Impact of fracture shear dilation on long-term heat extraction in Enhanced Geothermal Systems: Insights from a fully-coupled thermo-hydro-mechanical simulation

Abstract

Shear dilation of fractures has been recognized as a main mechanism of permeability enhancement by hydraulic stimulation in Enhanced Geothermal Systems (EGSs); however, the interactive role of fracture shear dilation and thermo-hydro-mechanical (THM) coupling processes in long-term heat extraction performance of EGSs remains unclear. In this study, we develop a novel THM coupling model based on the discrete fracture network approach, which can realistically capture important processes including hybrid normal-shear deformation of fractures, thermal expansion of rocks, fluid flow in both fractures and rocks, and heat convection/conduction as well as local thermal non-equilibrium effect and changes in physical parameters of fluid. We quantitatively investigate the effects of fracture network geometries and geomechanical boundary constraints on fracture shear dilatancy, and the resulting heat transfer characteristics of EGSs. Numerical results reveal that shear dilation of fractures can be triggered by transient pore pressurization and thermal stress under anisotropic in-situ stress condition, and would severely engender flow channeling as well as anisotropic heat transfer, which strongly impact the heat extraction performance. The production temperature tends to be overestimated while the thermal production rate may be underestimated, if the shear dilatational behavior is not incorporated. Increased in-situ stress ratio and injection/production pressure would magnify the effects of shear dilation, and lead to considerable enhancement of fracture permeability, eventually resulting in much earlier and quicker temperature drop. Excessive increase of fracture density and the location of injection-production wells parallel to potential channelized flow paths, formed by intersected fractures preferentially oriented for shear sliding, tend to form short circulating flow paths and reduce the heat extraction performance. Our study demonstrates the importance of considering fracture shear dilation and fully-coupled THM behaviors when evaluating the long-term performance and efficiency of heat extraction in EGSs.