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Sabodh K Garg1, Jim Combs2
(1) Maxwell Technologies, Inc, San Diego, CA
(2) Geo Hills Associates, Reno, NV

Abstract: Use of slim holes for geothermal exploration and reservoir assessment

Compared to conventional 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 exploration and reservoir assessment. In geothermal exploration, slim holes have been used for (1) obtaining core for geological studies, (2) defining the stratigraphic structure, (3) and characterizing reservoir fluid temperature, pressure and composition. To establish the utility of slim holes for reservoir assessment, it is essential to demonstrate that the discharge characteristics of large-diameter wells may be predicted based on discharge and/or injection tests on small-diameter boreholes. Production and injection test data from slim holes and large-diameter wells at five geothermal fields (Oguni, Sumikawa, Takigami, and Kirishima, Japan; Steamboat Hills, NV, 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 producing from liquid feedzones, the productivity and injectivity indices are more or less equal, and are independent of borehole diameter. Therefore, the discharge capacity of large-diameter wells with liquid feedzones can be predicted using either injection or production test data from slim holes. Analysis of injection and production data from boreholes for which discharge is accompanied by in situ boiling indicates that the productivity index is about an order of magnitude smaller than the injectivity index. Preliminary results from Japanese geothermal fields imply that the discharge data from slim holes can also be used to predict the discharge rate for large-diameter wells with two-phase feedzones.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana