The effects of thermal deformation on flow in a jointed geothermal reservoir

Abstract

In a Hot Dry Rock (HDR) geothermal reservoir, heat is removed primarily by fluid flowing through joints in relatively impermeable rock. Cooling causes the rock to thermally contract and deform, changing the joint opening; fluid viscosity can change by an order of magnitude over typical temperature ranges. In this paper we examine these effects on flow in both a single joint and in a more complex reservoir. We show that thermal deformation can either reduce flow due to local pinching of a joint or can increase flow due to global thermal opening of a joint. Which effect will dominate is dependent on the peripheral boundary conditions. In a reservoir, it is likely that both behaviors occur. Data from the HDR site at Fenton Hill indicates a generally decreasing flow (pinching). However, there have been periodic step increases in flow that more than compensate for the reduced flow (global joint opening).