The Drilling Experience of K6-2, the High-Temperature and Crooked Geothermal Well in Kakkonda, Japan

Well K6-2 was drilled for geothermal production for the Kakkonda No. 2 Power Plant (to be built in 1995) at the Kakkonda geothermal field, northern Honshu Island, Japan from 1988 through 1989. The well was planned to be vertical and the target area was a 100-m radius of 2800 m. Mainly, because of the formation inclination, strong bit walk tendency was encountered below 1200 m. Even with packed-hole bottom-hole assemblies (BHA), the well inclination buildup rate was over 1 deg per 30 m. With this buildup rate, the well inclination would be over 50 deg at 2800 m, and not only miss the target area, but could not reach total depth because of severe rotation drag in the very abrasive formation (tertiary: shale, dacitic tuff and andesitic tuff-breccia; pre-tertiary: slate, sandstone and andesitic tuff). Because a pendulum BHA did not help to drop the inclination, downhole motors with bent subs were employed. Totals of six and seven downhole motors for 12 1/4 and 8 1/2-in. hole sections, respectively, were run. The estimated formation temperature was over 350°C below 1900 m, so two mud cooling towers and 500 m³ pit were used to cool the returned mud. These systems worked well, but at 2245 m the estimated mud circulation temperature on bottom went up to 150°C and the stator rubber of the downhole motors unbonded and broke up after a 1-h run. Below that depth, only a packed hole BHA was employed, and the inclination increased from 6 deg at 2300 m to 14 deg at 2800 m. At 2799 m, lost circulation was encountered and drilling terminated at 2818 m.