Stress-induced fracture deformation is the principal cause for permeability change in geothermal systems. This study focuses on the influence of the nonlinear deformation and dilation effect of fractures on the geothermal system under the action of in-situ stress. By adopting a nonlinear constitutive model of rock fractures and embedding discrete fracture networks, numerical studies are first conducted to investigate the effects of different in-situ stress schemes on fracture aperture evolution using a rigid-body spring method. Based on the anisotropic aperture field of the fracture network caused by the in-situ stress, a finite element method is then used to study the flow and heat transfer process. The effects of different stress schemes on the heat flow transfer process are analyzed. Numerical simulation results show that when the ratio of horizontal to vertical stresses is not sufficient to cause shear dilation effects, the nonlinear normal deformation is the main factor affecting flow and heat transfer. In this case, the heat extraction efficiency is reduced. As the stress ratio increases, the shear dilation gradually becomes the dominant mechanism, and the heat extraction performance is improved. The obtained results provide a practical guide for geothermal site siting and optimizing heat extraction efficiency in geothermal reservoirs.

Understanding the influence of shear displacement on heat transport in rock fractures is important for evaluating and optimizing heat extraction in enhanced geothermal systems. This study presents quantitative characterization of the heat transfer evolution in single fractures subject to shear displacement, aiming to demonstrate the impact of shear displacement on heat transport in natural rock fractures. The direct shear of rock fractures is directly simulated using the finite element method and the Mohr-Coulomb yield criterion. The shear simulation method is validated against laboratory shear test data from the literature. Shear simulations under different mechanical conditions, including different normal stresses and shear displacements, are conducted. The sheared fractures are then used to simulate fluid flow and heat transfer processes by directly solving the Navier-Stokes equations and the heat transport equation. The results show that shear displacements can cause significant changes in fracture aperture and subsequently enhance the heterogeneity of flow fields and temperature fields in the fracture. The heat transfer coefficient increases with the increasing of normal stress and Peclet number, while it decreases with the increase of shear displacement. The plastic deformation of fracture surfaces can significantly affect the heat transfer rate. The findings can help understand the heat transfer characteristics in natural rock fractures.