Geothermal exploration and development of the Ogiri field in the Kirishima geothermal area

Abstract

Geothermal exploration in the Kirishima including geological and geophygical surveys, was conducted by Nittetsu Mining Company Ltd.(NMC) from 1973 and by a joint survey between Nippon Steel Corporation (NSC) and NMC starting in 1979. In 1989, a new geothermal development project was agreed formally upon Kyusyu Electric Power Co., Inc. (KEPCo), NSC, and NMC. Nittetsu Kagoshima Geothermal Company Ltd.(NKG), established in 1990 to produce the geo-thermal steam, carried out discharge tests in ten production wells in 1991. and confirmed the existence of a geothermal reservoir which is enough for generating a 30MW geothermal power plant. In November 1994, NKG started the construc-tion of the Ogiri Power Plant, which was scheduled for completion by March 1996. A total of fifty exploratory wells have been drilled with total composite length of 56, 848m; Of these, 35 wells (42, 692m in total) were drilled by the private companies, and 15 wells (14, 156m in total) were drilled by governmental organization.
The Ginyu Fault Reservoir was delineated by the exploratory drillings: It is a simple fracture-type reservoir which is 800 m in length and -77m to -483m in depth and trends with a strike of N71.4°-74.9°E, dipping of 63.2°-72.3°N. Reservoir temperature measured by well logging and calculated by silica geothermometry is 232°C and 240-245°C, respectively. The value of Kh is variable from 20 to 352dm with an average of 66 dm from ten wells. Discharge rate of steam and hot water per well, calculated to be at 5kg/cm2A of wellhead pressure, is 35.1 to 40.4t/h and 178 to 211t/h, respectively. The hot water is Na-K-Cl-SO4 type with pH from 8.6 to 8.8. Concentration of non-condensible gas in the steam is considerably low below 0.01 vol%.
Simulation experiments indicate that the Ginyu fault reservoir has a potential to generate steam with a discharge rate of 350t/h over the next 30 years. Single-flash system and the separated steam flow transmission system have been adopted in the plant design. In order to avoid the deposition of silica scale, a method which injects hot water into the underground by using line pressure and the difference of water head was adopted. The operation of the power plant is faciliated by the central control system unit.