Effect of water imbibition on hydration induced fracture and permeability of shale cores

During a hydraulic fracturing operation with water-based fracturing fluids, in-situ compressive stress was applied on shale. To study whether or not hydration induced fracture can be created to improve oil and gas recovery under isotropic compressive stress conditions, time-elapsed computerized tomography (CT) was used to obtain cross section images of shale cores with confining pressures loaded. Based on CT data, cut faces parallel to the core axial through the middle of core and 3D fracture images were reconstructed. To study the effects of hydration on shale pore fluid flowing under isotropic compressive stress conditions, shale permeability was measured with Nitrogen (N2), distilled water, 4% KCl solution, and 8% KCl solution. Hydration induced fractures came into being for Mancos shale with low confining pressure (15 psi) loaded. However, with high confining pressure (3000 psi) loaded, fractures tended to close eventually due to hydration. For the Eagleford shale, with either a low or a high confining pressure loaded, fractures eventually became nearly closed. In the Mancos shale which contained more swelling clay minerals, larger reduction of fracture apertures or permeability was observed than that in the Eagleford shale with confining pressure applied. Adding KCl into the water-based fracturing fluid during fracturing could decrease shale hydration and reduce shale permeability damage. Our study shows that 8% KCl solution could help reduce the permeability damage of Mancos shale, and 4% KCl solution was feasible to reduce the permeability damage of the Eagleford shale under isotropic compressive stress conditions.