Geomechanics Mechanisms in Microseismicity Generation Near Natural Fractures Induced by Hydraulic Fracturing

Abstract Economic success 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 natural fracture network is often identified using the microseismicity (MS) monitored during the stimulation. However, the fundamental mechanisms of MS generation and focal mechanisms inferred from geophysical analysis near hydraulic fractures 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 natural fractures 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 microseimic event are not well understood either, thus requiring a “bridge” between geomechanics and geophysics. To bridge this gap, a set of experiments will be performed in a geomechanics laboratory to observe whether, when, where, and how the acoustic emission (AE) events are generated under various scenarios. The experiments are designed under the guidance of 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, the local slip along natural fractures could be induced by both fluid leakoff and stress changes.