Geochemical Evaluation of the Hijiori HDR Reservoir at Yamagata, Japan

Abstract

A deep HDR reservoir granitic basement rock at the Hijiori test site, Japan has been evaluated by using results of geochemical monitoring of circulation fluids and tracer experiments in the 1995 and 1996 circulation tests. Na-CI experiments in the d-O-CI plots of the circulation fluids and river water clearly indicate the dilution process of the indigenous fluid by river water injected as the makeup water. Although dilution is a major process controlling the circulating fluid geochemistry, water-rock interaction processes, such as anhydrite dissolution, also occur in the reservoir. Changes in the chlorine and sulfate concentrations during the two-well production stage of HDR-2a and HDR-3 indicate that the connection from the infection well HDR-1 to HDR 2a is better than to HDR-3. Tracer experiments also suggest a difference in the fracture fluid between HDR-1 and the two production wells, HDR-2a and HDR-3.