Investigation of Hydromechanical Mechanisms in Microseismicity Generation in Natural Fractures Induced by Hydraulic Fracturing

Summary Economic success in producing oil/gas from low-permeability shale often relies on pre-existing natural fractures (NFs) to be activated or connected by hydraulic-fracturing (HF) stimulation. In practice, the reactivated NF network is often identified with the microseismicity (MS) monitored during the stimulation. However, the fundamental mechanisms of MS generation and focal mechanisms inferred from geophysical analysis near HFs are not currently well-understood (e.g., it is not clear whether the MS observed in the field can be mainly attributed to local shear slip along NFs when leakoff occurs and/or induced by stress changes as a result of HF propagation or fluid leakoff). The exact slippage area and amount of slip displacement generating the microseismic event are not well-understood either, thus requiring a “bridge” between geomechanics and geophysics. To bridge this gap, a set of experiments is performed in a geomechanics laboratory to observe whether, when, where, and how the acoustic-emission events are generated during various scenarios. The experiments are designed with the guidance of the fracture-flow/discrete-element method (DEM), coupling geomechanical simulations. In this paper, the reliability and accuracy of the fracture-flow/DEM coupling approach is validated through solving a few fundamental problems and comparing the numerical-simulation results with the corresponding analytical solutions. The coupling approach is then applied to simulate and optimize two fundamental laboratory experiments. The simulations indicate that, despite the difference in the magnitude, one could induce the local slip along NFs by both fluid leakoff and stress changes.