Reservoir Engineering Studies at the Matsukawa Geothermal Field

Abstract

Matsukawa was the first geothermal power plant established in Japan. It started power production in October 1966, and is the only vapor-dominated geothermal field developed to date. The power plant has been continuously producing full power, 22MWe, for almost 27 years. Matsukawa is located about 600km northeast of Tokyo and about 27km northwest of Morioka, Japan. It is in the Hachimant*i volcanic region, one of the most active volcanic regions in Japan. This paper reviews reservoir engineering studies at Matsukawa. This includes a study of the current state of the reservoir studied mainly by pressure buildup tests, a study of the initial state of the reservoir studied mainly by reconstruction of a reservoir pressure profile, and a numerical modeling study of the natural state of the reservoir. Continued pressure buildup tests since 1986 have revealed that there is a lateral steam flow from southwest to northeast in the Matsukawa vapor-dominated reservoir, and most of the steam is suplied from southwest of the development area. This result suggests that the vapor-dominated reservoir extends further southwest than the area now being exploited. These conclusions are supported by production records and chemical data of produced steam. The study on the natural state of the Matsukawa geothermal reservoir has revealed that there was a thin vapor-dominated zone at the shallow part of the reservoir (around 300m to 400m depth) and the current production zone (800m to 1300m depth) was filled with liquid before exploitation. Early production wells produced wet steam with some hot water at first, but they turned to produce only dry steam after a production period of 6 months to 1 year, because of the existence of low permeability aureole around the reservoir and high heat flow. Estimated conductive heat flux, 1.5W/m₂ is as high as that of The Geysers. The natural state modeling study showed that the model of the initial state of the reservoir, described above, was feasible. The results also indicated that the low permeability aureole was very important for the evolution of its natural state and also for production of superheated steam from the liquid zone below the thin vapor-dominated zone in the shallow part of the reservoir. Initial temperature distribution and the results of the simulation study suggest that there is an extensive heat source in the southwestern part of the reservoir.