| Title | Geothermal Resource Characterization and Evaluation at Astor Pass, Nevada |
|---|---|
| Authors | Reeves, Donald M.; Pohll, Greg; Lyles, Brad; Faulds, Jim; Louie, John; Ehni, Bill; Kratt, Chris; Cooper, Clay; Parashar, Rishi; Pullammanappallil, Satish; Noel, Donna |
| Year | 2012 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Geothermal characterization; resource evaluation; fracture permeability; reservoir testing; borehole flow testing; distributed temperature sensor |
| Abstract | A blind geothermal system adjacent to a large tufa tower at Astor Pass was characterized and evaluated according to a comprehensive field investigation consisting of geophysical and shallow temperature surveys, structural geological analysis, and reservoir characterization techniques. The geothermal reservoir system, located in basalts, basaltic-andesites and rhyolites of the Lower Pyramid Sequence, likely originated as the result of transfer of dextral shear to normal fault displacement creating a localized region of enhanced extension and fracture permeability. Shallow temperature and magnetotelluric surveys and seismic image interpretation informed the location of two exploration wells. These wells reached down to the underlying granodiorite basement, confirmed fault structures seen in the seismic images, and were used to characterize the reservoir through borehole fracture characterization and a long duration hydraulic reservoir test. High fracture densities (0.7 m average spacing) were encountered in the reservoir rocks, and results from the hydraulic test indicate that the reservoir is well-connected with drawdown responses observed in a shallow water well located approximately 1 km from the pumping well. Borehole spinner log surveys with temperature probes and high resolution DTS measurements indicate that fracture inflows are relatively constant with isothermal temperatures ranging between 92 to 95°C along the exploration wells. Analysis of the reservoir drawdown response yields transmissivity values of 4.8×10-4 to 6.7×10-5 m2/s, with fracture transmissivity values ranging between 1.0×10-4 to 7.2×10-7 m2/s. This comprehensive set of data is used to conclude that the Astor Pass geothermal reservoir is a small, low-enthalpy system with relatively isothermal temperatures and high fracture connectivity and permeability. |