Research On Anisotropic Mechanical Behavior Of Shale And Its Influence On Borehole Stability

For a long time,China's external dependence on energy remains high level.Unconventional oil and gas resources are of great significance for optimizing energy structure and ensuring energy security.Shale gas has attracted wide attention in recent years due to its cleanliness and large reserves.The geological environment of the shale gas enrichment area is often complex and deep buried,coupled with the mechanical anisotropy of shale,which brings challenges to water fracturing and gas migration.The safe,economic and efficient development of shale gas depends on fully understanding of the mechanical properties of shale in deep environment.In this paper,the influence of bedding angle on the mechanical properties of shale and borehole stability is studied by means of laboratory tests and numerical simulation.The main research works are as follows.(1)Cylindrical and disc specimens for laboratory tests were fabricated from an outcrop shale blocks from Jiangxi Province.Conventional triaxial compression tests and Brazilian tests were carried out to investigate the strength and deformation characteristics of shales with different bedding angle.Combining with acoustic emission(AE)and digital image correlation system,the fracture process and failure pattern of specimens were analyzed.(2)Aiming at the engineering problems such as tunnel excavation,unloading confining pressure test with two different stress paths for shale are designed.Evolution characteristics of stress and strain of shale specimens under unloading confining pressure test are obtained.The variation of strength and failure modes of shale under different unloading conditions is studied.(3)Using particle flow code(PFC),the shale numerical model containing the characteristics of bedding structure are established.A set of meso-parameters which can reflect the mechanical characteristics of shale are obtained and verified by comparison with the laboratory test results.Finally,the formation-borehole model of shale is established based on the parameters,the influence of bedding angle on borehole stability of shale is studied.Relevant conclusions have certain reference value for engineering practice.