Cyclic Experimental Studies on Damage Evolution Behaviors of Shale Dependent on Structural Orientations and Confining Pressures

Damage process of shale is of great importance when considering the wellbore stability and reservoir stimulation during the work of shale gas recovery. As shale has typical transversely isotropic structures and may be under different stress states in the reservoir, the damage process should be studied while considering both loading directions and confining pressures. A series of cyclic uniaxial and tri-axial compressive tests have been carried out on the shale samples with different oriented weak planes and confining pressures. The dissipated strain energy and its ratio to the releasable elastic strain energy have been studied comparing with the fracturing patterns of the samples. Based on the strain energy dissipation, damage variable is defined and the damage evolution equation is built to describe the damage process of shale samples dependent on loading directions and confining pressures. The damage equation shows that the damage of the shale samples increases as a power function of the axial stress. Under higher confining pressures, the damage is limited in the early loading stage, while it increases significantly with the formation of macro shear fractures when the peak strength is approaching. The change of increasing rate is weak for the shale samples ( β = 60°) as the main failure pattern is slip along the weak planes. This study is helpful for understanding the damage process and failure of wellbore, as well as the stimulation effect of the shale gas reservoir.