Numerical simulation results for problems 1, 2, 4, 6, and 7 of the geothermal code comparison study
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This report summarizes work done for a geothermal code comparison study (GTO-CCS) organized by the Geothermal Technologies Office. The project evaluated a wide range of numerical simulators by performing a series of benchmark problems. Numerical simulators for Enhanced Geothermal Systems (EGS) enable solution of problems related to coupled thermal (T), hydrologic (H), geomechanical (M), and geochemical (C) mechanisms. Our simulations in this study were performed using a discrete fracture network (DFN) simulator, Complex Fracturing ReseArch Code (CFRAC). Results of five comparison problems are presented in this report. Problems 1, 6, and 7 are solved as H-M cases. Problem 1 is an effective continuum problem that uses a correlation between fluid pressure and permeability. Results are compared on the basis of time series and spatial distribution plots of pressure, permeability, and fluid density. Problem 6 investigates changes in aperture and shear displacement along a fracture during injection. Also, normal and shear stress distributions are compared. Problem 7 examines the surface vertical displacement caused by injecting water into an extremely shallow fracture. In addition, the mode-I stress intensity factors along the fracture edges are calculated. Problems 2 and 4 are T-H-M cases. Some groups included thermoelastic stresses, but we could not because of the limitations of CFRAC. Problem 2 investigates fluid pressure and thermal transport in either a DFN or a continuum approximation to a fractured medium. Problem 4, inspired by the first EGS site in the United States at Fenton Hill, New Mexico, involves a single fracture intercepted by an injection well and a production well. The simulated fluid injection pressure and production temperature are compared with measured field data. This project shows that numerical simulation is a pragmatic tool to analyze the complexity of the fluid injection and deformation processes. Physical characteristics including fluid pressure, permeability, fracture deformation, flow rate, and temperature distribution can be calculated and provide a clearer picture of a geothermal reservoir.