Direction Of Maximum Principal Stress Research Articles

DEM numerical simulation study on fracture propagation of synchronous fracturing in a double fracture rock mass

The rational choice of perforation arrangement and injection angle is of great significance to improve the efficiency of synchronous fracturing and to achieve the stimulation of the shale reservoir. In this paper, the influence of perforation angle and perforation arrangement on the crack propagation mechanism and rock failure mode of synchronous fracturing is studied based on the particle discrete element software PFC. Results show that due to the influence of the stress shadow effect, elliptical cutting cracks are produced between perforations during the synchronous fracturing. When the perforations are arranged in alignment, the fracture mode of rock mass between the perforations is single crack cutting failure under low injection angle, and double-crack cutting failure under high injection angle. Meanwhile, when the injection angle tends to the direction of maximum principal stress ( $$\uptheta$$ ≥ 60°), the fracturing effect can be greatly improved. In addition, the small perforation spacing under the high principal stress difference is more conducive to the fracturing of the central rock mass. When the perforation is staggered arrangement and the horizontal distance between the perforation centers is the perforation length, it is easy to form the intersection phenomenon of wing cracks under the high principal stress difference, which is more conducive to the realization of fracturing in the central rock mass. In addition, high injection angle is more conducive to the formation of penetrating cracks, but not easy to achieve the central rock mass fracturing. The findings of this study can help for a better understanding of crack propagation law and rock failure mode under different perforation arrangements, which can provide some theoretical guidance for the field synchronous fracturing construction.

A Modeling Method of Continuous Fiber Paths for Additive Manufacturing (3D Printing) of Variable Stiffness Composite Structures

A modeling method of variable stiffness composite structures (VSCSs) with curved fiber trajectories has been developed. The fiber trajectories are aligned in the direction of maximum principal stress, and the VSCSs with variable fiber orientation and the variable fiber volume fraction are modeled on the basis of these trajectories. A material property degradation method taking into account the heterogeneity of material properties of the VSCSs is used to predict the ultimate load and model the progressive failure for a composite plate with a hole under tensile loading. It is shown that a transition from rectilinear reinforcement to curvilinear results in an increase in the ultimate load of the plate. The opportunity for simulation of a continuous fiber path for the VSCSs is presented, and the path could be used to produce the VSCSs by additive manufacturing (3D printing).

Experimental study on permeability enhancement of coal seam with high mineral content by acid fracturing

ABSTRACT The permeability enhancement is the key to increase the efficiency of gas drainage from low-permeability coal seam. In this paper, the acid fracturing technology is proposed to solve the problem of low permeability of coal seam with high mineral content. The self-developed coal-rock acid fracturing experimental system is used to study the whole process of coal fracturing under different stress conditions and the permeability of coal specimens with varying permeability enhancing methods. The surface characteristics and mineral content of coal specimens before and after acid fracturing were tested by SEM and XRF technologies. The test results show that the effect of coal permeability enhancement by acidizing fracturing is mainly dominated by initial permeability, mineral content, and stress environment. The higher initial permeability, soluble mineral content, and in situ stress difference will lead to greater enhancement of coal permeability after acid fracturing. The fracturing induced fractures extend along the direction of maximum principal stress. Acidification causes the dissolution of Ca and Mg minerals in coal specimens, which provides the priority of fractures for further extension. Therefore, acid fracturing in coal seam is the result of the joint action of hydraulic fracturing (dominant) and acidizing (auxiliary). In this study, acid fracturing increases the permeability of coal specimens by 26.5–28.3 times.

Numerical and Theoretical Analysis on Hydraulic Fracture of Rock Masses Based on Contour Integral and Auto-remeshing Technology

The initiation and formation of hydraulic fracture are vital to the prevention of the failures of rock mass structures and the increase of oil production etc. In order to further understand the law and mechanism of hydraulic fracture, The Python language is used to compile the programme for the hydraulic crack propagation based on the Abaqus platform, the auto-remeshing technology is used and and the crack propagation process is realized. The numerical test of hydraulic fracture of No.15 coal roof rock mass in Wangtaipu coal mine under the condition of single and double pre-existing fractures is carried out, and the theoretical mechanism is also discussed, results show that: The “wing” crack first initiates from the crack tip and then develops towards the direction of the maximum principal stress. The crack initiation angle decreases with the increase of the value of hydraulic fracturing factor D ((P − σ1)/(σ1 − σ3)), and the wing crack combination mode includes the tensile-shear mode and the pure tensile mode. KI at the outer side of the crack tip increases during crack propagation and KII changes little, while KI at the inner side increases first and then decreases, and the KII increases after the crack tips coalescence, which indicates that shear effect exists. The research results will provide a reference for the understanding of the mechanism of single and double cracks propagation under hydraulic fracturing.

Experimental Investigation of Drilling Lateral Boreholes in Chalk Rocks with High-Pressure Jets

Increasing reservoir connectivity to the wellbore and bypassing the damaged area is crucial in improving the productivity of the wells and enhancing the swept area. This has become feasible by a new technology called radial jet drilling (RJD), in which relatively long, small-diameter laterals can be drilled radially from the main wellbore. In this study, the authors attempt to gain a better understanding of the efficiency of a high-velocity jet drilling on chalk destruction, and also identify parameters controlling the jet drilling. For this purpose, two distinct outcrop chalks from Austin, Texas (US) and Northern Province, Welton (UK) are used in this study, which are analogs to the reservoir chalk in the North Sea. In conjunction with the jet drilling experiments, basic rock mechanics testing is carried out in order to correlate the rock strength and stiffness properties to the jet drilling performance. Jet drilling of boreholes is evaluated not only by varying the fluid and nozzle type and the fluid pressure at the nozzle, but also varying the jet drilling setup under unconfined and also confined stress fields resembling reservoir condition. Results of our study show a clear correlation of the rock strength (and stiffness) on the threshold pressure and specific energy required to break the rock. Tight chalk requires more than 30% higher pump pressure than used in soft chalk for breaking the chalk, having more than twice the strength properties. Soft chalk presents larger borehole size and better rate of penetration, both with water and acid-aided fluid, owing to its higher matrix permeability value, as well as lower mechanical properties that favor diffusion of the jet drilling fluid into the rock and faster erosion/breakage compared with tight chalk. Static nozzles create a larger surface area compared with rotating nozzles. The penetration rate of the nozzle is improved significantly under stress confinement. In addition, jet drilling in the direction of minimum principal stress (σ3) appears to be faster owing to localization of shear failure around the drilled hole induced by the differential stresses compared with the jet drilling in the direction of maximum principal stress (σ1) under isotropic stress or ambient conditions.

Fracture initiation pressure and failure modes of tree-type hydraulic fracturing in gas-bearing coal seams

Tree-type hydraulic fracturing is a new technology that can uniformly increase the permeability of coal seams. The existing initiation pressure prediction models do not fully consider the characteristics of coal seams and are not suitable for tree-type hydraulic fracturing. In this paper, an initiation pressure prediction model considering coal seam joints and gas pressure is established. In this model, the fracture failure modes of coal seam fracturing are calculated separately to accurately predict the failure mode and initiation pressure. Statistics and reasons are obtained for the fracture initiation position and the failure mode of tree-type hydraulic fracturing under different conditions. The fracture failure modes of a gas-bearing coal seam are mainly tensile failure and coal matrix failure, and tensile failure along bedding planes is the most common. The effects of 7 factors, including coal seam geological parameters, fracturing hole arrangement parameters and tree-type branch borehole arrangement parameters, on the fracture initiation pressure of tree-type hydraulic fracturing are analyzed. Among them, the angle between the tree-type branch borehole and the maximum principal ground stress direction is the only major influential factor that can be artificially controlled. The fracture initiation pressure is lowest when the tree-type branch borehole lies along the direction of the minimum principal ground stress. The results of a field experiment show that the theoretical initiation pressure and the actual initiation pressure are within 6.3%. In addition, compared with traditional fracturing technology, tree-type hydraulic fracturing technology can reduce the initiation pressure. The proposed model can provide a reference for field applications of tree-type hydraulic fracturing technology.

Effect of rock brittleness on propagation of hydraulic fractures in shale reservoirs with bedding‐planes

AbstractHydraulic fracturing forms complex hydraulic fracture networks (HFNs) in shale reservoirs and significantly improves the permeability of shale reservoirs. Although rock brittleness is a major factor in determining whether a shale reservoir can be fractured, the relationship between HFNs and rock brittleness remains unclear. To investigate this relationship in a shale reservoir with bedding planes, this paper presents a series of hydraulic fracturing simulations based on a hydromechanically coupled discrete element model. In addition, we analyzed the sensitivity of the difference in rock brittleness to bedding‐plane angle and density. The parameters used in the model were verified by comparing the simulated results with experimental results and a theoretical equation. The results showed that breakdown pressure and injection pressure increased with increasing rock brittleness. The tensile hydraulic fracture of a shale reservoir (THFSR) was always the most abundant type of hydraulic fracture (HF)—almost 2.5 times the sum of the other three types of HFs. The distribution of areas with higher fluid pressure deviated from the direction of the maximum principal stress when the angle between the bedding plane and maximum principal stress directions was large. Upon increasing this bedding‐plane angle, the breakdown pressure and rock brittleness index first decreased and then increased. However, regardless of bedding angle, the relative proportions of the various types of HFs remained essentially constant, and the seepage area expanded in the direction of the maximum principal stress. Increased bedding‐plane density resulted in a gradual increase in the total number of HFs, with significantly fewer of the THFSR type, and the large seepage areas connected with each other. This study thus provides useful information for preparing strategies for hydraulic fracturing.

Numerical simulation of blasting in confined fractured rocks using an immersed-body fluid-solid interaction model

We model blast-induced fracturing and fragmentation processes in fractured rocks using a fully coupled fluid-solid interaction model. This model links a finite-discrete element solid solver with a control volume-finite element fluid solver through an immersed-body method. The solid simulator can capture the deformation of intact rocks, interaction of matrix blocks, displacement of existing fractures and propagation of new cracks. The fluid simulator can simulate the highly compressible gas flow involved in the blasting and explosion process, which is assumed to follow the John-Wilkins-Lee equation of state. We design numerical experiments as follows. First, we generate a series of 1 m × 1 m discrete fracture networks associated with different fracture density and mean length values to consider various scenarios of distributed pre-existing fractures in rock. We apply isotropic/anisotropic in-situ stresses to the rock such that the system reaches an equilibrium state. Then we release the compressible gas associated with a prescribed high pressure in the borehole to simulate explosion, which engenders stress wave propagation and new crack generation in the system. We observe that the presence of natural fractures has a significant impact on the blast behaviour of fractured rocks such that new cracks tend to be arrested by pre-existing discontinuities which however accommodate wing cracks at their tips linking with other structures. Blast-driven cracks attempt to propagate along the maximum principal stress direction if an anisotropic stress condition is imposed. Our research findings have important implications for the design and assessment of blasting for underground excavation in fractured formations.

Peridynamics simulation of surrounding rock damage characteristics during tunnel excavation

The improved peridynamics is introduced to analyze the stability of surrounding rock in tunnel excavation process. By introducing short-range repulsive force into peridynamic equation of motion, the tensile and compression failure characteristics of rock materials is simulated. In order to simulate the process of tunnel excavation, the material point dormancy method is proposed in this paper. By simulating the distribution characteristics of excavation damage zone (EDZ) during the excavation of a circular tunnel with high stress difference, the relationship between the distribution position of EDZ and the direction of maximum principal stress is revealed. The v-shaped notches developed on the tunnel periphery at the direction of perpendicular to the maximum principal stress. The predicted EDZ distribution characteristics is consistent with the results of previous studies. And the displacement field of surrounding rock after tunnel excavation is also in good agreement with FEM simulation result. The simulation results show that this method not only has good stability, but also has high computational efficiency. By analyzing the deformation, damage and failure characteristics of surrounding rock under different buried depths, different lateral pressure coefficients, different excavation methods and different section shapes, the evolution process of rock instability caused by tunnel excavation and unloading is revealed. Which provides reference for the design and optimization of surrounding rock support scheme in deep buried tunnels.

Stress-based tool-path planning methodology for fused filament fabrication

Tool-path planning has a considerable impact on the quality of components printed by fused filament fabrication (FFF). This research proposes a path generation strategy based on the orientations of the maximum principal stresses. According to stress calculations from finite element analysis (FEA) of the components, tool-paths, which are programmed as parallel to the maximum principal stress directions, are constructed with the depth-first search (DFS) method and a connection criterion. The breakpoints in the tool-paths are then eliminated by connecting adjacent tool-paths. The Dijkstra algorithm is engaged to reduce the nozzle jump distance and shorten the production time. Stretching tests of different specimens printed with the developed path generation algorithms demonstrate that the model with the stress-based path has better mechanical performance. The digital image correlation (DIC) method and scanning electron microscopy (SEM) are employed to observe the fracture processes and fracture surfaces, respectively. Corresponding results of DIC and SEM reveal that different path filling forms exhibit variable failure patterns because of filament anisotropy. The filling fraction is calculated and indicates that the deposition quality of the advanced path is not compromised. This work provides a synthesis methodology for improving the mechanical performance of 3D printing products.

Study of the Rock Crack Propagation Induced by Blasting with a Decoupled Charge under High In Situ Stress

The high in situ stress can significantly affect the blast‐induced rock fragmentation and cause difficulties in deep mining and civil engineering where the drilling and blasting technique is applied. In this study, the rock crack propagation induced by blasting under in situ stress is first analyzed theoretically, and then a numerical model with a decoupled charge in LS‐DYNA is developed to reveal how the initiation and propagation of rock cracks are under high in situ stress. Through simulation, the mechanisms of blast‐induced crack evolution under various hydrostatic pressures and nonhydrostatic pressures are investigated, and the differences in crack evolution with specific decoupling coefficients are compared. According to the simulation, three damage zones, i.e., the crushed zone, the nonlinear fracture zone, and the radial crack propagation zone, are formed, and the radial crack evolution is greatly suppressed by the high in situ stress which has no much influence on the crack propagation in the crushed and the nonlinear fracture zones. The velocity of crack propagation is slightly reduced, and the process of crack propagation is stopped early when the rock is subjected to high in situ stress. Furthermore, the numerical analysis indicates that the crack grows preferentially in the direction of maximum principal stress, and the radial crack propagation is predominantly controlled by the preloaded pressure, which is vertical to the crack propagation direction. Based on the numerical results, it is suggested that the optimal decoupling coefficients for rock cracking are 2.65, 1.87, 1.37, and 1.22 for 0, 10, 20, and 30 MPa, respectively. This study provides not only an analysis of the rock crack evolution under high in situ stress but also a reference for resolving excavation difficulties in deep mining.

Experimental Study on the Uniaxial Compression Failure and Ultrasonic Transmission of Brittle Shale Considering the Bedding Effect

The rock failure process, especially the crack propagation process in rock, can essentially reveal many mechanical problems in the rock. The bedding structure of shale is one of the main influencing factors of rock failure. This study performed an ultrasonic dynamic transmission test on Longmaxi Formation shale specimens with different bedding angles under uniaxial compression and comprehensively analyzed the mechanical and ultrasonic evolution characteristics. The deformation and failure of the shale samples indicated obvious brittle characteristics, and the final failure mode was dominated by shear cracks along the bedding planes and tensile-compressive cracks along the maximum principal stress direction. The difference index of the bedding calculated by the ultrasonic response speed showed that with increasing deformation and failure, the influence of the bedding structure on the failure decreased. In the process of uniaxial compression, the ultrasonic transmission velocity changed periodically. Relying on the obvious jumping points of the ultrasonic velocity curve, we could initially judge the possible key points of the changes in the pores and cracks during the rock failure process. A new method to calculate the rock brittleness index based on the characteristic response velocity of ultrasonic waves was proposed. Compared with the classical method, this method revealed a good applicability. This study can further clarify the relationship between the acoustic characteristics of ultrasonic transmission and rock deformation and failure, and can lay a certain theoretical foundation for the subsequent study of ultrasonic transmission and rock structure, along with its mechanical, physical and chemical properties.

Simulation of anisotropic crack growth behavior of nickel base alloys under thermomechanical fatigue

Allvac®718Plus is a nickel base superalloy possessing outstanding abilities to withstand both mechanical and thermal loads, which make this material ideal for turbine disks and blades in aero engines. The thermomechanical fatigue (TMF) crack growth behavior depends on temperature, in-service load cycles and material orientation. In particular, the crack growth at high temperature dwell time shows significant influence of material anisotropy due to platelet-like precipitations, which leads to crack deflection from the maximum principal stress direction. These specific properties must be taken into account properly to ensure a safe design and the desired lifetime of components. The focus of this paper is to develop an appropriate fracture criterion to describe crack deflection for such kind of anisotropic material resistance. This criterion and the associated crack growth laws for thermomechanical fatigue are implemented in the FEM software Procrack for fully automated three-dimensional crack propagation simulation. The numerical simulation of experiments on CT-specimens with various material orientations provided evidence that the software can capture the actual fracture behavior. Moreover, a suitable set of material parameters could be identified. The performance of the presented approach to predict realistic three-dimensional TMF crack propagation is demonstrated by an application to a typical turbine component made from Allvac® 718Plus.

Determination of stress distribution on active fault by means of Casagrande method; an innovative approach

The investigation deals with the stress distributions of undisturbed soil samples taken by preconsolidation press method from paleoseismological studies in the Gerede (Bolu) segment of the North Anatolian Fault System (NAFS), which is one of the most important intra-continental active transform faults in the world. Taking into consideration of the theoretical principal stress directions causing the faulting in the soil samples, consolidation experiments were carried out in different directions and the preconsolidation pressure values (PPV) of the samples were found by Casagrande method. Relations between the prime stress directions which are effective in the mechanism of fault formation and the values of PPV found by the Casagrande method were revealed. With all these calculations, the principal directions and magnitudes of the principal stresses causing faulting in the near-surface depths on the Gerede segment of the NAFS have been determined by a new method. The Gerede segment of the maximum principal stress (σ1) value at N 55.50 W and N 600 W was found 554.08 kPa and 327.54 kPa respectively. The maximum principal stress (σ1) direction of the fault was found to be N 55.50 W and the angle value was 44.50.

Hydrate‐filled Fracture Formation at Keathley Canyon 151, Gulf of Mexico, and Implications for Non‐vent Sites

AbstractNear‐vertical hydrate‐filled fractures are found in subseafloor marine muds, at both advective methane vent sites and at sites without obvious methane and fluid advection (non‐vent sites). At non‐vent sites, the mechanisms that transport methane to the fractures and control how hydrate‐filled fractures form are not well understood. However, these mechanisms are important to establish, as most of Earth's natural gas hydrate is likely bound in marine mud systems. Herein, we focus on understanding the origin of hydrate and how fracture form at non‐vent sites by examining previously hydrate‐bearing fractures in conventional cores taken from Keathley Canyon 151, U.S. northern Gulf of Mexico, drilled by the Gas Hydrate Joint Industry Project in 2005. We combine information from well logs, sediment cores, and science party results and add new X‐ray computed tomography of archival sections and scanning electron microcopy of core samples to develop a conceptual model. We propose that locally generated microbial methane is transported via diffusion from small pores in marine mud into biomineralized burrows with larger pore size in a process called short‐range migration. Hydrate forms in burrows once the methane diffuses into them and the dissolved methane concentration exceeds the solubility threshold. When hydrate fills a burrow, heave from additional hydrate growth places stress on the burrow edges, expands the fracture, and creates additional void space in which methane can diffuse and continue forming hydrate. Fractures slowly propagate in the direction of maximum principal stress.

Mathematical model for calculating the horizontal principal stress of a faulted monoclinal structure in southwest Qaidam Basin, China

Faulted monoclinal structures incorporate folds and faults. Their displacements cause stress release of differing degrees in various areas. Furthermore, the faults commonly extend to the deep strata, which may lead to the flow of boiling inclusions and localized thermal stresses. A complex distribution of in situ stress appears in such areas, which makes it challenging to drill wellbores. In this study, we first analyzed the geological structure and the factors influencing in situ stresses of a faulted monoclinal structure in southwest Qaidam Basin. Second, based on Huang’s model, a mathematical model for calculating the horizontal principal stress of the faulted monoclinal structure was developed considering the stress release from fault and fold displacements and the outflow from boiling inclusions. Third, the magnitudes of the horizontal principal stresses of the study area were obtained using this new mathematical model, and the predicted values had an average error of 2.63% according to a comparison with the results of the hydraulic fracturing field measurements. Fourth, an in situ stress distribution in this area was established by combining the horizontal principal stresses’ magnitudes and the maximum horizontal principal stress direction. Finally, based on the in situ stress distribution of the study area, the mud density and trajectory for drilling the third stage of well Z207 were determined. The field application revealed that the design parameters met the engineering requirements satisfactorily, which indicates that this mathematical model can predict the horizontal principal stress of the faulted monoclinal structure in southwest Qaidam Basin.

Research on influence of the maximum principal stress direction on stability of roadway surrounding rock

Research on influence of the maximum principal stress direction on stability of roadway surrounding rock

The relationship between stress changes and strong earthquakes since 1900 around the Bayan Har block in northern Tibet, China

A 3D viscoelastic finite-element model of the Qinghai-Tibet Plateau, including the Bayan Har block, is established in this article. This model takes into consideration differences in the regional geological structure, the main active fault zone, the active tectonic block (hereafter referred to as a block) and boundary fault zone, the irregular topography and layered lithospheric structure, and the seismic velocity structure of the crust and mantle. Moreover, the present tectonic background stress field in the research area is reconstructed using the constraints of the observed values for the rate of crustal horizontal movement and the direction of maximum principal compressive stress. On these bases, the historical strong earthquake sequences with Ms ≥ 7 that have occurred around the Bayan Har block since 1900 AD are simulated. Based on the Coulomb failure stress and equivalent stress data, the relationships between stress evolution and strong earthquakes, the interactions between strong earthquakes are investigated. In addition, we have also explored the changes in stress on the Longmenshan fault zone before and after the 2008 Wenchuan earthquake, as well as the relationships between stress evolution and regional seismicity. The results show that the occurrence of strong earthquakes around the Bayan Har block may be related to the increase in the total stress in the source area. The influence of long-term tectonic stress loading on strong earthquakes cannot be ignored. The 2008 Wenchuan earthquake induced an increase in stress in the southern section of the Longmenshan fault zone, indicating that the 2008 Wenchuan earthquake has an obvious promoting effect on the 2013 Lushan earthquake. The seismicity of the Bayan Har block and its adjacent areas may be affected by the regional total stress state.

Length scale dependent elasticity in random three-dimensional fiber networks

The mechanics of fibrous materials are complicated by complex deformation mechanisms that result from the discrete fibrous structure. Though it is known that fibrous materials deform primarily by rotating and bending of fibers, the implications of rotating and bending on the mechanics of the network are not fully clear. Previous studies have investigated some effects of the fiber rotation and bending, observing fibers to rotate into directions of maximum principal stress, resulting in strain stiffening, and fibers under compression to buckle, resulting in compression softening. Nonlinear constitutive models have recently been developed to account for these deformation mechanisms, but the classical constitutive models that account for only stress and strain cannot fully account for fiber rotations and bending. Here, we interpret the mechanics of fibrous materials through micropolar elasticity, also called Cosserat elasticity, which differs from classical elasticity in that it accounts for local moments caused by rotation of points within a material. The resulting equations can be written in terms of characteristic lengths that cause stress to depend on both strain and the length scale of the material. We simulated three-dimensional networks of fibers and observed a strong effect of length scale on the stiffness of networks in bending with a more mild effect in torsion. The length-scale dependence of stiffness is consistent with micropolar elasticity and can be described in terms of characteristic lengths of the material. Factors affecting the characteristic length were investigated by altering the fiber density, alignment, and bending stiffness. Although density was found to have no effect on characteristic length, increased fiber alignment led to a decrease in characteristic length, and increased fiber bending stiffness led to an increase in characteristic length. These findings suggest that the characteristic length is increased by factors that increase bending moments supported by the fibers. Although our parameter study manipulated the magnitude of characteristic length, no combination of parameters gave a characteristic length of zero, indicating that the mechanics of fibrous materials depend on length scale.

Numerical simulation of hydraulic fracturing in transversely isotropic rock masses based on PFC-2D

In order to make a better understanding of the hydraulic fracturing in transversely isotropic rock masses, the modified particle flow modeling method was used by embedding the smooth joint models within an area of certain thickness, and the optimized fluid-mechanical coupling mechanism was applied in hydraulic fracturing modeling. On this basis, the influence of the injection rates, in-situ stress ratios and inclination angles of the bedding planes on the breakdown pressure and propagation of the hydraulic fractures was analyzed. The simulation indicated that: 1) Excessive small or large injection rates would lead to the increase of the breakdown pressure of the hydraulic fractures. 2) Under different inclination angles of the bedding planes, the crack breakdown pressure increased linearly with the increasing of the in-situ stress ratios. And under conditions of different in-situ stress ratios, the crack breakdown pressure changed as a ‘wave’ type with the increasing inclination angles of bedding planes. 3) Both the in-situ stress ratios and the inclination angle of bedding planes affected the propagation of the hydraulic fractures. The existence of the bedding planes would induce the hydraulic fractures to propagate along the bedding planes. The large inclinations of the bedding planes would cause the hydraulic fractures to keep propagating with the direction of maximum principal stress.