Reservoir Behaviour in a Stimulated Hot Dry Rock System

Research into the stimulation of hot dry rock (HDR) systems in crystalline rock has been underway in Cornwall, England for several years. Two deviated wells were drilled to a depth of 2100 m in 1981 with an interwell separation of 300 m. These wells were connected by massive hydraulic injections using water, but the interconnection was insufficient to permit long term circulation without excessive water losses. In 1985 a third well was drilled to a depth of 2600 m in a direction chosen from the analysis of the reservoir behavior during the previous circulation. A massive stimulation (200 l/s, 75 bbl/min) of gel was used to connect the wells and circulation was re-established in August 1985. Reservoir models have been developed from hydraulic analyses, thermal behavior, microseismic mapping, tracer dispersion and chemical modeling. The system behaves like an interconnected network of flow paths with a few dominant routes acting as flow conduits. The storage is associated with pressure dependent joint compliance, but it is isolated from the dominant flow paths. No unique physical model has yet been derived but the various techniques have been used to establish constraints on the geometry and nature of the heat transfer regions. The experiments are still in progress