Experimental and DEM Simulation Study on the Mechanical Characteristic and Strain Energy Evolution of Longmaxi Shale under a Confining Pressure Unloading Path

Drilling vertical and horizontal wellbores in the shale reservoir may trigger the in-situ stress release around the wellbore walls and change the original stress equilibrium state, leading the wellbores to instability. This stress change in the wellbore corresponds to the stress paths of confining pressure unloading and axial stress loading under laboratory conditions. In this paper, according to the conventional triaxial compression test results, laboratory experiments and DEM simulations by PFC 2D were conducted to deeply study the strength, failure, strain energy evolution, and micro-crack damage mechanism of shale specimens under confining pressure unloading conditions. The shale specimens at different bedding inclinations were tested under different initial axial stress levels and confining pressure unloading rates, with fixed initial unloading confining pressure. This research revealed that confining pressure unloading induces greater plastic deformation, more micro-crack damage and strain energy dissipation, and a more complex failure pattern. The strain energy dissipation and dilatation under confining pressure unloading conditions are mainly induced by the generation and accumulation of tensile cracks. Moreover, the unloading rate has a significant effect on the mechanical properties, and the high unloading rate enhances the failure strength and induces more strain energy dissipation and micro tensile cracks. For the wellbore drilling in shale formations, when the buried depth and vertical stress are fixed, the lower the lateral stress is, the easier it is to form tensile failure around the wellbore wall in the drilling process, and the more induced fractures will be generated in the formation around the wellbore.