Restoration from a Large Scale Steam Explosion at the Well Site of the Onikobe Geothermal Power Station

Abstract

The Onikobe single-flash steam turbine geothermal power plant, nominal output 15MWe (gross), has been supplying electricity to the grid since 1975. Numerous natural geothermal surface manifestations were present in the field prior to development. New fumaroles accompanied by hot liquid discharges spontaneously appeared at the well site on 8 September 2010. The fumaroles continued to grow until a large scale steam explosion occurred on 17 October 2010. A large steam cloud with entrained solid explosion products rose high into the air. A crater formed and the Well 128 wellhead became submerged in the crater lake. Two other production wells were engulfed in the crater. 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. Steam and hot water had been discharging until a relief well encountered Well 128. Monitoring of surface and subsurface condition with vibrometers, seismographs and tilt meters was started after steam explosion. This paper describes the sequence of events, the probable causes, monitoring activity and relief wells drilling which finally succeeded in ceasing discharge.