Competing effects of proppant and surface roughness on the frictional stability of propped fractures

Abstract

Proppant is often used to enhance reservoir stimulations, such as hydraulic fracturing and hydraulic shearing; however, the influence of proppant on the shear deformation of fractures and the potential consequent-induced earthquakes are rarely explored. We explore the systematics of frictional behavior, deformability and dilatancy of proppant-filled fractures to define the complex response to different fracture roughness and proppant mass loadings. Shear experiments on rough granite fractures show that proppant reduces cohesion and internal friction, reduces the shear stiffness, delays the shear displacement to a diminished peak strength, reduces the magnitude of shear dilation, and promotes ductile shear failure that is analogous to aseismic creep. A systematic transition in shear behavior occurs from fracture-roughness-dominant to proppant-dominant with increased proppant mass loading that is augmented by increased grain size. Long-wavelength fracture undulations may engage at large shear displacements, causing increased frictional resistance—identifying an intrinsic-scale effect. The presence of proppant reduces the shear dilation. Thus, the convolved interactions between proppant and fracture roughness require careful assessment in their impact on creating and sustaining permeability and modes of aseismic versus seismic ruptures.