Cumming Grc Exploration Geophysics 2006-09-09

Geophysical Exploration for Geothermal Resources by

William Cumming Cumming Geoscience, Santa Rosa CA [email protected]

Cumming Geoscience

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

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

Geothermal Development Characteristics Affecting Geophysics For >210°C • • •

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 Reservoir thickness 300 to 3000 m Testable wells usually >$1.5 million Commercial wells usually >$3 million

For <180°C • • •

Production by pumping hot water Binary generation 100% injection

• • • •

Reservoir top usually 100 to 500 m deep Reservoir thickness 100 to 1000 m Testable wells usually >$0.5 million Commercial wells usually $1 to $2 million

Issue

Deeper Thicker Wells cost more

Issue

Shallower Thinner Wells cost less

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

Acid Sulphate Fumarole

Chloride Spring

Unaltered Sm

ite ect

y Cla

Basin Clays

Propylitic Alteration in Fractured Geothermal Reservoir

Heat and Gas from Magma

Isotherms 1 KM 1:1

after Cumming et al. 2000

Cumming Geoscience

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

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

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

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

Geophysical Techniques in 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

Geophysical Techniques in Geothermal Exploration “Special”

“Standard”

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

“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

Acid Sulphate Fumarole

Chloride Spring

Unaltered Sm

eC ctit

lay

e

Basin Clays

Propylitic Alteration in Fractured Geothermal Reservoir

Heat and Gas from Magma

Isotherms 1 KM 1:1 Cumming, 2006

“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

MT Objectives in Geothermal Exploration 1. Map structure and conductance of <180°C low resistivity smectite clay zone capping the relatively resistive >200°C propylitic reservoir 2. Integrate with geochemistry and geology to – Estimate resource capacity – Target wells for high temperature permeability Cumming Geoscience

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

MT Physics

Geophys.washington.edu

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

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

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

Cumming Geoscience

T-MT Profiling

Quantech, 2003

T-MT Profiling • Continuous line of T stations with one MT station • 100 m spacing used in minerals is seldom costeffective 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

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 Geoscience

Cumming, 2003

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

TDEM Survey Types

From: MINDECO

TEM at Krafla • Detects base of clay • Maps reservoir top • MT not needed • Shallow reservoir • 300 to 1000 m loops

• Cost >$600/station • 1 to 3 stations/day • Geonics Protem / EM37 Cumming Geoscience

From: Arnason et al 2000

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

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.

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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

VES and Dipole-dipole Resistivity at Cerro Prieto

Charre-Meza et al 2000

Resistivity Imaging in Geothermal Exploration 1. Map base and conductance of low resistivity clay zone capping relatively resistive reservoir 2. Integrate with geochemistry and geology to – Estimate resource capacity – Target wells for high temperature permeability

Awibengkok Geothermal Field MT Cross-section MT Resistivity with MeB Smectite & Isotherms from Wells

1000

-1000

1 Km Cumming Geoscience

from: Gunderson, Cumming, Astra and Harvey (2000)

Karaha Bodas MT (Moore,2006)

Cumming Geoscience

from: Moore (2006)

MeB Analysis of Cuttings Grind Cuttings

1.

Suspend Powder

2.

Add MeB Increments

Detect Excess MeB

3.

4. from: Gunderson, Cumming, Astra and Harvey (2000)

El Tatio Schlumberger Profiling 1973

Lahsen and Trujillo (1976)

La Torta Conceptual Cross-section with MT Resistivity

Cumming, Vieytes, Ramirez and Sussman (2002)

La Torta 3D MT Resistivity Structure (Elevation of base of clay)

Cumming, Vieytes, Ramirez and Sussman (2002)

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

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.

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2D interpretations focus on lithology, structure and alteration.

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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

Gravity Bradys Hot Springs and Desert Peak Interpretation

from Oppliger, 7 May 03

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.

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SP pattern mainly reflects electro-kinetic effect, water flow in shallowest aquifer.

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In geothermal prospects, thermo-electric effect is significant but ambiguous.

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SP “anomalies” may indicate faults, or aquifer geometry. Cumming Geoscience

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

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

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.

Method

Cost / 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

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

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 hightemperature 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

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

Geothermal Geophysics Interpretation Pitfall Example MT Observation • MT resistivity cross-section contours often appear to define a low resistivity zone extending nearvertically 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

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

Geothermal Geophysics Research Topics • Reflection Seismic • Earthquake Tomography

•Velocity •Attenuation

•S-wave splitting • 3D Integrated Resistivity Cumming Geoscience

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

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

Reflection Seismic • Coso reflection seismic section (annotated)

Pullammanappallil et al. 2001 and Unruh et al. 2001

Refraction Seismic • Coso refraction tomography section showing velocity variation in color

Pullammanappallil et al. 2001 and Unruh et al. 2001

Reflection Seismic Rye Patch

Majer and Gritto, 2003

Reflection Seismic Rye Patch LBNL Modeling of Rye Patch

P-wave

Majer and Gritto, 2003

Microearthquakes (MEQ) in Geothermal Exploration •

Numerous conventional (not noise mapping) MEQ exploration surveys had little or no success at wildcat geothermal prospects.

•

Limited exploration successes (Simiyu and Malin, WGC 2000) were on the margins of developed fields.

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Most geothermal fields that have been monitored prior to production are relatively aseismic over a decade or so.

•

Tests to check if this was due to an unusual number of small events relative to larger earthquakes have not found this to be the case.

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After production, most fields that have deep injection have an increase in local earthquakes but several large fields with deep injection and production remain relatively quiet. Most shallow fields remain aseismic.

•

Although MEQ monitoring is a common geothermal development tool at fields where many MEQ’s are detected, the episodic data and high cost make it a risky exploration tool.

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3D MT Resolution Research

Cumming and Mackie, 2003

Cumming Geoscience

New Geophysical Technology Question: •

What investment in geophysical technology is likely to pay out in geothermal exploration?

Past answers and successes: •

Leverage petroleum and mineral technology •

•

University / Lab basic science and method validation • • • •

•

TDEM - MT static correction validated for geothermal MT - 1D and 2D smooth inversion imaging for geothermal MT - remote-reference processing for noise leveraged dGPS - leveraged to make all surveys cheaper and better

Audit geophysics with geology, geochemistry, etc • •

•

MT, TDEM, VES, CSMT, Gravity etc but adaptation crucial

Illite-smectite clay model for resistivity interpretation Top down modeling for gravity and magnetic interpretation

Integrate using case histories and risk assessment •

Cumming Geoscience

Integrate geophysics into resource risk assessment by interpreting in context of likelihood of conceptual model cases

Audit Geophysics with Geology e.g. Joe Moore pointed out lithologic permeability at Bulalo

Litho/Structural Facies Model of Bulalo Reservoir

Moore, 2006

Cumming Geoscience

Geophysical Exploration for Geothermal Resources by

William Cumming Cumming Geoscience, Santa Rosa CA [email protected]

Cumming Geoscience

Discussion Session • JAXA L-band satellite interferometry for deformation • Re-do geological ground truthing of geophysics aided by remote sensing • Gas seep and spring surveys should never be assumed to be complete • Exploring for injection can be different • …

Cumming Geoscience