OPTIMIZING CASING DESIGNS OF SLIM HOLES FOR MAXIMUM DISCHARGE OF GEOTHERMAL FLUIDS FROM LIQUID FEEDZONES

Because of their portability, low cost, and limited environmental impact, it is desirable to use diamond-bit coring (or small hybrid) rigs for drilling boreholes to supply geothermal fluid for small off-grid geothermal power plants. To-date, coring rigs have been principally used for drilling slim holes for geothermal exploration and reservoir assessment. Since these slim holes were not intended to serve as long-term production wells, little attention has been paid to optimizing casing design for obtaining maximum discharge of geothermal fluids. For example, many existing slim holes are completed with a 114 mm casing (~100 mm ID) and a 100 mm open hole (or 80 mm slotted liner in 100 mm open hole) below the cased interval. The latter design restricts the maximum discharge rate from boreholes with liquid feedzones to about 10 kg/s (~36 tons/hour). In this paper, alternate completion schemes for slim holes drilled by coring (or hybrid) rigs are examined. It is shown that with proper casing design, the discharge capacity of slim holes can be increased by as much as 200%.