Numerical investigation of the impact of fracture aperture anisotropy on EGS thermal performance

•Natural fractures exhibit geometric anisotropy.•Fracture surfaces in a perpendicular flow configuration have a higher tendency for favorable thermal performance than fracture surfaces in a parallel flow configuration.•The flow-wetted surface area and volumetric flowrate contribute to the amount of heat extracted from an enhanced geothermal system.•High geometric anisotropy results in a high difference in thermal performance between the perpendicular and parallel flow configurations. The impact of anisotropy within spatially-varying fracture apertures on heat transfer in the context of Enhanced Geothermal Systems was investigated. The anisotropic fracture aperture distributions studied included samples from laboratory-scale fractures and additional 100 artificial fracture aperture distributions generated using Sequential Gaussian Simulation. Two flow configurations were investigated. The perpendicular flow configuration had a flow perpendicular to the lateral shear offset direction, while the parallel flow configuration had a flow parallel to the lateral shear offset direction. The thermal performance was simulated numerically using a thermohydraulic model with a 50 mm by 50 mm sheared fracture, typical granitic rock properties, and temperature dependence of fluid density and viscosity. The results showed that 70% of the fracture aperture distributions studied gave rise to favorable thermal performance in the perpendicular flow configuration. It was also observed from this study that the flow-wetted surface area had a direct and significant contribution to the amount of heat extracted. In addition, larger volumetric flow through preferential paths also contributed to more heat extraction and could offset surfaces without large flow-wetted surface areas. The results of this study suggest that placing an injector well in the direction perpendicular to shear or slip may result in favorable thermal performance over a parallel flow configuration. Geometric anisotropy was deduced from the joint roughness coefficient (JRC) and variogram modeling parameters of the datasets studied. High geometric anisotropy results in high differences in thermal drawdown between the flow configurations. Combining the difference in geometric anisotropy and difference in JRCs between the flow configuration could aid in identifying if the thermal performance will be favorable in a given flow configuration.