Hydrologic Properties of the Dixie Valley, Nevada, Geothermal Reservoir from Well-test Analyses

HYDROLOGIC PROPERTIES OF THE DIXIE VALLEY, NEVADA, GEOTHERMAL RESERVOIR FROM WELL-TEST ANALYSES R.H. Morinl, S.H. Hickman2, C.A. Barton3, A.M. Shapird, W.R. Benoits, J.H. Sass6 1U.S. Geological Survey, Denver, CO 80225, USA [email protected] 2U.S. Geological Survey, Menlo Park, CA 94025, USA [email protected] 3Stanford University, Dept. of Geophysics, Stanford, CA 94305, USA barton@ pangea. stanford.edu 4U.S. Geological Survey, Reston, VA 20192, USA ashapi ro @ usgs. gov SOxbow Geothermal Corp., Reno, NV 89502, USA [email protected] .com 6U.S. Geological Survey, Flagstaff, AZ 86001, USA jsass@flagmail. wr.usgs.gov Temperature, pressure, and spinner (TPS) logs have been recorded in several wells from the Dixie Valley Geothermal Reservoir in west central Nevada. A variety of well-test analyses has been performed with these data to quantify the hydrologic properties of this fault-dominated geothermal resource. Four complementary analytical techniques were employed, their individual application depending upon availability and quality of data and validity of scientific assumptions. In some instances, redundancy in methodologies was used to decouple interrelated terms. The methods were (1) step drawdown, variable-discharge test; (2) recovery analysis; (3) damped-oscillation response; and (4) injection test. To date, TPS logs from five wells have been examined and results fall into two distinct categories. Productive, economically viable wells have permeability-thickness values on the order of 105 millidarcy-meter (mD-m) and storativities of about 10-3. Low-productivity wells, sometimes located only a few kilometers from their permeable counterparts, are artesian and display a sharp reduction in permeability-thickness to about 101 mD-m with storativities on the order of 10-4. These results demonstrate that the hydrologic characteristics of this liquid-dominated geothermal system exhibit a significant spatial variability along the rangebounding normal fault that forms the predominant aquifer. A large-scale, coherent model of the Dixie Valley Geothermal Reservoir will require an understanding of the nature of this heterogeneity and the parameters that control it. The Dixie Valley Geothermal Field is located within the western Basin and Range in west central Nevada, USA. The area is marked by high localized heat flow (Williams et al., 1997) and late Cenozoic extensional faulting and volcanism (Okaya and Thompson, 1985). The regional hydrologic system is comprised of over 5000 km* of surface drainage (Parchman and Knox, 1981), with groundwater recharge occurring primarily from infiltration of meteoric water through permeable channels within the consolidated rocks of the surrounding mountain ranges. The Stillwater Fault is a major range-bounding normal fault that dips moderately (= 52") to the east-southeast and forms the predominant aquifer in the region. Most of the geothermal heat and mass transport encountered in Dixie Valley occurs within this fault zone (Benoit, 1992). Numerous wells have been drilled to exploit this liquid-dominated geothermal resource. In this report, temperature, pressure, and spinner (TPS) geophysical logs obtained from five wells drilled along the fault zone are analyzed to quantify the hydrologic properties of the reservoir. The analytical methods adopted here are similar to conventional groundwater techniques, though the presence of flashing fluid in the wellbore during self-sustaining geothermal discharge introduces additional complications (e.& Narasimhan and Witherspoon, 1979; Grant et al., 1982). The pertinent wells are identified as 73B-7, 74-7, 62-21, 66-21 and 45-14, and their locations are shown in Figure 1. The results represent reservoir characteristics in the vicinity of the individual wells that