Numerical Simulations of Supercritical Geothermal Resource Utilization

Abstract

Summary Geothermal wells drilled into a supercritical geothermal resource, where temperature exceeds the critical temperature of water (∼374 ºC) could generate an order of magnitude higher power output than the majority of conventional geothermal wells. However, the best strategy for supercritical resource utilization - direct production of single phase fluid, or injection of cold water to expand the potential resource, or a combination of both - has remained unclear. Due to the strong, non-linear temperature and pressure dependence of fluid properties at supercritical conditions as well as the strongly varying rock properties near a magmatic heat source, numerical simulation of such resource utilization has remained challenging as these characteristics preclude common simplifications typically used in geothermal reservoir modelling. In this work we address these challenges and present reconnaissance 3D numerical simulations of a geothermal system evolution from the time of magma emplacement and through a subsequent formation of a supercritical geothermal resource. The simulations of two-phase liquid-vapor flow with boiling are performed using the 3D extension of the Control Volume Finite Element Method (CVFEM) [1] within the CSMP++ software library [2] together with an accurate equation of state for H2O-NaCl valid in a broad temperature (0 to 1000 ºC) and pressure (1 to 5000 bar) range [3]. The CVFEM scheme is locally mass-conservative and is able to capture strong gradients in fluid properties arising at the contact between a magmatic intrusion and the host rock. The magmatic heat source is explicitly represented as a region of an unstructured mesh and its permeability varies with temperature mimicking the brittle-ductile transition of a basaltic rock. The potential supercritical resource is targeted by a well or a group of wells, direct production from and cold water injection into the geothermal reservoir is modelled. To do so we have implemented a Peaceman-like well model for the 3D CVFEM. We benchmarked our numerical well model against an analytical solution and tested it for mesh size convergence at supercritical conditions. Our reconnaissance simulations provide the first glimpse into the response of a supercritical resource to operation with wells. P. Weis et al., Geofluids 2014, 14, 3. S.K. Matthäi et al., Geological Society Special Publication 2007, 292. T. Driesner, C.A. Heinrich, Geochimica et Cosmochimica Acta 2007, 71.