Three Dimensional Heat and Mass Balance Modelling of Convection in Structurally Controlled Conduits in Basin and Range Geothermal Systems

Abstract

The Basin and Range province is host to numerous geothermal systems with observed temperatures > 200 ? 280°C at 2-3 km depth. Most of these were blind discoveries with no known magmatic heat source. A recent review of the structural setting of these systems indicates that a large number of these systems occur in regions characterized by fault intersections and stepovers in fault strands associated with increased fracture density and permeability. Using TOUGH2-EOS1sc, we performed heat and mass balance modelling on a 3 dimensional (40 x 26 x 12 km) polygonal model of an idealized Basin and Range system. We modelled the evolution of geothermal circulation in a permeable, steeply plunging conduit extending to a depth of 8 km, representative of a fault stepover/intersection setting. The effect of various combinations of host rock and fault zone permeabilities was evaluated with respect to temperature, volume of upflow, presence/ absence of downflow, temporal evolution, and the 3D extent of geothermal anomaly. Results were compared with observed fluid temperatures and estimated pre-production fluid flow at Dixie Valley. Several model configurations matched transient fluid temperatures and flow rates that are observed at the Dixie Valley geothermal field. Timing of the peak and duration of thermal fluid flow is strongly dependent on the ?conductance? (i.e. cross sectional area * permeability) of the conduit, and the presence or absence of convection and downflow of fluids in the conduit. Time of peak discharge can vary from a few hundred to several hundred thousand years. In all but the longest-lived systems the footprint of subsurface heat extraction is limited to a few km away from the conduit. This suggests that multiple geothermal systems can be spaced 10 - 20 km apart along major structures without significant interference.