Use of Electrical Surveys for Geothermal Reservoir Characterization: Beowawe Geothermal Field

1. Use of Electrical Surveys for Geothermal Reservoir Characterization: Beowawe Geothermal Field Sabodh K. Garg, John W. Pritchett, Philip E. Wannamaker, Jim Combs GRC 2007 Annual Meeting Sparks, Nevada

3. Outline Three-dimensional numerical model of the Beowawe geothermal field, north-central Nevada. Computed temperature and salinity distribution used to compute pore fluid resistivity. Archie�s law relates formation resistivity to pore fluid resistivity. STAR DC, MT and SP postprocessors used to calculate expected response corresponding to available electrical surveys.

4. Topography, Wells, DC resistivity lines (yellow), MT stations (green) Electrical surveys: 1. dipole-dipole resistivity (McPhar Geophysics), 6 NS lines, 2 within (lines 2 and 1) within the developed area. 2. MT (Geotronics) 12 tensor MT soundings. Two stations Ben-3 and Ben-12 straddle DC line 2 3. SP survey. Terraphysics 10 NS lines. De Moully and Corwin used a denser new work . Positive to the north and west of the Geysers Terrace, and negative to the southeast.Electrical surveys: 1. dipole-dipole resistivity (McPhar Geophysics), 6 NS lines, 2 within (lines 2 and 1) within the developed area. 2. MT (Geotronics) 12 tensor MT soundings. Two stations Ben-3 and Ben-12 straddle DC line 2 3. SP survey. Terraphysics 10 NS lines. De Moully and Corwin used a denser new work . Positive to the north and west of the Geysers Terrace, and negative to the southeast.

5. Numerical Grid (x-y plane) for natural state simulation In the vertical direction, grid extends from -2000 masl to 1650 masl. Reservoir fluid treated as pure water with a tracer used to track salinity. Top surface: 1 bar pressure and conductive/radiative boundary. Bottom Surface: Both heat and mass fluxes imposed. Mass upflow enters the Malpais fault zone in the vicinity of Ginn wells.In the vertical direction, grid extends from -2000 masl to 1650 masl. Reservoir fluid treated as pure water with a tracer used to track salinity. Top surface: 1 bar pressure and conductive/radiative boundary. Bottom Surface: Both heat and mass fluxes imposed. Mass upflow enters the Malpais fault zone in the vicinity of Ginn wells.

6. Geologic Section:east-west plane (j=6) Well heads for Beowawe production wells Ginn-1, Ginn-2 and 77-13 lie just to the north of this section.Well heads for Beowawe production wells Ginn-1, Ginn-2 and 77-13 lie just to the north of this section.

7. Computed pressures and temperatures (j=6)

8. Computed tracer (salinity) distribution (j=6)

9. Comparison between computed and measured feedzone pressures Except for well Ginn1-13, computed and measured pressures are in good agreement. Even for well Ginn1-13, the discrepancy is not that great.Except for well Ginn1-13, computed and measured pressures are in good agreement. Even for well Ginn1-13, the discrepancy is not that great.

10. Computed (dashed) and measured (solid) temperatures in well Ginn1-13 Measured temperatures in other Beowawe wells are also in fair agreement with the computed values. Based on a comparison between measured and computed pressures and temperatures, we conclude that the model results are an adequate representation of the natural state of the Beowawe geothermal field.Measured temperatures in other Beowawe wells are also in fair agreement with the computed values. Based on a comparison between measured and computed pressures and temperatures, we conclude that the model results are an adequate representation of the natural state of the Beowawe geothermal field.

11. Electrical Grid All of the electrical postprocessors use a spatially-extensive computational grid (the �electrical grid�) within which the STAR grid is embedded. The electrical grid covers an area of 400 square kilometers. In the vertical direction, the electrical grid extends to about 12 km below the highest point along the surface. Larger box: STAR grid. Smaller box: SP survey area.All of the electrical postprocessors use a spatially-extensive computational grid (the �electrical grid�) within which the STAR grid is embedded. The electrical grid covers an area of 400 square kilometers. In the vertical direction, the electrical grid extends to about 12 km below the highest point along the surface. Larger box: STAR grid. Smaller box: SP survey area.

12. Dipole-dipole resistivity survey In the volume common to the STAR and electric grids, Archie�s law adopted to relate formation electrical resistivity to pore fluid resistivity and porosity. Results of dipole-dipole survey used to specify formation resistivity in parts of the electrical grid that are disjoint from the STAR grid. The DC resistivity survey maps the so-called �apparent resistivity� distribution. The computed profiles reproduce most of the important features of measured profiles.

13. Dipole-dipole Line 2

14. Dipole-dipole resistivity (line 2)n = 1, 2, 4, 5 Apparent resistivity varies considerably both along the horizontal and vertical directions. The resistivity at depth is lower than at shallow levels.Apparent resistivity varies considerably both along the horizontal and vertical directions. The resistivity at depth is lower than at shallow levels.

15. Magnetotelluric (MT) Survey The magnetotelluric (MT) method uses naturally occurring electromagnetic (EM) waves as sources to map the resistivity structure. EM time-series data are decomposed into spectra, providing �apparent resistivity� as a function of frequency. The depth of penetration is inversely proportional to frequency; thus lower frequencies can be used to map the deeper resistivity structure. Twelve tensor MT soundings were collected on and around the Beowawe KGRA in 1976; only two MT stations (Ben-3 and Ben-12) lie within the currently exploited geothermal field.

16. MT Station Ben-3

17. MT Survey (continued) Although the computed and observed MT sounding curves have the same general shape and show a decline in resistivity with increasing depth, there exists a quantitative discrepancy between the observed and predicted MT responses. The observed results need to be multiplied by a factor of 3. Possible reasons for this difference include �static shift� and anisotropic (horizontal and/or vertical) resistivity distribution. For Ben-2, the multiplicative factor is 2.For Ben-2, the multiplicative factor is 2.

18. Self-Potential (SP) Survey The self-potential (SP) method measures variations in natural DC voltages over the surface of the earth caused by the movement of underground fluids. Anomalies of several hundred millivolts of this type have been observed at several geothermal fields. Two self-potential surveys were performed at Beowawe in the1970s. The SP anomaly at Beowawe is positive to the northwest and negative to the southeast of the Geysers Terrace.

19. SP Survey (computed) Computed results show that SP is positive to the west and north of the Geysers Terrace, and negative to the southeast.Computed results show that SP is positive to the west and north of the Geysers Terrace, and negative to the southeast.

20. SP survey (continued) The SP anomaly at Beowawe is positive to the northwest and negative to the southeast of the Geysers Terrace. Although the computed and measured SP distributions disagree in detail, both display the same general behavior (positive to the northwest and negative to the southeast).

21. Conclusions (1) A suite of electrical surveys may be used to define the characteristics of hidden �Basin and Range� type geothermal systems. None of the methods taken alone can provide unambiguous indication of a geothermal system. Both the hot and permeable geothermal reservoir rocks, and impermeable clays and shales have low resistivities. However, clays and shales cannot support the vigorous upflow needed to sustain geothermal systems.

22. Conclusions (2) A self-potential anomaly can be caused by topography driven cold water flow, but ordinary cold ground water aquifers do not normally exhibit low resistivities. Therefore if the regional heat flow is high, and the electrical resistivity and self-potential anomalies coincide, the possibilities of finding a productive geothermal reservoir are enhanced.

23. Acknowledgment This work was supported by the U.S. Department of Energy Geothermal Technologies Program under Contract No. 18084 Amendment 8 between Battelle Energy Alliance, LLC (operator of Idaho National Laboratory) and Science Applications International Corporation.