RESERVOIR CONDITIONS AT 3-6 KM DEPTH IN THE HELLISHEIDI GEOTHERMAL FIELD, SW-ICELAND, ESTIMATED BY DEEP DRILLING, COLD WATER INJECTION AND SEISMIC MONITORING

摘要

Reykjavik Energy recently drilled a 2808 m deep exploration well HE-8 in the Hellisheidi high-temperature field. The well is located at the western boundary of a NNE trending fissure swarm. Two main feedzones were encountered at 1350 og 2200 m depth. The bottomhole temperature appears not to exceed 300℃, suggesting considerable fluid convection at this depth. When stimulating the well by cold water injection at the end of drilling and again after 3 months of warm-up, the well's injectivity increased from 1-2 up to 6-7 kg/s/bar. An intrinsic reservoir permeability of 3-6 milli-Darcys was estimated by modeling the transient pressure data collected. A total of 22 small quakes accompanied the cold water injection into well HE-8, mostly at 4-6 km depth. Fluid pressure changes inside the reservoir fracture network, during injection, are strongly suspected as a trigger for these quakes. It also implies that there is a pressure communication and good permeability between the feedzones at 1350 and 2000 m depth and the 4-6 km depth of the quake centers. Large normal faults, dipping to the east, are suspected as likely surfaces of quake generation. These downhole and seismic data suggest considerably deeper fluid convection cells than previously assumed and may result in an increased generating potential estimate for the Hellisheidi resource. The field therefore appears feasible as a target for drilling a very deep exploration well. The study shows that the regional stress field and the permeability field are related. Seismic monitoring during injection may become a valuable tool for locating permeable fault surfaces at great depths.