Joint influence of in-situ stress and fracture network geometry on heat transfer in fractured geothermal reservoirs

Abstract

We quantitatively investigate the superimposed influence of geometrical properties and geomechanical deformations of two-dimensional fracture networks on heat transfer in fractured geothermal reservoirs. The reservoir model consists of rock matrix with uniform properties and natural fractures following power law length scaling. We model its thermo-hydro-mechanical behavior based on sequentially linked solvers that resolve the solid deformation, fluid flow and heat transport processes. The fracture aperture is spatially variable and dependent on the stress loading conditions, based on which stress-dependent fluid flow is further simulated. Our numerical results show a good correlation between the heat extraction efficiency and the fracture network connectivity. The connectivity seems to be a prerequisite for geomechanical factors to impact heat transfer. Depending on the hydraulic boundary condition, the stress loading may lead to different geothermal responses: under a high hydraulic gradient, an increased stress ratio results in a decrease in the heat extraction efficiency; in contrast, under a low hydraulic gradient, an increased stress ratio corresponds to an enhanced heat production. The observed effects of in-situ stress and fracture network geometry on heat transport are quantitatively characterized by a dimensionless analysis using the fracture-matrix Péclet number, which is defined as the ratio of convection timescale in the fracture to conduction timescale in the matrix. Our research findings have important implications for understanding the heat exchange performance of fractured rocks in geothermal reservoirs naturally subjected to complex in-situ stress conditions.