Summary

Wellbore image data recorded in wells penetrating a geothermal reservoir associated with an active normal fault at Dixie Valley, Nevada, was used in conjunction with hydrologic tests and in situ stress measurements (Hickman and Zoback, 1997) to investigate the relationship between reservoir productivity and the contemporary in situ stress field. The Dixie Valley Geothermal Field is a fault-controlled geothermal reservoir located in the Basin and Range Province of the Western U.S. The Stillwater Fault, a basin bounding normal fault, is the producing reservoir for a geothermal plant operated by Oxbow Geothermal Corporation. However, there are well-documented lateral variations in productivity along the fault that are not fully understood.

An extensive reservoir-scale open-hole logging program was conducted from October 1995 through April 1997, during which we obtained sets of borehole televiewer, precision temperature, and spinner flowmeter logs from wells within the primary zone of geothermal production (transmissivities on the order of 1 m²/min) and from wells within a few km of the producing zone that were relatively impermeable and, hence, not commercially viable (transmissivities of about 10^-4 m²/min). Using these logs, we have located and oriented faults and fractures and studied their hydrologic properties through comparison with fracture-related thermal and flow anomalies. The measurements made in these wells provide complete data for a systematic, comparative study of the effects of in situ stress on fracture permeability along producing and nonproducing segments of the fault.

Wellbore image data recorded in each well show pervasive macroscopic fractures with a wide range of orientations throughout the logged intervals. Fracture analysis results yield significant fracture populations parallel to the local trend of the Stillwater Fault in each wellbore. There are, however, statistical differences in the orientation of these macroscopic fracture populations between the producing wells and nonproducing wells located 8-20 km to the southwest. Nonproducing wells have fairly well-developed sets of northeast and southwest steep to moderately dipping fractures and relatively few shallow dipping fractures. Fractures in producing wells have a wider span of fracture dips and more diverse fracture orientations.

Fracture and fluid flow analyses indicate that in both the producing and nonproducing wells there are relatively few fractures that dominate flow. Hydraulically conductive fractures along the producing segments of the fault have orientations that are distinct from the overall orientation of fractures and faults in the producing wells. Conductive fractures along the nonproducing fault segments do not have unique orientations from the dominant fracture rends in the nonproductive wells. By utilizing results from hydraulic fracturing stress measurements and observations of wellbore failure in these wells we have determined the proximity of these fractures and faults to frictional failure.

Results indicate that fracture zones with high measured permeabilities within the producing segment of the fault are parallel to the local trend of the Stillwater Fault and are critically stressed for frictional failure in the ESE extensional stress regime measured at the site. In contrast, the non-producing segments of the Stillwater Fault are severely misoriented to the measured stress regime for frictional failure. Some of the relatively permeable fractures within the nonproductive segments of the fault are also critically stressed for frictional failure, however, the misorientation of the main fault zone and the high horizontal deviatoric stresess measured in these wells (Hickman and Zoback, 1997) appear to dominate the overall potential for fluid flow.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah