Modeling Discretely Fractured Geothermal Reservoirs: A Focus on Thermal Recovery, Fracture Intersections, and Fracture Roughness

摘要

We present a numerical model to address several key reservoir structures of Engineered or Enhanced Geothermal Systems, including intersecting fractures and fracture roughness. The numerically simulated thermal recovery of a depleted model fractured EGS reservoir was similar to previously published findings. Recovery was initially rapid but slows down progressively and is primarily dominated by conduction in one direction. The mean fracture surface temperature can be easily estimated by knowing the total amount of thermal energy extracted and the thermophysical properties of the confining reservoir rocks. We demonstrated the capability of our model to simulate fracture intersections by showing two types of intersections in a two fracture system, an intersection parallel and perpendicular to the line connecting the injection and production well. Depending on the nature of the intersection different thermal performances occurred with the parallel intersection resulting in better performance due to better sweeping of the fracture surface. By studying a distribution of surface roughness within the aperture of a simulated fracture we observed that significant flow channeling occurred within the fracture and greatly affected the thermal performance of the reservoir. The surface roughness was assumed to have a self-affine geometry. For example, for dipole flow, flow channeling led to a better thermal performance in 25% of the simulated reservoirs by having higher production temperatures than the constant aperture case. In future work, we intend to use simulated tracer results predicted by our numerical model as a method to infer reservoir geometry and subsurface thermal conditions.