Discharge Capability and Geothermal Reservoir Assessment Using Data from Slim Holes

Abstract

Compared to conventional rotary-drilled large-diameter wells, the drilling costs for slim holes are relatively modest. Because of this cost differential, it is desirable to use slim holes for geothermal reservoir assessment. Production and injection data from slim holes and large-diameter wells at five geothermal fields (Oguni, Sumikawa, Takigami, and Kirishima, Japan; Steamboat Hills, U.S.A.) were examined to establish relationships (1) between productivity and injectivity indices, and (2) between discharge capacity of slim holes and large diameter wells. For boreholes with liquid feedzones, the productivity and injectivity indices are more or less equal, and the discharge rate of large-diameter wells can be predicted using either production or injection test data from slim holes. Analysis of injection and production data from boreholes for which discharge is accompanied by in situ boiling indicates that productivity index is about an order of magnitude smaller than the injectivity index. A wellbore simulator (WELBOR) was employed to investigate the effect of borehole diameter on the discharge capacity of boreholes with in situ boiling. Preliminary results imply that the discharge data from slim holes together with a wellbore simulator can be used to estimate the discharge capacity of large-diameter wells with two-phase feedzones.