| Abstract |
The Ohnuma geothermal field is located in the northern part of the Hachimantai volcanic region, Northeast Japan. The Ohnuma geothermal power station has been operated by Mitsubishi Material Corporation (MMC) since 1974. Since then, only two make up production wells were drilled in 1976 and 1983, while several reinjection wells were drilled between 1980 and 1992. The rated plant generating capacity has been increased progressively from 6 MWe in 1974 ti 9.5 MWe in 1986. Temporal changes in production flow rate production enthalpy and chloride concentrations are observed in response to changes in the operating conditions of the production and reinjection wells. Decline in production enthalpies and increase in chloride concentrations were observed but with less effect on both parameters when injection is done further away from the production sector. Quartz temperature, however, remains comparatively stable. It is estimated that production fluids being returned from reinjection wells has not been fully re-equilibrated with respect to quartz. Recently, stable production flow rates and declining enthalpies were observed. This results in the decline of the steam flowrates and the reduction in the generating capacity of the plant to 7.4 MWe. Over 25 years, the observed low decline in the steam flow has been controlled because of proper reservoir management. Drilling of make-up wells were limited since meeting the rated plant capacity is not being forces. The temperature of the Ohnuma geothermal reservoir is moderate and the reservoir fluids are low salinity. These reservoir characteristics cause to prolong the longevity of the Ohnuma production wells. |