Predicted and observed shear on preexisting faults during hydraulic fracture stimulation

Multi-stage hydraulic fracturing increases production from extremely low-permeability unconventional reservoirs by simultaneously inducing slip on pre-existing fracture planes. Here we illustrate how the high pore pressure generated during hydraulic fracturing operations induces slip on preexisting fractures and faults with a wide range of orientations, thus creating an interconnected permeable fracture network. We demonstrate the basic principles of stimulating slip on poorly oriented faults using the stress state for a horizontal well in the Barnett Shale where fracture orientation data are also available from an image log. We compare this analysis with independent fracture orientation data obtained from earthquake focal plane mechanisms. Using the stress data, we are able to determine which nodal plane slipped in each microseismic event. As the two analyses yield essentially identical results, they show the basic processes by which slip on planes of varied orientations occurs during hydraulic stimulation. We extend this analysis to address some misconceptions about the likelihood of slip on horizontal bedding planes and planes parallel and perpendicular to horizontal principal stress directions. Notably, we show that inducing slip on horizontal or sub-horizontal bedding planes is nearly impossible except in compressive (reverse to strike-slip/reverse) stress states (SHmax ≥ Shmin ≈ SV) or when ambient pore pressure is extremely high. The latter case results in very small differences in the magnitudes of the three principal stresses, regardless of the regional stress state.