The performance of a Hot Dry Rock system is a function of the flow- geometry within the volume of rock that acts as a heat exchanger (the reservoir). This geometry will be controlled by the natural fracture system and the in situ stress field. A three-dimensional fracture network model was used to investigate the nature of such a reservoir and to make quantitative estimates of the physical parameters of the natural fracture system.
This paper describes the work carried out as part of a study of the application of Discrete Fracture Network Models to Hot Dry Rock geothermal systems (Lanyon et al, 1991). The aims of the study were to investigate the feasibility of such an approach and to create a model of a particular reservoir which integrates results from hydraulic, petrophysical, thermal and geochemical analyses.
Hot Dry Rock Technology
Hot Dry Rock (HDR) is a geothermal technology currently under investigation in several countries (Batchelor, 1989). Exploitation of geothermal energy by HDR technology is a "heat mining" process. The objective is to move heat from the rock to the surface using fluid circulation. The design of such systems is controlled by the heat extraction requirements. Major programmes in the USA and UK have developed zones of increased permeability at depths of 2-3 km in fractured crystalline rock. Water has been circulated between deep boreholes and heat extracted from the circulation fluid. Reservoir performance, however, has been disappointing and the economic viability of the technology has not yet been demonstrated. This paper describes a study related to the UK Geothermal Energy Project at Rosemanowes in Cornwall. (Parker, 1989) The site is located on the Cammenellis granite batholith. The boreholes RH11 and RH12 were drilled as part of the first reservoir creation experiments. The open hole section of RH11 was approximately 250m above that of RH12.. The open hole sections were inclined at 30 ø to the vertical at an azimuth of 300 ø. It was not possible to create a good connection between these two wells and the second phase of experiments included the drilling of the deepest borehole, RH15, which was drilled to pass approximately 250m below RH12 with an open hole section again inclined at approximately 30 ø to the vertical and with an azimuth of 10 ø. Massive hydraulic fracturing was used to stimulate the natural fracture system and to create a connection between RH12 and RH15.
