex fracture morphology with significantly higher permeability. When compared with water, the fracture aperture of ScCO2 was decreased by 4.10%–72.33%, the tortuosity of ScCO2 was increased by 5.41%–70.98% and the fractal dimension of ScCO2 was increased by 4.55%–8.41%. The breakdown pressure of sandstone is more sensitive to the nature of the fracturing fluid, but fracture aperture is less sensitive to fracturing fluid than for shale and coal. Compared with granite, the tortuosity of sedimentary rock is more sensitive to the fracturing fluid and the fracture fractal dimension is less sensitive to the fracturing fluid. Existing research shows that ScCO2 has the advantages of low breakdown pressure, good fracture creation and environmental protection. It is recommended that research be conducted in terms of sample terms, experimental conditions, effectiveness evaluation and theoretical derivation in order to promote the application of ScCO2 reformed reservoirs in the future.

Liquefaction behavior of inherently anisotropic sand under cyclic simple shear: Insights from three-dimensional DEM simulations

Inherent fabric anisotropy has been widely recognized to have significant effects on the liquefaction behavior of sand. A discrete element method (DEM) study was conducted to investigate the liquefaction behavior of inherently anisotropic sand under cyclic simple shear conditions. In the DEM model, the under-compaction method was modified to generate uniform and inherently anisotropic specimens. A series of DEM simulations of undrained cyclic simple shear tests were performed to investigate the effects of bedding plane angles with a full spectrum ranging from −90° to +90°. The relative magnitudes of the three effective normal stress components were compared and analyzed. The internal structure evolution was quantified through a contact-normal-based fabric tensor that could describe the load-bearing structure of granular specimens. The simulation results show that the effect of bedding plane angle on the liquefaction resistance of anisotropic sand is not monotonic. For example, as the bedding plane angle of specimen increases from 0° to 90°, the liquefaction resistance first decreases and then increases, with the most detrimental scenario occurs at 45°. This could be explained by correlating the magnitudes of shear modulus with the discrepancy between the loading direction and the direction of fabric during each cycle.

Numerical simulation on hydraulic fracture propagation in laminated shale based on thermo-hydro-mechanical-damage coupling model

The temperature of a deep shale reservoir may reach more than 100°C, and the effect of thermal shock on shale hydraulic fracturing has rarely not been considered in previous studies. Based on mesoscopic damage mechanics and the finite element method, a thermo-hydro-mechanical-damage (THMD) coupling model considering temperature, seepage, stress, and damage fields was constructed to investigate the effects of reservoir temperature, convective heat transfer coefficient ( h), in-situ stress difference and bedding plane angle ( αθ) on shale hydraulic fracturing. The results show that multiple hydraulic fractures (HFs) can occur under thermal shock and that HFs control the distribution of seepage, temperature, and stress fields. Reservoir temperature, in-situ stress difference and αθ are primary factors affecting hydraulic fracturing, whereas h is a secondary factor. When the reservoir temperature rises from 50°C to 150°C, the initiation and breakdown pressures decrease by 65.5% and 16.7%, respectively. HFs cross the bedding plane more easily, and fracture complexity is obviously enhanced. A higher h is favourable for slightly reducing the initiation and breakdown pressures, but it has little influence on the fracture complexity. Once the in-situ stress difference is low, there is a high fracture complexity, but HFs are more easily captured by bedding planes to limit the propagation of fracture height. When the in-situ stress difference is high, HFs are more likely to form bi-wing fractures. Whether αθ is too large or small, it is not conducive to improving the fracture complexity. In this study, when αθ is 30°, HFs and bedding planes intersect to form a fracture network. Essentially, thermal shock plays a key role in reducing the initiation pressure and forming multiple HFs during the fracturing process, and fracture propagation mainly depends on the injection pressure. The results can serve as reasonable suggestions for the optimization of shale hydraulic fracturing.

The invasion law of drilling fluid along bedding fractures of shale

In the process of drilling, the drilling fluid will invade into the bedding plane of shale under the action of pressure difference that will cause hydration collapse and wellbore instability. In order to ensure the wellbore stability during shale oil and gas drilling, it is necessary to clarify the invasion law of drilling fluid along bedding fractures during the drilling process. The immersion experiment method is often used to study the invasion law of drilling fluid, which is quite different from the actual invasion process of drilling fluid underground. In this paper, the depth of drilling fluid invasion into shale under different confining pressures and displacement times is intuitively and accurately determined by the displacement experiment and NMR scanning first. Also, then the mathematical relationships between drilling fluid invasion depth and invasion time, invasion pressure difference, confining pressure, bedding angle, and drilling fluid viscosity were established. The errors between the calculated values of the drilling fluid invasion depth and the experimental values were less than 15%, and the calculation accuracy was high. In addition to the influence of invasion time, formation pressure difference and confining pressure on invasion depth were researched through the method of numerical simulation. The results showed that the liquid invasion depth increased logarithmically with the increase of invasion time and formation pressure difference, but it grew slowly in the later period and tended to be stable; the invasion depth decreased exponentially with the increase of confining pressure, bedding plane angle, and drilling fluid viscosity. The results in the paper provide a basis for the subsequent determination of the collapse pressure and collapse period of bedding shale.

The Effect of Bedding Plane Angle on Hydraulic Fracture Propagation in Mineral Heterogeneity Model

The bedding planes of unconventional oil and gas reservoirs are relatively well developed. Bedding planes directly interfere with hydraulic fracture expansion. Determining how bedding planes influence hydraulic fractures is key for understanding the formation and evolution of hydraulic fracturing networks. After conducting X-ray diffraction analysis of shale, we used Python programming to establish a numerical model of mineral heterogeneity with a 0-thickness cohesive element and a bedding plane that was globally embedded. The influence of the bedding-plane angle on hydraulic fracture propagation was studied. Acoustic emission (AE) data were simulated using MATLAB programming to study fracture propagation in detail. The numerical simulation and AE data showed that the propagation paths of hydraulic fractures were determined by the maximum principal stress and bedding plane. Clearer bedding effects were observed with smaller angles between the bedding surface and the maximum principal stress. However, the bedding effect led to continuous bedding slip fractures, which is not conducive to forming a complex fracture network. At moderate bedding plane angles, cross-layer and bedding fractures alternately appeared, characteristic of intermittent dislocation fracture and a complex fracture network. During hydraulic fracturing, tensile fractures represented the dominant fracture type and manifested in cross-layer fractures. We observed large fracture widths, which are conducive to proppant migration and filling. However, the shear fractures mostly manifested as bedding slip fractures with small fracture widths. Combining the fracture-network, AE, and fractal dimension data showed that a complex fracture network was most readily generated when the angle between the bedding plane and the maximum principal stress was 30°. The numerical simulation results provide important technical information for fracturing construction, which should support the efficient extraction of unconventional tight oil and gas.

Bedding Plane Angles and a Horizontal Prefabricated Flaw Affecting the Cracking Characteristics of Shale Specimens

Bedding Plane Angles and a Horizontal Prefabricated Flaw Affecting the Cracking Characteristics of Shale Specimens

Experimental investigation on the mixed-mode fracture behavior of rock-like material with bedding plane

To investigate the combined effects of bedding plane angle and notch angle on the mixed-mode fracture behavior of rock-like material, three-point bending tests of semi-circular bend (SCB) rock-like specimens with two relative locations between bedding plane and notch were first conducted accompanied by acoustic emission (AE) tests. The fracture surface characteristics of SCB rock-like specimens were then scanned and analyzed. The experimental results revealed that both the bedding plane angle and notch angle showed strong influences on the fracture behavior of rock-like material. Under each bedding plane angle, most of the fracture load continuously increased with the increasing notch angle. Under each notch angle, the fracture load continuously decreased with the increasing bedding plane angle. The fracture load of SCB rock-like specimens when bedding plane pro-dip to the notch was larger than that when bedding plane anti-dip to the notch. With the increasing bedding plane angle, most of the values of mode I, mode II and effective fracture toughness continuously decreased. With the increasing bedding plane angle, the fracture modes of SCB rock-like specimens transformed to bedding plane dominant fracture. The fracture surface properties are closely related with the fracture mode of SCB rock-like specimens. When bedding plane anti-dip to the notch, joint roughness coefficient (JRC) values almost linearly decreased with the increasing bedding plane angle. With different bedding plane angles and notch angles, the relationships between fracture load and accumulated AE energy could be well described by the linear function.

Experimental Investigation on the Initiation of Hydraulic Fractures from a Simulated Wellbore in Laminated Shale

Abstract Understanding the formation mechanisms of complex fracture networks is vitally important for hydraulic fracturing operations in shale formation. For this purpose, a hydraulic fracturing experiment under a core-plunger scale is conducted to investigate the impact of the bedding plane angle, borehole size, and injection rate on fracture initiation behaviors of laminated shale rock. The results on rock properties demonstrate that the anisotropic characteristics of shale rock are reflected not only in elastic modulus but also in tensile strength. The results of fracturing experiments show that the bedding plane dip angle and borehole size have significant effects on fracture initiation behaviors, in that fracture initiation pressure (FIP) decreases with the increase of those two factors. The impact of injection rate, by contrast, has no obvious variety regulation. The above data is further used to validate our previously proposed fully anisotropic FIP model, which shows better agreement with experimental results than those using other models under various parameter combinations. Finally, a postfracturing analysis is performed to identify the fracture growth patterns and the microstructures on the fracture surfaces. The results show that the hydraulic fractures (HFs) always grow along mechanically favorable directions, and the potential interaction between HFs and bedding planes mainly manifests as fracture arrest. Meanwhile, the roughness of fracture surfaces is physically different from each other, which in turn results in the difficulties of fluid flow and proppant migration. The findings of this study can help for a better understanding of the fracture initiation behavior of laminated shale rock and the corresponding fracture morphology.

Effects of bedding planes on mechanical characteristics and crack evolution of rocks containing a single pre-existing flaw

The existence of bedding planes in rock materials has an obvious impact on the material mechanical properties and the failure process of engineering projects. In this study, a series of pre-cracked models with different bedding plane properties (i.e., strength reduction factor, bedding plane angle and spacing) were established and simulated to fail under the uniaxial compression. The results indicate that the increases of the strength reduction factor and the bedding plane spacing lead to the increasing peak strength, the change of peak strength regarding the bedding plane angle presents a concave upward trend. In terms of crack evolution, the proportion of cracks formed in rock matrix increases concerning the increasing strength reduction factor and bedding plane spacing, the abovementioned crack proportion in models with increasing bedding plane angles firstly decreases and then increases. The rock matrix has higher mechanical characteristics than bedding planes, the change of crack proportion can account for the changing peak strength from the microscopic viewpoint. By summarizing the failure patterns, the failure of models with the 45° ~ 75° bedding plane angle is induced by cracks formed in bedding planes, which can be depicted and discussed by the plane of weakness model. The above results imply that the rock slope stability can be evaluated by analyzing bedding plane properties in the field.

Multiscale modelling and analysis of footing resting on an anisotropic sand

The influence of the inherent fabric anisotropy of sand and its evolution on the footing bearing capacity and failure mechanism has been modelled and analysed with a multiscale approach that couples the finite-element method (FEM) and the discrete-element method (DEM). In this approach, the material constitutive responses are captured by DEM simulations on representative volume elements (RVEs) assigned to the FEM Gauss points. The inherent fabric anisotropy of sand is reflected by the bedding plane angle α of the RVEs composed of elliptic particles, where α measures the average angle between the long axes of the particles and the horizontal direction. The bearing capacity is found to decrease with an increasing α and the reduction can reach as high as 25%. Asymmetric deformation has been observed in the α other than 0° or 90° cases, which is mainly attributable to the different relative principal directions between the stress and the fabric at the two sides of the domain. Further multiscale analyses reveal that the load-bearing structure of the material expressed by the contact-normal-based fabric tensor is more fragile under the footing loading at one side of the domain and in larger α cases, which offers a micromechanical explanation to the observation of the asymmetric deformation and the decreased bearing capacity with increased α.

Full-field deformation measurements from Brazilian disc tests on anisotropic phyllite under impact loads

To explore the dynamic fracturing behaviours and failure mechanisms, transversely isotropic phyllite rocks with five bedding plane angles were investigated for full-field strain Brazilian disc tests under different strain rates using a split Hopkinson pressure bar and high-speed digital image correlation (DIC) technology. Two kinds of crack evolution processes: the cracks in specimens with low bedding angles activated near the non-loading point, and other cracks initiated at the centre of the disc prior to exceeding the peak stress were observed. Considering the four types of failure modes characterized by different mechanisms, the dynamic failure mechanism of transversely isotropic rock was discussed. The Brazilian disc strength of transversely isotropic rock subjected to impact loads was computed by introducing the patchy weakness model to reflect the dominant role of the rock matrix in the dynamic test. The DIC results demonstrated that additional random orientation defects are activated and the competition for evolving into cracks becomes intense, indicating that cracks develop less along bedding planes as the strain rate increases. Moreover, the fracture patterns of the transversely isotropic phyllite specimens changed under static and dynamic conditions. Therefore, it is necessary to study the properties of transversely isotropic rocks under dynamic impacts rather than simply using inferences from static results.

The effect of shale bedding on supercritical CO2 jet fracturing: A experimental study

The bedding plane of shale is related to anisotropic mechanical property, which apparently affects the shale fracturing. To investigate the influence of shale bedding on the fracture morphology and perforation damage under Supercritical Carbon Dioxide (SC–CO2) jet fracturing, the experiments were comprehensively studied via the macro fracture analysis, strain monitoring, Scanning Electron Microscope (SEM) test and Energy Dispersive Spectroscopy (EDS) test. The results demonstrated that the average Young's Modulus (YM) and Uniaxial Compressive Strength (UCS) of shale specimen with the bedding plane angle of 0° is 9.37 times and 2.89 times of the shale specimen with bedding plane angle of 90°, respectively. During jet fracturing, both the jet pressure and existing bedding are beneficial for fracture initiating and propagating naturally along the weak bedding planes, and shear fracture can enhance the fracture complexity. When the perforation distributes in the direction perpendicular to the bedding plane, the less strain can be obtained, and the mass loss and CO2 absorption of shale specimen significantly increase by 350% and 300%, respectively, as the jet pressure increases from 25 MPa to 50 MPa, which is larger than the shale specimen with bedding plane angle of 0°. The perforation damage is explored after jet fracturing via SEM and EDS, and it is found the direction of perforation perpendicular to the bedding plane is beneficial for micro-cracks generation, and leads to 10% decline of carbon (C) and oxygen (O) elements in the samples. These studies clearly show the influence of the bedding plane angle on the fracture morphology and perforation damage, which are crucial for SC-CO2 fracturing and CO2 storage in shale reservoir.

Directional strength and stiffness characteristics of inherently anisotropic sand: The influence of deposition inclination

Evolution of soil fabric during deposition, sedimentation and subsequent shearing has been the focus of innumerous experimental and numerical studies in the past. However, the effect of inclined bedding plane on the mechanical behavior of granular materials has not been well recognized mostly due to the difficulties associated with the relevant simulations in the laboratory. In this study, a comprehensive experimental program is carried out to rigorously examine the significant influence of inclined bedding plane and deposition angle on the mechanical behavior of sands over a wide range of strains, from very small to large levels. To this end, specimens at different bedding plane angles are first prepared, frozen and then subjected to several consolidated drained (CD), consolidated undrained (CU) and bender element tests. Results show that the small strain shear stiffness and tangent axial stiffness are heavily influenced by the major principal stress direction and the deposition inclination. In terms of tangent axial stiffness, this effect is highlighted particularly before peak shear strength where the mechanical behavior of the granular material is dominated by particle interlocking. Besides, the anisotropic response of the soil is more pronounced at higher isotropic confining pressures. The volumetric behavior, internal friction angle and shear band direction are also observed to be remarkably dependent on the angle of bedding plane.

The Analytical Model for Horizontal Wellbore Stability in Anisotropic Shale Reservoir

Wellbore collapse is a frequent problem during drilling in shale gas reservoir, restricting efficient exploration of shale gas. Due to the presence of bedding plane, conventional homogeneity model is inappropriate to conduct wellbore stability analysis in shale reservoir. Therefore, in this study, under the influence of bedding plane, stress distributions including in situ stress components, pore pressure, seepage stress have been calculated. Meanwhile, shale strength is expressed using the Jaeger's criterion, which assumes shale is isotropic body with one group of weak planes. In combination with stress distribution and shale strength, an analytical model has been developed to investigate wellbore failure region. By using this model, influence factors of wellbore stability have been fully analyzed. Results demonstrate that the difference between bedding plane and rock matrix will modify in situ stress components, transport equations and rock mechanical properties around wellbore. As a result, there are heterogeneous distributions of shale strength and stress state. When the shear failure along bedding plane happens, shale strength has clearly reduction and breakout region has increment, changed from two-lobed failure to four-lobed failure shape. The different intersections between bedding plane and wellbore make variable anisotropy in wellbore plane, modifying the pore pressure propagation and seepage stress distribution. When the bedding plane angle in wellbore plane is low or high value, shear failure along bedding plane doesn’t happen and wellbore stability is depended on rock matrix strength. For middle bedding plane angle, failure region clearly increases and shear failure along bedding plane occurs, aggravating the instability. Furthermore, drilling fluid reduces shale strength, thus increasing region area and changing failure mode. This drilling fluid impact is strong in initial stage and becomes stable after 96 h. By having comprehensive investigation on bedding plane influence, findings in this study are advantageous for drilling operations in shale reservoir.

Numerical simulation analysis of vertical propagation of hydraulic fracture in bedding plane

Hydraulic fracturing is one of the principle technologies employed for achieving the efficient development of shale oil and gas reservoirs, and the bedding plane of the reservoir is a key factor affecting the geometrical distribution of hydraulic fractures in three dimensions. The present study adopts cohesive force unit theory to construct a multi-layer hydraulic fracture propagation model with coupling stress damage filtration, and the effects of changes in the reservoir stress field, angle of the bedding planes relative to vertically propagating hydraulic fractures, and tensile strength of the bedding planes on the vertical propagation of hydraulic fractures are analyzed. The simulation results show that the hydraulic fracturing process of penetrating the bedding planes can be divided into three stages. Here, the hydraulic fracture propagates in the rock matrix and intersects with the bedding planes with continuous injection of fracturing fluid in stages I and II, while the vertical stress, bedding plane angle, and tensile strength of the bedding planes determine the direction of hydraulic fracture propagation in the third stage. Reservoirs with low vertical stress differences and nearly horizontal bedding planes with low dip angles are found to be favorable for opening the bedding planes, and reservoirs with high vertical stress differences and bedding plane dip angles are favorable for the longitudinal expansion of hydraulic fractures. Simultaneously, the degree of opening of the bedding planes also increases as the tensile strength of the bedding planes decreases. This is mainly because the energy consumption of the bedding layer opening process decreases as the tensile strength of the bedding planes decreases.