Kinematic dilation during the hydraulic stimulation of pre-fractured rocks

Abstract

A single planar fracture geometry dominates the process of hydraulic fracturing in homogeneous, isotropic and cohesive materials. However, this fracture geometry cannot explain the high recovery efficiency observed in shale gas and enhanced geothermal energy. Experimental and numerical studies reported here demonstrate that pre-fractured, structured reservoirs experience extensive geometric distortion and dilation around the main opening-mode discontinuity generated by high-pressure fluid injection. This kinematic dilation may decrease at high confining stresses because blocks deform and split. Parameters such as the dominant fracture set orientation, block size and slenderness and blocks overlap length characterise a pre-structured medium and determine its deformation pattern and hydromechanical behaviour. Kinematically controlled dilational distortion greatly improves fluid conductivity in the pre-structured medium. A sixth-power relationship is anticipated between the enhanced hydraulic conductivity and the roundness of the main opening.