| Abstract |
An extensive exploration program consisting of geological, geochemical, and geophysical surveys, core hole drilling, well drilling, and pressure transient testing has been carried out at the Wasabizawa geothermal field, located in northeastern Honshu, in Akita prefecture. Construction of a new 46.199 MWe geothermal power station was commenced in 2015, and commercial operation was initiated in May 2019. This paper presents a newly developed conceptual model based on the available data, which provided technical basis for planning the power station. The field is located in mountainous area with surface elevation varying between +600m ASL and +900m ASL. The subsurface stratigraphy consists of a sequence of volcanic rocks overlying naturally-fractured granitic basement rocks. The geothermal reservoir is hosted by naturally-fractured granitic rocks, and the reservoir top is located between about +200 m ASL and -200 m ASL depending on the horizontal location, with reservoir temperatures of 280 - 290 ï‚°C. Correlation of pressure with equivalent feedpoint elevation shows that almost all wells in the area are in good pressure communication with each other, except two wells (YO-8 and AY-5) located to the west near Akinomiya hot spring area: the feedpoint pressure of these two wells are about 1 MPa lower than those of other wells relative to the same elevation. The feedpoint pressures of the latter wells suggests that the reservoir is isolated from nearby Akinomiya hot spring area by a relatively impermeable zone, and this inference is also supported by results of isotope analysis of hot spring water and brine. Inferred temperature distribution in the area delineates the horizontal reservoir extension. It is likely that the reservoir is also isolated from nearby Kawarage fumarole and Doroyu hot spring located to the east, based on the inferred temperature distribution and chemical composition of these hot springs. Fault systems striking along NW-SE, NE-SW and E-W trends are indicated by geophysical surveys (gravity and MT & CSAMT surveys). Reservoir fluid is neutral NaCl type brine with chloride concentration between 400 mg/L and 2,400 mg/L. The low Cl concentration of water may be due to dilution by steam condensate and/or possible recharge water from the eastern part. Reservoir temperature and pressure profiles as well as the chloride concentration suggest that a two phase region (steam cap) may exist just below the cap rock in eastern part of the field. Based on the conceptual model, and considering the surface topography and location of existing exploratory wells, the location of production well pads and reinjection well pads were planned to be set about 2 km apart from each other, and the production and injection strategy should allow sustainable operation in the future. A numerical reservoir simulation based on the conceptual model also showed that 46 MWe will be sustainable. Total of 11 production and reinjection wells, other than five existing exploration wells which will be converted to production and injection wells, were drilled during construction of the power station, and an extensive heterogeneity of fracture distribution in the granitic basement rock was observed. Large excess total production capacity was obtained relative to required flow rate for 46.199 MWe output. |