ASSESSMENT OF HDR RESERVOIR STIMULATION AND PERFORMANCE USING SIMPLE STOCHASTIC-MODELS

Experience in developing and circulating HDR reservoirs at Rosemanowes in Cornwall, UK, and more recently at Soultz-Sous-Forets in Alsace, F rance, suggests that the natural fracture system controls fluid flow a nd that artificial fractures are relatively unimportant. Two simple mo dels, based on standard analytic solutions for the mechanical behaviou r of cracks combined with a simple representation of the frictional pr operties of fracture surfaces, have been developed. The models describ e the stimulation and circulation of HDR reservoirs in naturally fract ured basement. In these models a connected fracture network is assumed , but the details of the spatial relationships between fractures withi n a conceptually ellipsoidal stimulated zone are ignored. On the other hand the models take close account of the size and orientation distri butions of the natural fracture sets when calculating changes in fract ure volume and reservoir impedance. The models provide estimates of th e rock volume to fluid volume ratios required for stimulations, the in jection flow rate for differing injection pressures, the fluid flux/fr acture area partitioning, and hence thermal drawdown behaviour. The tw o models have been applied to field data from the Rosemanowes HDR rese arch site gathered during circulation from 1985 to 1992. The first mod el enables the calculation of adequate stimulation pressures and fluid volumes; the second, reservoir injectivity curves and fluid flux/frac ture surface area distributions within the reservoir.