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Geomechanical Characterization for Stimulation of Geothermal Systems, Desert Peak Geothermal Field, NW Basin and Range, NV

Davatzes, Nicholas C.1; Hickman, Steve 2
1 Earth and Environmental Science, Temple University, Philadelphia, PA.
2 Earthquake Hazards Team, U.S. Geological Survey, Menlo Park, CA.

A complete suite of geophysical logs has been acquired for structural and stress analysis in preparation for stimulating shearing of natural fractures intersected by well 27-15 to enhance permeability in volcanic, meta-sedimentary, and igneous rocks adjacent to the Desert Peak Geothermal Field, NV. Logs acquired to date include: temperature/pressure/spinner (TPS) logs to reveal fluid entry/exit points, image logs to characterize natural fractures and stress-induced borehole failure, and velocity and density logs to constrain rock strength and SV. TPS logs indicate permeability in the system is stratified: A shallow, high temperature gradient extends from the water table to 3500 ft MD coinciding with smectitic alteration. A deeper nearly isothermal zone extends from ~3500 to TD at 5627 ft MD associated with chlorite-illite-quartz alteration. Locally high permeability is evident from an injecting (3 barrels/minute) TPS log at a transition from shale to diorite at 4800 ft MD associated with enhanced illite-chlorite, quartz, and calcite alteration. Other geologically significant permeable zones are revealed by temperature anomalies in static equilibrium and non-equilibrium TPS logs. Image logs, including Advanced Logic Technologies BHTV modified for use in 12-15” diameter wells and Schlumberger hot-hole FMS, show tensile fractures indicating a modern Shmin azimuth of 114±17 below the casing shoe at 3007 ft MD—similar to previous results in well 23-1 to the east. This is consistent with normal slip on mapped ESE and WNW dipping normal faults. Most imaged formations include sub-populations of fractures sharing the modern normal faulting orientation; however, fracture density is strongly related to formation. Further analysis in conjunction with a mini-hydraulic fracturing experiment to determine the magnitudes of horizontal principal stresses will be completed prior to stimulation.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009