Modelling Very Deep Production and Reinjection Using a Regional Scale Model of a TVZ-Like Geothermal Field

Abstract

The rock-mechanics code SULEC and the fluid flow code TOUGH2 have been combined to produce a thermal/fluid flow model of a two-dimensional west to east section across the TVZ which connects processes which occur at ~100 km depths to those which produce plumes of high temperature fluids within a few kilometers of the surface. The model predicts that the brittle-ductile transition occurs at ~7-9 km depth, and that permeabilities in the brittle crust above this of ~1 mD) are sufficient to support convective plumes which transport heat and fluid to the surface. We provisionally interpret these plumes as TVZ-like geothermal fields because of their similarity to them in terms of their total heat and mass flows, their temperatures and their narrow width (~ a few km). By construction, the model represents the complete hydrological system associated with the convective plumes and can be used to address questions concerning the long term sustainability of these as renewable energy sources. In particular, we model very deep (~5 km) fluid extraction (with no reinjection) for a period of 150 years, and examine the effects of this on the geothermal field itself, on its surface heat and mass flows, and on neighbouring fields.