Study on plastic-brittleness mechanical properties and failure law of deep brittle shale under complex mechanical environment of wellbore

Abstract

China has abundant deep shale gas resources, but the complex problem of wellbore collapse in horizontal well drilling is prominent. Aiming at the problem of wellbore collapse in deep brittle shale, deep shale’s mechanical loading simulation experiment scheme under a complex wellbore alternating mechanical environment was carried out. Triaxial mechanical tests of shale under different confining pressures are carried out in this paper. The stress-strain and acoustic characteristics of shale under different confining pressures during loading are analyzed, and rock’s stress-strain and dynamic acoustic response mechanisms are discussed. The mechanical constitutive relationship, plastic brittle mechanical properties, and anisotropic characteristics of deep brittle shale under a complex mechanical environment of the wellbore are studied. The collapse failure law of deep brittle shale wellbore is also analyzed. The results show that the deep brittle shale has high mechanical strength, is highly brittle, and developed bedding fractures. Under the condition of low confining pressure, the stress-strain curve of shale before fracture is mainly elastic deformation, and the plastic mechanical damage is not obvious. With the increase of confining pressure, shale’s compressive strength and elastic modulus increase, and its plastic damage gradually become apparent. In the process of triaxial loading, the change of acoustic wave velocity increases first and then decreases. With the increase of confining pressure, the peak stress point becomes smaller and smaller, revealing the plastic failure characteristics of rock under high confining pressure. The brittle features of shale under low confining pressure are obvious. The fracture morphology of rock after stress is mainly the splitting tensile fracture, and the number and size of cracks are significant. Under high confining pressure, shale exhibits plastic failure, and plastic damage characteristics are evident in the process of rock sample failure. Rock sample failure is mainly manifested as single shear failure or conjugate shear failure with tensile fracture. The experimental results show that the failure mode of brittle shale under a complex wellbore mechanical environment is mainly tensile splitting and shear coupling failure. The research results of this paper have particular guiding significance for the study of wellbore stability during drilling in the deep brittle shale formation.