FRACTURING OF SHALES AS FUNCTION OF STRESS AND ANISOTROPY

Understanding the hydro-mechanical properties of shale formations is becoming increasingly important as they play important roles in the production of unconventional oil and gas resources as well as in geological carbon sequestration where shales typically form the cap rock above sequestration reservoirs. Fractures can provide high permeability pathways for fluid flow and leakage in very low permeability shales. The layered microstructure of shales can result strong hydro-mechanical anisotropy along with the stress dependence in shale fracturing behavior. This paper aims at providing a better understanding of: 1) fracturing behavior of shales due to shearing in relation to stress and anisotropy, and 2) the effects of fracturing to the changes of permeability in shales. Triaxial compression tests were performed on shale sample cored horizontally by simultaneously measuring the permeability and P and S wave velocities. Structural anisotropy of shale is evaluated in terms of Thomsen's (1986) shear wave splitting parameter. The relationship between permeability and splitting parameter suggests that brittle fractures are suddenly formed along bedding plane immediately after reaching macroscopic yield point at low confining stress, whereas shear fractures are formed gradually after yield point. It is demonstrated that the shear-wave splitting parameter is a useful index for permeability of horizontal shale subjected to shearing.