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Geophysical Exploration for Geothermal Resources

Geophysical Exploration for Geothermal Resources. by William Cumming Cumming Geoscience, Santa Rosa CA wcumming@wcumming.com. Cumming Geoscience. Geophysics Outline. Types of reservoirs Types of geophysical methods High temperature versus low temperature How resistivity methods work

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Geophysical Exploration for Geothermal Resources

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  1. Geophysical Exploration for Geothermal Resources by William Cumming Cumming Geoscience, Santa Rosa CA wcumming@wcumming.com Cumming Geoscience

  2. Geophysics Outline • Types of reservoirs • Types of geophysical methods • High temperature versus low temperature • How resistivity methods work • MT, T-MT, TDEM, CSMT, VES • Applications of resistivity methods • Other methods • Gravity, SP, Magnetics • Cost of geophysics • Example pitfall in geophysics interpretation • New Methods and Research

  3. Geothermal Geophysics • Paul Brophy’s “types” have similar rock physics • Almost all geothermal reservoir types host temperature sensitive clays that can be imaged using resistivity • O&G geophysics is dominated by seismic imaging of permeability “traps” and, recently, reservoir properties. • Geothermal geophysics is dominated by resistivity imaging of the permeability “traps” and a key reservoir property, the natural state isotherm pattern, that is the starting point for most geothermal reservoir models. • Surface resistivity cannot image individual entries but can image the permeable volume of the reservoir and, with geology, geochemistry etc, can significantly reduce well targeting risk in many cases. • Even if resistivity “works” for shallow low temperature resources, other approaches may be more cost-effective. • There are many “special” methods for “special” issues

  4. Geothermal DevelopmentCharacteristics Affecting Geophysics For >210°C Issue • Production by flash lift of water-steam • Flash and/or binary generation • 50 to 100% injection at new fields • 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 <180°C Issue • Production by pumping hot water • Binary generation • 100% injection • Reservoir top usually 100 to 500 m deep Shallower • Reservoir thickness 100 to 1000 m Thinner • Testable wells usually >$0.5 million Wells cost less • Commercial wells usually $1 to $2 million

  5. 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 after Cumming et al. 2000 Cumming Geoscience

  6. Geophysical Exploration of <180°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. Cumming Geoscience

  7. Geothermal Geophysics Technology • Geophysical exploration technology is 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. Cumming Geoscience

  8. Geophysical Acronyms MT Magnetotellurics AMT Audiomagnetotellurics T-MT Telluric-Magnetotellurics CSAMT Controlled Source Audiomagnetotellurics HEM Helicopter Electromagnetics TDEM Time Domain Electromagnetics TEM same as TDEM VES Vertical Electrical Sounding SP Self-Potential dGPS Differential Global Positioning System MEQ Microearthquake Cumming Geoscience

  9. Geophysical Techniques Geothermal Exploration Standard:MT, T-MT, TDEM, Gravity Legacy: Dipole-Dipole, Tensor Dipole-Bipole Special: VES, AMT, CSAMT, SP, HEM Aeromagnetics, Precision Ground Magnetics Research: Reflection / Refraction Seismic Special Applications Development: Microgravity, Microearthquake, Subsidence Proprietary: E-Scan, E-Map Unreviewed: Aquatrack Suspect: Seismic Noise, Low Res Ground Magnetics Plausible methods with weak technical support Cumming Geoscience

  10. Geophysical Techniquesin Geothermal Exploration • Infer geothermal resource characteristics for well targeting and resource capacity estimation by remotely constraining rock properties such as: • Resistivity: using MT, TDEM, VES, CSAMT, HEM • Density: using gravity and seismic reflection • Magnetic susceptibility: using magnetic field • Seismic velocity: Refraction and reflection seismic • Natural electrical potential (V): using SP • et al (e.g. crack density from MEQ) Cumming Geoscience

  11. Geophysical Techniquesin Geothermal Exploration “Special” “Standard” • CSMT for noisy areas or where limitations do not matter and low cost does • Magnetics for alteration & unit boundary patterns • SP for shallow <180°C • DC profiling and HEM for reconnaissance mapping • MT for base of clay cap • TDEM for statics and detail • Gravity for lithology and large structure Cumming Geoscience

  12. “Standard” Geophysical Plan >200°C Geothermal Exploration • MT to map base of clay “cap” • TDEM for MT statics and detail • Gas and fluid geochemistry for conceptual target • Maybe gravity for lithology and large structure Cumming Geoscience Cumming, 2006

  13. “Standard” Geophysical Plan <180°C Geothermal Exploration • TDEM or other low-cost resistivity for clay cap • SP if target shallow and topography gentle • Other methods to support geology, geochemistry • Temperature Gradient Wells if access and drilling are low cost. • More like exploration for aquifers than for minerals or petroleum. Cumming Geoscience

  14. MT Objectives in Geothermal Exploration • Map structure and conductance of <180°C low resistivity smectite clay zone capping the relatively resistive >200°C propylitic reservoir • Integrate with geochemistry and geology to • Estimate resource capacity • Target wells for high temperature permeability Cumming Geoscience

  15. MT Method • E 2 dipoles ~100 m • H 3 magnetometers • EM signal from sun and electrical storms • MT resistivity at 1 Hz is about 1 km down • Blue zone is low resistivity smectite • Topo and shallow conductors give different resistivity on 2 dipoles, i.e. statics Cumming Geoscience

  16. MT Physics Geophys.washington.edu

  17. MT Acquisition Issues • AC power line noise is usually mitigated by a ~200 to 400 m standoff • DC power lines and electric trains can limit depth of investigation to <1000 m • Pipes, fences and similar metal features usually require a 200 to 1000 m standoff • Although the equipment is portable, cost rises steeply if access to sites is poor Cumming Geoscience

  18. MT Field Layout • Uses natural EM signal • > 5 km depth • Records 7 to 20 hours • 2-5 man portable system • One or two stations/day • T-MT uses 2 to 3 MT stations with 2-10 T-only stations for lower cost where lateral changes are smooth. Cumming Geoscience

  19. MT versus T-MT Digging holes for magnetometers is time-consuming so costs are reduced by doing T-MT in areas with smooth near-surface resistivity variations. Cumming Geoscience

  20. T-MT Profiling Quantech, 2003

  21. T-MT Profiling • Continuous line of T stations with one MT station • 100 m spacing used in minerals is seldom cost-effective for deeper and/or larger geothermal targets • Cost is sometimes less than MT stations for smaller, shallower targets, like those in minerals exploration • Real time processing and display for noise reduction • Statics due to topography on continuous T-MT can be corrected when surface resistivity is uniform • Having T but not MT at some stations may limit resolution but this is seldom an issue in geothermal Cumming Geoscience

  22. TDEM / TEM • Pulse current in outer loop, measure signal in inner loop from “smoke rings” of current induced by magnetic field. • TDEM depth < MT • No electrodes so no static distortion • Focused so less 2D/3D distortion Cumming, 2003 Cumming Geoscience

  23. TDEM • Record in minutes • Very portable when using batteries • 1 to 7 stations/day • Cost $200 to >$600 per station From: Geosystem Cumming Geoscience

  24. TDEM Survey Types From: MINDECO

  25. TEM at Krafla • Detects base of clay • Maps reservoir top • MT not needed • Shallow reservoir • 300 to 1000 m loops • Cost >$600/station • 1 to3 stations/day • Geonics Protem / EM37 From: Arnason et al 2000 Cumming Geoscience

  26. CSMT Profiling • Scalar MT profiling using a wire transmitter • Costs < MT • Active source better near some noise sources • Cannot as reliably detect or correct static and 2D/3D distortion • “Near field” transmitter distortion • Higher frequency so depth < 200 to < 1000 m • Fewer imaging and processing options Cumming Geoscience

  27. VES Resistivity • Vertical Electrical Soundings ( also known as Schlumberger or DC Soundings ) transmit current in one expanding dipole and measure voltage across a smaller centered dipole. • Use 2D images from VES for well targeting and resource capacity, single dipole spacing for reconnaissance • In geothermal areas, depth of resolution is about 15 to 25% of transmitter dipole length. Transmitter dipoles sometimes must be >5 km long to resolve top of relatively resistive reservoir. • Reprocessing old VES data to 1D/2D smooth images is often worthwhile if transmit dipole large enough (AB/2 > 2 km) • Environmental issues, cost and logistics limit new surveys Cumming Geoscience

  28. VES and Dipole-dipole Resistivity at Cerro Prieto Charre-Meza et al 2000

  29. Resistivity Imaging in Geothermal Exploration • Map base and conductance of low resistivity clay zone capping relatively resistive reservoir • Integrate with geochemistry and geology to • Estimate resource capacity • Target wells for high temperature permeability

  30. Awibengkok Geothermal FieldMT Cross-sectionMT Resistivity with MeB Smectite & Isotherms from Wells 1000 Meters -1000 1 Km Cumming Geoscience from: Gunderson, Cumming, Astra and Harvey (2000)

  31. Karaha Bodas MT (Moore,2006) from: Moore (2006) Cumming Geoscience

  32. MeB Analysis of Cuttings Grind Cuttings Suspend Powder 1. 2. Add MeB Increments Detect Excess MeB 3. 4. from: Gunderson, Cumming, Astra and Harvey (2000)

  33. El TatioSchlumbergerProfiling1973 Lahsen and Trujillo (1976)

  34. La Torta Conceptual Cross-section with MT Resistivity Cumming, Vieytes, Ramirez and Sussman (2002)

  35. La Torta3D MT Resistivity Structure (Elevation of base of clay) Cumming, Vieytes, Ramirez and Sussman (2002)

  36. Gravity • 1 or 2 people • Scintrex automatic meter reduced error compared to L&R (which are OK) • dGPS reduced cost and error by half • Responds to rock density variation, mainly related to rock porosity. • Interpreted for lithology, structure and alteration. Cumming Geoscience

  37. Gravity Interpretation • Density in geothermal exploration models is determined by porosity and, to a lesser extent, mineral grain density. • Pore fluid changes detected by precision gravity for development monitoring are usually insignificant in exploration surveys. • 2D interpretations focus on lithology, structure and alteration. • Large, shallow density contrasts overwhelm subtle ones so sinter may be undetectable near lava domes surrounded by pumice tuff. • Use top-down interpretation in models because the gravity effect of a deeper density contrast is more spread out and indistinct and, more importantly, rock density contrasts decrease with depth: • At 100 m, 30°C, lava can be 2.7 and tuff 1.1 g/cm3 • At 1000 m, 250°C, lava can be 2.7 and tuff 2.4 g/cm3 • Contrast at 100 m is ~10 times larger than at 1000 m. • Because of its greater ambiguity, gravity is often more effective in extending models developed using sounding methods like MT. Cumming Geoscience

  38. Gravity Bradys Hot Springs and Desert Peak Interpretation from Oppliger, 7 May 03

  39. SP • Self Potential (SP) profiling measures voltage across a dipole to map V/m. • Low cost; requires 2 people with wire, volt-ohmmeter and electrodes. • SP pattern mainly reflects electro-kinetic effect, water flow in shallowest aquifer. • In geothermal prospects, thermo-electric effect is significant but ambiguous. • SP “anomalies” may indicate faults, or aquifer geometry. Cumming Geoscience

  40. SP • Case histories show SP can characterize upflow and shallow outflow aquifers in areas with gentle topography. • Near-surface groundwater signal is strongest so even rainfall significantly changes SP patterns. • Cost is relatively low but so is relevance, especially for deeper resources. • SP mainly used to characterize shallow low temperature systems. Mokai Cumming Geoscience Hochstein et al., 1990

  41. Magnetic Surveys • Map local variations in earth’s magnetic field that, in volcanics, correlate with magnetite content • Aeromagnetic survey: magnetometer in plane • Draped is better, constant elevation is easier • Used to: 1) map structure and lithology; and 2) characterize extent of alteration, especially related to SO4 destruction of magnetite • Ground magnetic survey: 1 person walks profiles • Proton precession magnetometer usually saturated and under-sampled near volcanics • Cesium-vapor magnetometer data every 50 cm using dGPS can map near-surface geology. Cumming Geoscience

  42. Cost for Geophysics Includes acquisition & some imaging but not integrated interpretation. MT <0.05 to >300 Hz Low cost: Sites < 500 m from vehicle. < 1 hr to easy camp, etc. High cost: >30% sites > 1 km from vehicle. > 1 hr to camp, etc. MethodCost / data unit Mob & misc MT $1k - $3k / MT $5k - $30k T- MT $0.2k - $1.2k / T $8k - $35k T-MT T-MT Profile $4k - $10k / line km $5k - $45k CSAMT $2k - $6k / line km $3k - $30k TDEM $0.2k - $0.6k / TDEM $3k - $15k Gravity+dGPS $30 - $90 / station $3 - $15k Cumming Geoscience

  43. Geophysical Exploration of <180°C Geothermal Systems Can geophysics be both useful and low cost. • Yes, if • Production aquifer is <500 m deep. • Method is matched to the situation; e.g. TDEM for <500 m, SP in gentle terrain. • Deep inferred from shallow Cumming Geoscience

  44. Geophysics Uncertainty in Geothermal Exploration MT -TDEM can image the base of the clay cap conforming to the top of the reservoir for most geothermal reservoirs >140°C but • Although the apex of this structure is often the shallowest permeability and sometimes becomes a steam cap, it is sometimes tight and it is often not located over the deep high-temperature upflow. • MT might not be the most cost-effective approach for shallow resources, especially for low-temperature cases. so • Check conceptual advantages of other methods • Integrate with geochemistry and geology • Drill a conceptual model, NOT an anomaly Cumming Geoscience

  45. Value of Information • Use case-oriented decision trees to estimate: • Value of resource based on risk weighted ENPV • Value of new information through its affect on case probabilities • 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 redundantly affect the same resource probabilities and how does it compare with respect to the above questions? Cumming Geoscience

  46. Geothermal Geophysics Interpretation Pitfall Example MT Observation • MT resistivity cross-section contours often appear to define a low resistivity zone extending near-vertically below 500 m depth. Interpretation Pitfall • Vertically trending low resistivity zones at >500 m depth are commonly misinterpreted as evidence of deep reservoir structural permeability Issue • Flaws in MT processing commonly produce false vertically-oriented low resistivity zones at depth. • Static distortion, noise, and inconsistent station projection are the most common problems. Recognition • Large contrasts in resistivity over large depth ranges at adjacent stations suggest a statics problem. Check for a split between MT apparent resistivity curves at high frequency. • Check for noise in the apparent resistivity and phase curves for stations near the vertical feature. • 2D inversions can be distorted when MT stations are projected onto the profile being imaged so that their relative geometry is not preserved. Remedies • Correct statics using TDEM, smoothing inversions or surface geology consistency. • Edit noise so that it does not bias the inversion to low resistivity at depth • Correct inconsistent station projections. • Reliable imaging of resistivity is usually relatively smooth horizontally so be skeptical when interpreting near vertical resistivity contours. • Review the plausibility of resistivity values with respect to realistic reservoir properties. Cumming Geoscience

  47. Geothermal Geophysics Interpretation Pitfall Example • Vertical contours in MT cross-section show deep low resistivity in red • Erroneously interpreted as reservoir fault zone • MT imaging of resistivity distorted by: • noise near station 1 • static at station 2 • MT cross-section without distortion shows classic geothermal cap geometry Cumming Geoscience

  48. Geothermal Geophysics Research Topics • Reflection Seismic • Earthquake Tomography • Velocity • Attenuation • S-wave splitting • 3D Integrated Resistivity Cumming Geoscience

  49. Reflection Seismic • Dominates petroleum exploration • However, $ billions in petroleum seismic research have not solved problems with: • P attenuation by shallow gas like CO2 in clay • Shallow dense rocks like lavas • Statics due to rugged topography with rapid seismic velocity changes (like lavas and tuffs) • Resolving closely spaced deep structures • Lack of rock contacts that coherently reflect • S-conversion interference • So MT, gravity etc used by oil companies Cumming Geoscience

  50. Geothermal Reflection Seismic • Goal is usually to image permeable zones • Clay cap and possibly reservoir imaged by refraction tomography with resolution usually poorer than resistivity and cost that is higher • Reservoir volume imaged by reflection seismic in the sense that it is usually a “no data” zone • Large scale structural setting of fields imaged • Few reservoir faults or entries imaged • Therefore, still a research topic for geothermal exploration applications • Potential development applications such as field-margin injection well targeting would be more cost-effective if acquisition cost was reduced. Cumming Geoscience

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