To understand causes for localized variations in stress and permeability in a geothermal reservoir associated with the Stillwater Fault Zone (SFZ), we collected borehole televiewer, temperature and flowmeter logs and conducted a hydraulic fracturing test in a well (82-5) that penetrated the SFZ within the known boundaries of the geothermal field but which failed to encounter significant permeability. Although stuck drill pipe prevented direct access to the SFZ, borehole breakouts and cooling cracks indicate a ~90? rotation in the azimuth of the least horizontal principal stress (Shmin) in well 82-5 at about 2.7 km depth; similar rotations were observed in a producing well located ~0.6 km to the northeast. This rotation, together with the low Shmin magnitude measured at 2.5 km depth in well 82-5, is most readily explained through the occurrence of one or more normal faulting earthquakes in the hanging wall of the SFZ in the northern part of the reservoir. The orientation of Shmin below 2.7 km (i.e., ~20 to 50 m above the top of the SFZ) is such that both the overall SFZ and natural fractures directly above the SFZ are optimally oriented for normal faulting failure. If these fracture and stress orientations persist into the SFZ itself, then the existence of a local stress relief zone (i.e., anomalously high Shmin magnitude) is the most likely explanation for the very low fault zone permeability encountered in well 82-5. Furthermore, under this condition a massive hydraulic fracture within the SFZ would propagate parallel to the strike of the SFZ and possibly intersect the highly permeable reservoir to the northeast. Thus, well 82-5 may be a good candidate for reservoir stimulation, although it would most likely require that a new leg be drilled to regain access to the SFZ.

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