Reservoir Characterization by Geochemical Method at the Ogachi HDR Site, Japan

Abstract

To create the artificial reservoirs for the Hot Dry Rock (HDR) geothermal energy, two hydraulic fracturing tests were performed at the depth of 1000 m and 7f 0 mat Ogachi, Akita prefecture, Japan, in 1991 and 1992. After these tests, a 22-day circulation test was carried out through both resewoirs in the late of 1993. During these tests, geochemical studies were conducted to evaluate the chemical behavior of the geothermal fluid at the reservoirs and then to estimate the reservoirsí character The flow volume dependent concentration change of the major dissolved components the flowback water and production water is caused by the mixing between the injected water and the existing formation water, and the interaction between the injected water and the host rock. According to the results of geochemical study and other prospecting methods during the hydraulic fracturing tests, the (lower) was controlled by the natural fracture system, whereas the 2nd (upper} was controlled by the ground stress. In the early time of a 22-day circulation test, the total volume of both reservoirs which connect an injection well to a production well is estimated to be about