Anisotropic Experimental Study Of Shale Failure Modes And Parameters In Southern Sichuan

The shale reservoir has typical bedding characteristics,and the mineral particles that make up the shale body are different in size,unevenly distributed and combined,resulting in significant anisotropy of the mechanical properties and failure mechanism of the shale.In the hydraulic fracturing process,the bedding plane is always earlier than the fracture of the rock mass,which leads to the fracture along the weak surface when the hydraulic fracture is gradually extended,thereby inhibiting the formation of the fracture network and forming irregular network cracks.This makes the efficiency of hydraulic fracturing slip,which ultimately leads to difficulties in the exploitation of shale gas.Therefore,it is of great practical significance to study the anisotropy characteristics of shale microstructure,physical and mechanical parameters and failure modes,and analyze its mechanical properties and failure mechanism,so as to analyze the wellbore stability encountered in shale gas exploitation and the expansion law of hydraulic fractures.The main contents and results of this paper include:(1)The influence of mineral components on anisotropy and the microstructure anisotropy of shale in south Sichuan are studied.In this area,there are more shale mineral content and clay minerals,and shale reservoirs have higher compressibility.Clay mineral content is an important factor for shale anisotropy.The anisotropy of microstructure is unevenly distributed and greatly different,so that the shale has obvious anisotropy characteristics,and the anisotropy of its microstructure is the intrinsic factor of the existence of shale anisotropy characteristics.(2)Tensile,uniaxial and triaxial compression tests were carried out on shale samples with different bedding angles.In the tensile test,the failure mode of 0° sample was mainly matrix tension splitting failure,and the comprehensive failure mode of 22.5°,45° and 67.5°samples was matrix and bedding plane shear and tensile failure,while the failure mode of 90°sample was bedding plane tension splitting failure.Uniaxial compression test,0° sample is mainly through the tensioning disruption of bedding,22.5 ° sample is along the bedding plane and throughout the bedding surface complex shear failure and perpendicular to the radial tensile damage,45 ° sample for along the bedding plane and shear damage through bedding face,67.5°sample for shear failure along the bedding surface and perpendicular to the radial tensile damage,90° sample along the bedding surface tension for fracturing.Triaxial compression test,0° sample is mainly through bedding tension damage and shear damage,22.5 ° sample for shear failure of bedding,45 ° sample for along the bedding plane and shear damage through bedding face,67.5 ° sample for shear damage along the bedding plane,the 90 ° sample for tension along the bedding surface damage,and the tensile damage through shear failure.The anisotropy of the failure mode is very obvious.(3)From the anisotropy test of shale physical and mechanical parameters,it can be concluded that,with the gradual increase of bedding angle from 0° to 90°,the dynamic parameters of shale samples have obvious changes,and their anisotropy is very obvious.The tensile strength of shale shows a trend of gradually decreasing,and the uniaxial compressive strength curve of shale samples shows a U-shaped change trend with high on both sides and low in the middle.The triaxial compressive strength curves of shale samples under various confining pressures show a U-shaped trend of high on both sides but low in the middle.The static elastic parameters of shale samples also change obviously with the change of bedding angle.According to the calculation of internal friction angle and cohesive force,it is found that the cohesive force of the shale matrix in this area is quite different from that of the weak bedding surface,while the internal friction angle is basically the same.(4)Numerical tests of shale uniaxial compression and triaxial compression with different bedding angles were carried out by RFPA software,showing the step-by-step failure process of shale with different bedding angles under loads of different sizes and directions,and analyzing the anisotropy of shale under numerical tests.The results show that the uniaxial and triaxial compressive strength obtained by the numerical test is consistent with the change trend of the compressive strength obtained by the physical test with the change trend of the bedding angle,showing a U-shaped change trend with high on both sides and low in the middle.The failure modes of shale models at different bedding angles are similar to those of samples in corresponding physical tests,and their anisotropy is also very obvious.