Tectonic Controls on Reservoir Permeability in the Dixie Valley, Nevada, Geothermal Field

To determine factors controlling permeability variations within and adjacent to a fault-hosted geothermal reservoir at Dixie Valley, Nevada, we conducted borehole televiewer observations of wellbore failure (breakouts and cooling cracks) together with hydraulic fracturing stress measurements in six wells drilled into the Stillwater fault zone at depths of 2-3 km. Measurements in highly permeable wells penetrating the main geothermal reservoir indicate that the local orientation of the least horizontal principal stress, Shmln, is nearly optimal for normal faulting on the Stillwater fault. Hydraulic fracturing tests from these wells further show that the magnitude of Shmin is low enough to lead to frictional failure on the Stillwater and nearby subparallel faults, suggesting that fault slip is responsible for the high reservoir productivity. Similar measurements were conducted in two wells penetrating a relatively impermeable segment of the Stillwater fault zone, located -8 and 20 km southwest of the geothermal reservoir (wells 66-21 and 45-14, respectively). The orientation of Shmin in well 66-21 is near optimal for normal faulting on the nearby Stillwater fault, but the magnitude of Shmln is too high to result in incipient frictional failure. In contrast, although the magnitude of Shmin in well 45-14 is low enough to lead to normal faulting on optimally oriented faults, the orientation of the Stillwater fault near this well is rotated by -40" from the optimal orientation for normal faulting. This misorientation, coupled with an apparent increase in the magnitude of the greatest horizontal principal stress in going from the producing to nonproducing wells, acts to inhibit frictional failure on the Stillwater fault zone in proximity to well 45-14. Taken together, data from the nonproducing and producing wells thus suggest that a necessary condition for high reservoir permeability is that the Stillwater fault zone be critically stressed for frictional failure in the current stress field. INTRODUCTION To evaluate the role of tectonic controls on the productivity of a fracture-dominated geothermal reservoir at Dixie Valley, Nevada, we are conducting an integrated study of fracturing, stress, and hydrologic properties in geothermal wells drilled into ad near the Stillwater fault zone (Figure 1). This fault is a major, active, range-bounding normal fault located in the western Basin and Range province, Nevada (see 1 Okaya and Thompson, 1985), and comprises the main reservoir for a -62 MW geothermal electric power I plant operated by Oxbow Geothermal Corporation. j Although earthquakes have not ruptured this segment of the Stillwater fault in historic times, large (M = 6.8-7.7) earthquakes have occurred within the past 80 years along range-bounding faults both to the northeast and southwest of the Dixie Valley Geothermal Field (DVGF), and geologic evidence shows that the Stillwater fault abutting the DVGF experienced two or more faulting episodes (total offset -9 m) during the past 12,000 years (Wallace and Whitney, 1984). 1 1 The principal goal of this study is to define the nature, distribution, and hydraulic properties of fractures associated with the DVGF, and to characterize the manner in which these fractures, and hence the overall reservoir hydrology, are related to the local stress field. This project was initiated in late 1995 with an extensive downhole measurements program conducted in a 2.6-km-deep geothermal production well (well 73B-7) drilled into the Stillwater fault zone by Oxbow Geothermal Corporaticin (see Hickman ad aback , 1997; Barton et al., 1997; Hickman et al., 1997). This study has since been expanded to include borehole televiewer logging, temperature/pressure/spinner (TPS) logging, and hydraulic fracturing stress measurements from additional wells within the primary zone of geothermal production (transmissivities on the order of 1 m*/min) and from wells within a few km of the producing zone that