Geothermal Geophysics Overview and Resistivity Techniques

1. Geothermal Geophysics Overview and Resistivity Techniques

2. Geothermal Exploration Geophysics Questions When geophysics is integrated with complete data set in a consistent geothermal conceptual model: What drilling target is lowest risk? What is the capacity of the reservoir in MW? Where should wells be targeted to prove that resource capacity? What is the likelihood of success for the next well(s) based on analogous downside, upside and most likely conceptual models? What range of resource capacity is consistent with the geoscience data set based on analogous downside, upside and most likely conceptual models?

3. Geothermal Geophysics Overview What surface methods are used? What do they measure? How do they contribute to the conceptual model of the resource? Why are resistivity methods relatively popular? Why is MT a popular resistivity method? What are some of its pitfalls? What are the advantages of conceptual targeting, as opposed to anomaly targeting?

4. Geothermal Geophysics Methods Mainly adapted from the petroleum and mining industries. BUT Mining has shallower, smaller targets. Petroleum has different imaging needs in a different geological setting, making reflection seismic the preferred technique. Petroleum and minerals have more value per explored volume than hot water.

5. Surface Geophysical Techniquesin Geothermal Exploration

6. Geophysical Acronyms

7. Surface Geophysical Techniques in Geothermal Exploration

8. Geophysical Exploration of Geothermal Systems

9. Geothermal Resource Characteristics Affecting Geophysics For >200�C Issue Reservoir top usually 300 to 1000 m deep Deeper Reservoir thickness 300 to 3000 m Thicker Testable wells usually >$1.5 million Wells cost more Commercial wells usually >$3 million For <200�C tabular Issue Reservoir top usually 100 to 800 m deep Shallower (for now) Reservoir thickness 100 to 1000 m Thinner Testable wells sometimes <$1 million Wells cost less Commercial wells usually $1.2 to $3 million

10. Geophysical Exploration of Geothermal Systems

11. Geothermal Resource Characteristics Affecting Geophysics 2 For <200�C upflow Issue Reservoir top usually 300 to 1000 m deep similar to >200�C Reservoir thickness ? maybe >1000 m Fault zone Testable wells usually >$1.5 million Wells cost more Commercial wells usually $1.5 to >$3 million For HDR/EGS Issue Reservoir top created Dynamic Reservoir thickness created Dynamic Testable wells cost more Wells cost more Commercial well cost a research issue

12. Geophysical Exploration of >200�C Geothermal Systems Resource image area > 1 km2, often > 4 km2 Exploration image area > 4 km2, often > 50 km2 Depth to reservoir top 300 to 2000 m Access often rugged Environmental issues

13. Conceptual Objectives for Exploration Geophysics Where is the reservoir? How big is it? Isotherm geometry, the overall permeability constraint for reservoir simulation, can be constrained using low resistivity, temperature sensitive clay alteration for >500 m depth: MT. Possibly VES, DC-T, CSMT etc. for <500 depth: TEM, CSMT, AMT, VES, DC-T etc What can extend surface geology deeper? Gravity: lithology, dense alteration, basin geometry Magnetics: near-surface sulfate alteration, volcanics Where are specific well entries? Rare validated success cases, few �partial technical successes� and numerous under-reported failures Reflection seismic in geothermal often poor quality. Can characterize structure. However, entries rarely imaged. Geochemistry and geology used with resistivity to map leaky clay cap alteration intensity and geometry (i.e. detect �sweet spots� not entries)

14. Surface Geothermal GeophysicsOther Than Resistivity

15. Geophysical Exploration of <200�C Geothermal Systems Resource image area > 1 km2, often > 4 km2 Exploration image area > 4 km2, often > 20 km2 Depth to reservoir top 100 to 1000 m More like exploration for aquifers than for minerals or petroleum.

16. Conceptual Objectives for Exploration Geophysics Where is the reservoir? How big is it? Isotherm geometry, the overall permeability constraint for reservoir simulation, can be constrained using low resistivity, temperature sensitive clay alteration for >500 m depth: MT. Possibly VES, DC-T, CSMT etc. for <500 depth: TEM, CSMT, AMT, VES, DC-T etc What can extend surface geology deeper? Gravity: lithology, dense alteration, basin geometry Magnetics: near-surface sulfate alteration, volcanics Where are specific well entries? Rare validated success cases, few �partial technical successes� and numerous under-reported failures Reflection seismic in geothermal often poor quality. Can characterize structure. However, entries rarely imaged. Geochemistry and geology used with resistivity to map leaky clay cap alteration intensity and geometry.

17. Surface Geophysics Methodsfor Resistivity

18. Geothermal Resistivity Pattern

19. Resistivity Acquisition Issues Noise Pipes, fences, power lines and similar metal features usually require a standoff, typically 100 to 1000 m depending on method Near power plants, passive methods like MT are doubtful and all methods suffer from higher noise DC power lines can limit MT depth of investigation to <1000 m at 30 km distance and <5000 m at 100 km distance Statics Static distortion affects all methods that use electrodes (all but TEM) Difficult to avoid in volcanics or rugged areas Static correction by inversion smoothing is sometimes unrealistic Access Cost rises steeply if access to sites is poor Faster methods like T-MT reduce cost only where access is easy Cable oriented methods require wide and continuous access so more suited to Nevada than New Zealand ..

20. MT Method

21. MT Physics

22. MT Physics

23. MT Method

24. T-MT Method

25. TDEM / TEM

26. TDEM / TEM

27. �Standard� Geophysical Plan >200�C Geothermal Exploration MT to map base of clay �cap� Gas and fluid geochemistry for conceptual target Maybe TEM for MT statics and detail Maybe gravity for lithology and large structure

28. Resistivity Objectives in Geothermal Exploration Map structure and conductance of <180�C low resistivity smectite clay zone capping the relatively resistive reservoir Integrate with geochemistry and geology to Estimate resource capacity Target wells for high temperature permeability Estimate risk probabilities ..

29. Geothermal Resistivity Pattern

30. Salak Geothermal FieldMT Cross-sectionMT Resistivity with MeB Smectite & Isotherms from Wells

32. �Standard� Geophysical Plan <200�C Geothermal Exploration Lowest cost resistivity to reach base of clay cap, maybe AMT, CSMT, DC, etc. Temperature Gradient Holes if access and drilling are low cost. Ground magnetics and gravity for geology and alteration mapping. SP if target simple, shallow and low relief Reflection seismic if structure is simple and manifestations are weak

33. Geothermal MT Interpretation Pitfalls

34. Exploration and Evaluation Stages and Styles

35. What To Target?Anomaly or Conceptual Model

36. Uncertainty in Geothermal Resistivity Interpretation Noise and 3D Distortion Anomalous parts of images should be checked for underlying data quality issues Acquisition and interpretation should be done by different entities Conceptual Interpretation Resistivity methods can image the intensity of hydrothermal clay alteration and the geometry of the base of the low resistivity clay cap conforming to the geothermal reservoir However, the apex of the clay cap may be over the shallowest permeability but not over the deep high-temperature upflow which must be inferred from less reliable alteration intensity. so Check conceptual advantages of other methods Integrate with geochemistry and geology Drill a conceptual model, NOT an anomaly Validate geophysical interpretation after drilling ..

37. Glass Mountain Geothermal FieldMT 1D-2D-3D Resistivity and Well 17A-6

38. Audit Geophysics with Well Data

39. Cost for Geophysics

40. Geothermal Geophysics Overview and Resistivity Techniques

41. VES and Dipole-dipole Resistivity at Cerro Prieto

42. VES Resistivity

43. VES and Dipole-dipole Resistivity at Cerro Prieto

44. CSMT Profiling

45. SP

46. SP

47. Microearthquakes (MEQ)in Geothermal Exploration

48. Magnetic Surveys

49. Reflection Seismic in Geothermal Exploration Dominates petroleum exploration $ billions in petroleum seismic research are making incremental progress on the issues that inhibit applications to geothermal exploration: P attenuation by shallow gas like CO2 in clay alteration Shallow dense rocks like lavas Statics due to rugged topography with rapid seismic velocity changes (like lavas and tuffs) Poor velocity constraints near target depths Scattering from closely spaced deep faults Lack of rock contacts that coherently reflect S-conversion interference In these situations, petroleum exploration companies use MT, EM, gravity etc

50. Reflection Seismic Geothermal Applications When goal is to image permeability; not all fault segments are permeable so ideally want stress setting, not just one segment volume velocity imaged by bending ray tomography will usually be too low resolution to resolve discrete faults at reservoir depth unless geometry and velocity are ideal. Faults are imaged by reflection seismic in geothermal prospects with; 1) layered geology, 2) low gas flux, 3) limited shallow lava, and 4) discrete structures Case histories image some reservoir faults but very few validated entries. Field-margin injection wells are easier targets If CO2 is trapped in clay cap, then perimeter of shallow reservoir permeability often matches �bad data� zone for reflection seismic Clay cap sometimes imaged by p-wave bending ray velocity analyses but poorer resolution and higher cost than MT resistivity S-wave splitting is still speculative Therefore, still a research topic for geothermal exploration Reflection seismic is more cost-effective when acquisition cost is lower without compromising acquisition .

51. Value of Geophysics Use decision trees to assess impact of new information Choose three likely outcomes of the resource decision Your best guess: what you think it is. Typical downside: considering similar situations that were disappointing. Likely upside: best similar case or maybe better with justification. Assess probability of each case based mostly on 100 prospects, with Monte Carlo ranges providing context for exceptional cases. How many prospects with diagnostic information like this had an outcome better than this case? Assess likely affect of new information on probability for each case, considering consistency and breadth of experience using similar information elsewhere. Use decision tables to assess new information: How much would the new information likely affect resource decision probabilities? How much does sufficiently reliable information cost? What other information would affect the same resource probabilities and is it more cost-effective?

52. Geophysical Confidence Levels