Borehole Geophysics i n Geothermal Wells--Problems and Progress

Abstract

Surface geophysical techniques are readily adaptable to exploration for and evaluation of geothermal reservoirs because existing equipment and interpretive models can be used. In contrast, the application of borehole geophysics for these same purposes requires the development of equipment to operate dependably in the very hostile environment of some geothermal wells. After equipment has been developed and tested, its response must be calibrated with respect to required parameters such as lithology and porosity. This is difficult in geologic environments where there is practically no experience in well-log interpretation. The desired final products are reliable data to guide exploration for geothermal systems, and to aid in reservoir evaluation, modeling, and development, in the ways in which well logs are routinely used in the petroleum industry. Researchers in geothermal exploration are still some years from achieving the level of application already attained in petroleum exploration and development. This deficiency is receiving some attention from the U.S. Energy Research and Development Agency (ERDA) and the U.S. Geological Survey. The two agencies cosponsored a workshop on Geophysical Measurements in Geothermal Wells in September 1975 (Baker, Baker, and Hughen, 1975). Sandia Laboratories had previously'summarized the state-of-the-art in a report on "Well-Logging Technology and Geothermal Applications" (Baker, Campbell, and Hughen, 1975). For several years the U.S. Geological Survey has had a research program to develop logging instrumentation and log-interpretation techniques for geothermal applications. This summary describes some of the results of that research effort.