The mapping of producing fractures in a geothermal field is an important technical objective in field development. Locating, orienting, and assessing
producing fractures can guide drilling programs and optimize the placement of production and injection wells. A long-offset multicomponent borehole induction resistivity tool capable of surviving the high temperatures encountered in geothermal wells has recently been developed in a NEDO project ėDeep seated Geothermal Resource Surveyíí and tested in a high temperature environment. Several characteristics of this device make it ideal for detecting producing fractures. Whereas commercial induction logging devices have source receiver separations of 1 m, this device has multiple sensors with separations up to 8 m, allowing for deeper penetration and the ability to straddle fracture-induced washout zones in boreholes. The three-component measurements also make it possible to map the strike and inclination of nearby fractures and other three-dimensional structures. This, in turn, allows for accurate projection of these structures into the space between wells.

In this paper, we describe the design of the tool and show results of a performance test carried out in an oil-field steam flood. Data from vertical sensors are compared to conventional logging results and indicate
the recent formation of a low-resistivity zone associated with high temperatures due to steam flood breakthrough. Horizontal field data indicate that the hgh-temperature zone is irregular in the vicinity of the borehole and more pronounced closest to the steam injector.

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