Geothermal Relief Well at the Onikobe Geothermal Power Station

Abstract

The Onikobe single-flash steam turbine geothermal power plant in Japan has been supplying electricity to the grid for 36 years. Power station output has increased over the years and reached 15 MWe (gross) in February 2010. Numerous natural geothermal manifestations were present in the field prior to development. New fumaroles accompanied by hot liquid discharges spontaneously appeared near Well 128 on 8 September 2010. Such spontaneous features had frequently appeared in the past. The new features were monitored carefully ? the manifestations intensified abruptly on 8 October, and engulfed Well 128 the same day. The fumaroles continued to grow until a large scale steam explosion occurred on 17 October 2010. A crater-lake formed and the Well 128 wellhead became submerged in hot water. After the steam explosion, steam and water continued flowing from the crater-lake. Fluid sampling was performed using a radio-controlled helicopter, and results of the chemical analysis of the fluid showed that the fluid in the crater was identical to production well 128 fluid. This suggests that Well 128 was damaged by the steam explosion incident, and that the residual flow from the crater-lake afterwards could be due to a casing failure in Well 128. This paper describes the sequence of crater-lake events and relief well drilling which finally succeeded in killing and plugging Well 128.