| Abstract |
The Sumikawa geothermal power plant is located at the Hachimantai volcanic region in Akita Prefecture, Northeastern Japan. The 50 MWe unit has been operating since March 1995. From the environmental viewpoint, thermal water and steam condensate are reinjected into each reinjection well. In these five years, many reinjection wells have encountered some problems. For example, the wellhead pressure of SD-1 was kept constant condition, while the amount of the thermal water decreased. To improve such state, we reinjected to the mixture of thermal water and steam condensate in order to recover reinjection capacity. Then the reinjection capacity of thermal water was recovered. Decrease of steam production is also observed in some production wells, which is strongly observed in the production wells S-4 and SC-1.The temperature decrease of production zone is presumed that the reinjected water may return into the production zone. Because of decrease of silica geothermometer and/or increasing Cl- concentration are observed in their brine. Tracer tests were conducted in order to reveal the influence of reinjected water on production zone in 1991 and 1999. In the case of 1999, the tracers, KI and KBr were introduced into the well SE-2 used for reinjection of steam condensate and the well SD-4 is for the thermal water. Iodine was detected most of the production wells which occur thermal water, except SC-2, and KBr was detected all of such kind of wells. Bromine was concentrated in the wells S-4 and SC-1 strongly. For example, the well S-4 is most affected by reinjection waters, 23% of discharged fluids (steam and thermal water) are reinjected water. To avoid reservoir temperature decrease, the most effective way to keep reinjection capacity higher is to drill new additional reinjection well far from production zone. Moreover, suitable utilization of reinjection wells is also needed. |