2001 Volume 23 Issue 2 Pages 107-125
The Sumikawa geothermal field is located in the Hachimantai volcanic region of northeastern Japan. The commercial operation of the power plant started in March 1995 with an installed capacity of 50 MWe. There, separated hot water and steam condensate, both are reinjected into the geothermal reservoir. In this paper, the influence of the reservoir on physicochemical characteristics due to reinjection fluids was studied. Changes in the chemical and hydrogen isotopic compositions with time were observed on production fluids during the exploitation. Some wells showed seasonal cyclic changes in Cl concentration and δD, which were inversely correlate It was inferred that such changes were due to return of steam condensate reinjected to production area. This inference was verified from tracer tests. In order to simulate the physicochemical changes in the reservoir caused by reinjection of steam condensate and separated water, a lumped parameter model was developed based on the conservations of mass and energy. As regarding three wells, SB-1, S-4 and SC-1, the model calculation was able to adequately simulate the seasonal cyclic changes in Cl concentration and δD in a production fluid, and also to determine returning ratios of steam condensate and two kinds of separated water reinjected, fluid mass and porosity in a production tank, that means a definite volume around the production well where the reinjected fluids are well mixed up with the original geothermal fluid coming up. The returning ratio of steam condensate is relatively high in the wells in which the seasonal changes were clearly observed. We also found that the returning ratios were different in individual reinjection section. Decreasing rate of the reservoir temperature around a well is dependent on volume of the production tank and the degree of the returning ratios in individual well. Decreasing rate is relatively rapid in the well that has small volume and high return ratio of steam condensate.
