The status of deep geothermal well drilling in Kakkonda, Japan

Abstract

K 6-3 was drilled in Kakkonda, northern Honshu Island by Tohoku Geothermal Energy Co., Ltd. as a production well for the Kakkonda No. 2 power plant; that will be operational in 1996. Initially in Kakkonda, about 70 wells with depths ranging from 1, 000 to 2, 000m depths were drilled and geothermal fluids were tapped from a tertiary formation. But recently 3 deeper wells were drilled into a pre-tertiary formation and neo-granitic rocks. These wells discovered a deeper promising reservoir. The planned depth of K6-3 was 2, 800m but it was deepened to 3, 000m when a fracture zone was found at 2, 764m. It took 180 days to drill from spud in to TD. This included 33 days for lost circulation treatments in the shallower depths and 27 days for 15 trajectory correction runs with downhole motors. Retrievable type MWD (ANADRILL SLIM-1), with a temperature limitation of 150°C, was first employed for the high temperature geothermal well at the depth range of 1, 531 to 2, 245m, where the formation is over 330°C. Twenty-seven MWD runs were recorded for both downhole motor drilling and rotary drilling. Total operating time was about 300 hours and total downtime was 13 hours; which was mainly due to battery parts failures. Two cooling towers and a 500kl cooling pit were used to cool the returned mud. These systems worked well, but the stator rubbers came off for the downhole motors used at about 2, 198m and 2, 245m depths. Below that depth, only rotary packed assemblies with long blades stabilizers were employed to keep the trajectory as straight as possible. Low content bentonite mulls with high temperature dispersant and lubricant were used as a drilling fluid. Various temperature data were taken, such as temperature logging at TD, logging at 2, 200m before running 95/8″ liner, mud temperature in and out of the well, mud circulation temperature recorded by MWD, and bottom hole temperature recorded by thermometers installed on top of the magnetic single shot tool. The following results were found: (a) Even if the formation temperature is 350°C drilling fluids can be cooled with proper cooling system and bottom hole circulation temperature (BHCT) can be kept about 80°C in a 121/4″ hole. (b) Even when pumping colder mud BHCT incre ased very much for the 81/2″ hole. This is maybe because of decreased pump rate for the 81/2″. (c) The BHCT decreased drastically while drilling with lost circulation.
Acid fluids were expected for this well, so a duplex tieback casing was installed from surface to 400m depth.