Tectonic controls on fracture permeability in a geothermal reservoir at Dixie Valley, Nevada

摘要

To help determine the nature and origins of permeability variations within a fault-hosted geothermal reservoir at Dixie Valley, Nevada, the authors conducted borehole televiewer logging and hydraulic fracturing stress measurements in six wells drilled into the Stillwater fault zone at depths of 2- -3 km. Televiewer logs from wells penetrating the highly permeable portion of the fault zone revealed extensive drilling-induced tensile fractures. As the Stillwater fault at this location dips S45(degree)E at (approximately) 53(degree) it is nearly at the optimal orientation for normal faulting in the current stress field. Hydraulic fracturing tests from these permeable wells show that the magnitude of S(sub hmin) is very low relative to the vertical stress S(sub v). Similar measurements conducted in two wells penetrating a relatively impermeable segment of the Stillwater fault zone 8 and 20 km southwest of the producing geothermal reservoir indicate that the orientation of S(sub hmin) is S20(degree)E and S41(degree)E, respectively, with S(sub hmin) /S(sub v) ranging from 0.55--0.64 at depths of 1.9--2.2 km. This stress orientation is near optimal for normal faulting on the Stillwater fault in the northernmost non-producing well, but (approximately) 40(degree) rotated from the optimal orientation for normal faulting in the southernmost well. The observation that borehole breakouts were present in these nonproducing wells, but absent in wells drilled into the permeable main reservoir, indicates a significant increase in the magnitude of maximum horizontal principal stress, S(sub Hmax), in going from the producing to non-producing segments of the fault. The increase in S(sub Hmaz), coupled with elevated S(sub hmin)/S(sub v) values and a misorientation of the Stillwater fault zone with respect to the principal stress directions, leads to a decrease in the proximity of the fault zone to Coulomb failure. This suggests 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.