Analysis of the response of the Raft River monitor wells to the 1979 injection tests

The geothermal resource for the Department of Energy's (DOE) Raft River Geothermal 5 MWe Power Project is located in a closed ground water basin in southcentral Idaho. Chemical analyses indicate the existence of natural communication along fractures between the geothermal reservoir and the shallower aquifers developed for irrigation. Much of the ground water that is presently used for irrigation is of poor quality. Injection of geothermal fluids at intermediate depths may increase communication between the reservoir and the aquifer, resulting in further degradation of shallow ground water quality over time. Seven monitor wells, ranging in depth from 150 m to 400 m, were drilled to evaluate the potential for this degradation. Monitoring of these wells during two 21-day injection tests at the Raft River Geothermal Injection Well-6 (RRGI-6) indicates two types of response in the shallow aquifer system. First, the water level in Monitor Well-4 (MW-4) increased an average of 0.4 m/week during injection, indicating direct fracture connection between the injection zone and the aquifer penetrated by MW-4. Second, water levels in MW-5, MW-6, and MW-7 showed a step function decrease which coincided with the period of the injection tests. Analyses indicate that this response may be caused by elasticmore » deformation in the aquifer matrix.« less