| Abstract |
The Sumikawa geothermal power station is located in the Hachimantai volcanic region of northeastern Japan, and started commercial operation in March 1995 with an installed capacity of 50 MW. As the result of geochemical monitoring since the beginning of operation, characteristic changes in chemical concentrations (Cl and SiO2) and oxygen and hydrogen isotopic compositions were observed in production fluids. In particular, some wells showed seasonal cyclic changes in the Cl concentration and the isotopic composition, and these two changes correlated inversely. These changes suggest strongly the admixing of reinjected waste water, i.e. hot water separated from steam, and steam condensate, into the production sections. Especially, the observed seasonal changes imply the mixing of steam condensate, because the reinjection rate of steam condensate seasonally varies according to cooling efficiency depending on due to atmospheric temperature. In order to simulate such changes and to estimate physical parameters in the reservoir, a lumped parameter model taking account of steam condensate reinjection was developed. This model is based on two mass balance equations with respect to the Cl concentration and the hydrogen isotopic composition, and also on the following manner of fluid mixing. The each production wells have individual fluid reservoirs. Two separated hot water separated in different separator systems and steam condensate are separately reinjected into three sections. These three reinjected fluids flow into each production section at returning ratios. The optimum values of unknown parameters, which are fluid mass in reservoir (W), and returning ratios of steam condensate (f1), and two kinds of hot water (f2 and f3) are obtained from matching calculated values of Cl and D with observed values. The result of well SB-1, for example, are as follow. W=7.7?107kg, f1=0.13, f2=0.15, f3=0.01 The seasonal cyclic changes were also well simulated with this model. |