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Birdsell, Daniel T ¹ ; Karra, Satish ² ; Rajaram, Hari ³

¹ University of Colorado, Civil Environmental & Architectural Engineering
² Los Alamos National Laboratory, Computational Earth Science Group
³ University of Colorado, Civil Environmental & Architectural Engineering

Earthquakes caused by injection of fluids underground have become much more common in the past decade in the Central and Eastern United States. This injection-induced seismicity (IIS) generally occurs in fractured and faulted crystalline basement rock; migrates away from the well according to a nonlinear diffusion process; and is a function of fault orientation, fluid pressure, and in-situ stress state. Nevertheless, much of the previous modeling of IIS has largely ignored fractures, faults, and in-situ stress state, focusing primarily on fluid pressure increase as the driver of IIS. In this work we present a discrete fracture network and matrix (DFNM) model that includes Darcy flow, fractures, faults, and geomechanics. The model is massively parallel, so it can capture domains of relevant length scale (~10 km) with thousands of fractures. The fractures can open and close as a function of pressure and normal stress, which alters their permeability and porosity. We apply this DFNM model to site-specific locations of IIS including at Greeley, Colorado, and we compare the results to previous models that do not include fractures, faults, and geomechanics. This exercise offers insights about IIS at Greeley, constrains realistic hydraulic diffusivity values, and shows that earthquake-causing pressure moves farther from a well when key fractures become more permeable due to their normal deformation. This DFNM modeling framework shows promise for applications where matrix and fracture flow are important and hydraulic diffusivity is a function of pressure, stress, and/or shear failure history (e.g. unconventional oil and gas production, IIS, geothermal energy, and geologic carbon sequestration).