Study On Anisotropic Mechanical Behavior Of Reservoir Shale In Shear Failure

As an unconventional oil and gas resource,shale gas is in line with the current trend of clean,low-carbon,safe and efficient energy utilization.As an important part of the"14th Five-Year Plan"and the 2035 long-term goal,the exploration and development of shale gas plays an important role in ensuring my country’s energy supply and advancing the energy revolution.In the development process of shale gas,the anisotropy of shale under shear failure is a problem that cannot be ignored.Because shale is extremely brittle,it is easy to be sheared during hydraulic fracturing,which will seriously affect the abundance and accuracy of fracture propagation,thereby restricting the migration path of shale gas in the reservoir,resulting in low shale gas production productivity;In addition,the anisotropic characteristics of shale will affect its strength and failure mode,and increase the risk of well wall collapse,which is not conducive to the safety and stability of shale gas development.This paper takes reservoir shale as the research object.Through triaxial compression test and direct shear test of shale,the mechanical response anisotropy characteristics of shale under complex load are described.At the same time,the shear fracture of shale is studied with acoustic emission technology.The energy evolution mechanism during the failure process and its failure mechanism are discussed.Based on the meso-mechanical analysis method,the meso-energy evolution is analyzed emphatically,and the anisotropic mechanical behavior in the shale shear failure process is revealed.（1）Research on the mechanical response of shale under complex loads is carried out.The triaxial compression test results of different bedding shale show that the strength and elastic modulus of shale are greatly affected by the bedding angle,and the compressive strength of shale presents a"U"-shaped change trend of high on both sides and low in the middle,60°shale has the lowest compressive strength,and 90°shale has the highest compressive strength.The bedding angle has little effect on the crack initiation stress of shale cracks,but it has a great influence on the crack damage stress.The bedding angle is the main factor that influences the strong anisotropy of the ultimate failure mode and failure mechanism of shale.The increase of confining pressure will increase the crack damage stress of shale,and the increase rate of 0°and90°shale is the fastest.The existence of bedding has a controlling effect on crack propagation.At the same time,the direct shear test of different bedding shale shows that the whole shearing process of shale shows obvious brittleness characteristics,and the four-stage characteristics are obvious.These four stages are the compaction stage where the shear displacement increases quickly and the shear stress increases slowly,the stage where the shear stress rises rapidly,the failure stage where the shear stress falls,and the residual strength stage.The cohesion and internal friction angle of shale with different bedding angles are different.The cohesive forces of 0°,30°,45°,and 60°shale are12.1MPa、12.8MPa、10.5MPa and 17.4MPa,and the friction angle is 22.5、20.6、26.6 and 22.8,respectively.Under the same normal stress conditions,shale has the highest shear strength and the largest cohesive force;when the normal stress increases,the peak shear stress and residual strength of the shale increase,that is,increasing the normal stress can increase shear strength of large specimens.（2）Combined with acoustic emission technology,two acoustic emission characteristic parameters of ringing count and energy are used to deeply study the damage change and energy evolution law of shale in the process of shear fracture.The test results show that the acoustic emission characteristics of shale shear failure process can be divided into compaction stage,crack initiation and stable propagation stage,and crack unsteady propagation stage.In the compaction stage,the cumulative ringing count and cumulative energy are small,and the number of cracks is small;When the shear stress increases to 1/3 of the peak stress,the cumulative ringing count gradually increases in a stepwise manner,while the cumulative energy slowly increases.At this time,the inside of the sample cracks and gradually expands;At the moment of sample failure,the shear stress drops sharply,and the cumulative ringing count and cumulative energy rise sharply.After reaching the residual strength stage,no new cracks are generated in the sample,and the cumulative ringing count and cumulative energy tend to be stable.In addition,the bedding angle has a great influence on the characteristic quantity of shale acoustic emission.The cumulative ringing count and energy release frequency of 15°and 75°shale are larger than other angles,and the cumulative ringing count generated after the failure of 60°shale is the least.When the normal stress gradually increases,the acoustic emission activity becomes more active,and the frequency of shale ringing at all angles becomes higher and higher,the cumulative energy becomes larger,and the damage inside the specimen increases.The failure time of 30°and 60°shale samples is advanced.When the shear rate increases,the failure time of the specimens at all angles will be shortened,and the increase of the shear rate has a certain weakening effect on the shear strength of the specimens.（3）Based on the experimental study of shale shear failure,the anisotropic mechanical behavior of shale fracture was studied based on the particle discrete element software PFC^2D.Numerical simulation results show that the fracture evolution process of shale with different bedding angles has obvious anisotropy characteristics.The cracks in shale basically start from the two ends of the shear surface,the cracks in the 0°shale are only generated on the horizontal shear surface,the fracture surface is relatively flat,and the specimen has shear slip failure on the horizontal shear surface;the fractures of30°,45°and 60°shale develop along the direction of the bedding angle,penetrate the bedding plane,and extend into the shale matrix.The specific process is as follows:the fractures of the 30°shale extend from the center of the sample to both ends,the left-end fractures of the 45°shale extend to the upper right,the right-end fractures extend to the lower left,and the 60°shale fractures extend to both ends on three parallel fracture zones.The shear fracture surface of the 90°shale is uneven,and the sawtooth marks caused by friction are obvious.It is under the combined action of tensile stress and shear stress.The macroscopic crack propagation law corresponds to the meso-level energy evolution mechanism.In the elastic compression deformation stage,the dissipated energy is almost zero,no cracks are generated in the sample,the boundary energy increases exponentially and all of it is converted into particle strain energy and parallel bonding strain energy and stored inside the sample;After that,the dissipated energy steadily rises,the frictional energy occupies all of the dissipated energy,and the cracks gradually crack and expand;After reaching the peak stress of the specimen,the dissipated energy rises rapidly and the strain energy drops instantaneously,the crack expands steadily and gradually penetrates,and the model specimen appears to slip between particles and eventually fails.