In-situ hydraulic parameters for the Carnmenellis granite hot dry rock geothermal energy research reservoir

A research hot dry rock (HDR) geothermal energy reservoir is currently under development in the Carnmenellis granite in Cornwall (U.K.) at a depth of about 2000 m. This paper details the conduct, analysis and interpretation of hydraulic tests using low to medium pressures and flow rates. These tests allowed an evaluation of the in-situ hydraulic parameters relevant to initial water storage and long term leakage. Tests were analysed using both diffusion theory and an explicit coupled hydraulic/mechanical computer code. At low fluid overpressures the permeabilities, in terms of uniform diffusion, were in the range 1-10 microdarcy (..mu..d), rising to about 60 ..mu..d at overpressures of about 5 MPa. Long term leakage from an HDR reservoir implied by these figures is minor. The computer model was able to simulate several cycles of injections and shut-ins at progressively higher pressures and flow rates with a single set of variables.