Effect of Water Imbibition on Fracture Generation in Mancos Shale under Isotropic and Anisotropic Stress Conditions

During hydraulic fracturing in shale oil and gas reservoirs, hydration swelling due to shale water imbibition is believed to create secondary fractures that improve permeability, as observed at ambient conditions. In this paper, the influence of hydration swelling on fracture generation and permeabilities in Mancos shale at different stress conditions was investigated. Hydration swelling was characterized by swelling strain tests. Generated fractures were observed by computed tomography (CT) scan tests. The permeability of the shale was measured before and after water imbibition. In shale with isotropic confining pressure increased to 2.0 MPa or more, water imbibition can cause fracture closing instead of propagation. As a result, permeability decreased significantly, up to two orders compared with that before water imbibition. On the other hand, the differential stress required to activate shear-induced fractures was greatly reduced after hydration, up to 50% in shale after water imbibition. For shale cores under higher confining pressure, with decreased hydration swelling magnitude, shear-induced fracture generation was reduced. Water permeability of shale with a shear-induced fracture was nearly 100 times higher than that of shale without a fracture. In other words, water imbibition can enhance the generation of shear-induced fractures under anisotropic stress to improve shale permeability, but it can cause fractures to close in shale under isotropic stress.