Mechanistically modeling of hydraulic fracture opening, closure and residual fracture permeability during cyclic flow injection tests

In order to address the challenges of subsurface energy-related process involving rock fracturing behavior and permeability enhancement, the United States Department of Energy (DOE), Geothermal Technologies Office (GTO) is funding a collaborative investigation of enhanced geothermal systems (EGS) processes at the intermediate-scale (~10-20 m). Referred to as the EGS Collab/SIGMA-V project, this project is envisioned to comprise three field experiments and the site for the first experiment is on the 4850ft Level below the ground in phyllite of the Precambrian Poorman formation within the Sanford Underground Research Facility (SURF), located at the former Homestake Gold Mine in Lead, South Dakota. The experiments will provide rich data sets that can be used to improve and validate the model and to further understandings of the relationships between stress, fracturing, induced seismicity and permeability enhancement. In this paper, aimed at supporting the design of flow tests after stimulations, a three-dimensional (3D) quasi-static discrete element model (DEM) for fractured rock mechanics is coupled with a fluid flow model that explicitly accounts for fluid flow within the fractures, fluid leakage from fractures into rock matrix and the follow-on Darcy flow with matrix. The initiation, propagation and arresting of hydraulic fracture by production well are investigated numerically before the field stimulation experiments. The follow-on openings and closures of the fracture, spatial-temporal evolutions of the fracture aperture/permeability during step flow tests are modeled and predicted before the field experiments, then will be compared with the actual field flow test data when available.