A Tracer-Based Model for Heat Transfer in a Hot Dry Rock Reservoir

Abstract

A model for heat transfer in a Hot Dry Rock (HDR) geothermal reservoir is developed which predicts produced fluid thermal performance based on tracer determined residence time distribution and an estimated value of fracture porosity. The tracer response is modeled as flow through several paths of highly fractured rock. Fracture porosity is used to convert fluid volumes to rock volumes, which is necessary to model the heat sweep. The produced fluid response is the mixing cup average of the individual path temperatures. The model adequately represents the measured thermal repose of an HDR reservoir operated at the Fenton Hill NM HDR geothermal site in the late 1970s. Application to the current Fenton Hill reservoir predicts a rapid initial thermal drawdown of about 50°C, followed by a very slow thermal decline thereafter. The model is most sensitive to fracture porosity an less to the flow path geometry to match the tracer response.