History of Drilling Operations in the Kirishima Geothermal Field

Abstract

The Kirishima geothermal area is located in the southwest part of the Kirishima volcano group in Kagoshima Prefecture, Japan. Exploration of the Kirishima geothermal area by the Nittetsu Mining Co. was started in 1973. After evaluation of the surveys, Nippon Steel Corp. and Nittetsu Mining Co. drilled 21 exploratory wells: 8 slim holes, 9 production- and 4 reinjection. wells to depths between 1,000 and 2,000 m. Several new drilling procedures and techniques were introduced in some exploratory wells, for example, sidetrack operations using a casing section mill, drilling with aerated mud, and drilling ahead without returns using produced geofluids. The Nittetu Kagoshima Geothermal Co. was established in 1990 by the Nippon Steel Corp. and Nittetsu Mining Co. Production drilling was concentrated in the Ohgiri District of the field along the Ginyu fault. Production wells were drilled into the fault zone. This zone was highly permeable and productive, and bit drop and loss of pumping pressure were noticed when the fault zone was intersected. A standard drilling plan was established; surface holes were predrilled and conductor pipes were set before using a large rig. Reinjection wells were directionally drilled to intersect a major fracture forming the western boundary of the fault zone. When sufficient steam production was proven, the Kyushu Electric Power Co. constructed the Ohgiri power plant with a capacity of 30 MW. The Nittetsu Kagoshima Geothermal Co. supplies steam for this plant which was commissioned on 1 March 1996. Drilling activities of exploration-, production-, and replacement wells after electricity production began in 1966 are described in this paper.