Stress and Permeability Heterogeneity within the Dixie Valley Geothermal211 Reservoir: Recent Results from Well 82-5

摘要

We collected borehole televiewer, temperature and flowmeter logs and conducted a211u001ehydraulic fracturing test in a well (82-5) that penetrated the SFZ within the 211u001eknown boundaries of the geothermal field but which failed to encounter 211u001esignificant permeability. Although stuck drill pipe prevented direct access to 211u001ethe SFZ, borehole breakouts and cooling cracks indicated a (approximately) 90 211u001edegree rotation in the azimuth of the least horizontal principal stress (Shmin) 211u001ein well 82-5 at about 2.7 km depth. This rotation, together with the low (Shmin) 211u001emagnitude measured at 2.5 km depth in well 82-5, is most readily explained 211u001ethrough the occurrences of one or more normal faulting earthquakes in the hanging 211u001ewall of the SFZ in the northern part of the reservoir. The orientation of (Shmin) 211u001ebelow 2.7 km (i.e., (approximately) 20 to 50 m above the top of the SFZ) is such 211u001ethat both the overall SFZ and natural fractures directly above the SFZ are 211u001eoptimally oriented for normal faulting failure. If these fracture and stress 211u001eorient ations persist into the SFZ itself, then the existence of a local stress 211u001erelief zone (i.e., anormalously high (Shmin) magnitude) is the most likely 211u001eexplanation for the very low fault zone permeability encountered in well 82-5.