Evaluating the Effect of Anisotropy on Hydraulic Stimulation in a Slate Geothermal Reservoir

Most geothermal reservoirs in Taiwan are distributed in metamorphic rock masses, especially slate. The presence of slate foliation causes rock masses to exhibit high anisotropy and heterogeneity. This study conducted an anisotropic coupled hydro-mechanical analysis to evaluate the influence area of hydraulic stimulation in slate for the operation of a geothermal power plant. A failure criterion for slate incorporated with a transversely isotropic poroelastic model were established to describe the behavior of slate, and a fracture aperture model was used to simulate the stress-dependent permeability of open fractures. The developed model was validated using a benchmark problem of geothermal techniques under an isotropic condition. The effects of anisotropy on the mechanical behavior of the fracture and rock matrix were also investigated. These results indicate that the analysis results indicated that anisotropy significantly affects stress and strain distributions. When the anisotropic angle is 135°, a substantial increase in pressure, deviatoric stress, and volumetric strain occur at the center of fracture walls. However, larger damage zones are generated in the case of the anisotropic angle is 0° and 90°. The proposed model was further applied to analyze the injection response in a slate geothermal site. The analysis results revealed that the proposed model can reasonably simulate the anisotropic coupled hydro-mechanical behavior of slate geothermal reservoir.