Influence of permeability anisotropy on heat transfer and permeability evolution in geothermal reservoir

Extracting heat energy from geothermal reservoirs essentially relies on circulating cold fluid within fractured hot rocks. The intrinsic anisotropic permeability in the reservoir directly affects the path of flow and the associated thermal drawdown from cooling procedure. Consequently, each individual component including thermal, hydraulic, and mechanical field needs to be considered, to evaluate the influence of permeability anisotropy on the thermal production and evolution of rock properties. In this work, the fully implicit coupled simulator TFReact was successfully implemented to generate results prototypical of an enhanced geothermal system. Anisotropic permeability values were generated from the variation of fracture spacing at three orthogonal principal directions, with identical initial fracture aperture. Five case scenarios of permeability anisotropy were simulated to evaluate the influence of anisotropic thermal drawdown in triggering permeability evolution. Analysis of the propagation of the thermal front from injector to producer indicated that low anisotropic permeability values will lead to late cold water breakthrough at producer, slower migration rate and wider sweep area than high anisotropic permeability values. Isotropic permeability scenario showed a lower thermal gradient profile, comparing against the scenarios of anisotropic permeability. The anisotropic value of 0.01 produced the highest power output, while isotropic permeability generated the least power output. Induced thermal stress resulted an unloading response by reducing compressive normal stress in sub-horizontal direction, and thereby increase fracture aperture in sub-horizontal direction. Invariably, the induced thermal expansion stress increased the compressive stress and reduced fracture aperture and permeability. After the same timing of injection-production cycle, the highest anisotropic permeability scenario resulted a factor of 2.5 increment in evolving fracture permeability, while lowest anisotropic permeability scenario lead to a factor of 0.35 decrease in changing fracture permeability. The generated thermal output suggested the most favorable strategy in maximizing thermal sweep across the reservoir, by prompting thermal transfer normal to direction of injector-producer.