Science-Geothermal Surface Exploration

7/25/2019 Science-Geothermal Surface Exploration

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Geothermal surface exploration

Thrinn Fridriksson

SOR Iceland GeoSurvey


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Introduction The objective is to obtain as much information

about the properties of the geothermal systemas possible, prior to drilling

Successful surface exploration will reduce the

cost of later stages in the development andthus save a lot of money in the end Geothermal surface exploration is a

multidisciplinary task Exploration strategy depends on many factors,including geological setting and temperature in

the system


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Surface exploration can provideinformation on:

temperature in the geothermal reservoir permeability of the reservoir

areal extent of the thermal anomaly

depth do useful temperatures location of the upflow zone

chemical composition of fluid


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Components of geothermal surfaceexploration

Geological mapping Geophysical exploration

Geochemical exploration


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Geological mapping

Volcanic history Defines the active and extinctgeothermal manifestations

Structural control over fluid flow in thesubsurface

Risk assessment


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Geological mapping


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Objectives of geophysical surveys

To obtain information about: Temperature Permeability

Porosity

Structure and size of the geothermal

system Etc.


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Fundamental/Obtainable Parameters

Temp-

erature

Resis

-tivity

Magnet

ization

Density Seismic

Velocity

Stream-

ing Pot.

Seism-

icity

Tempera-

ture

X X (x) (x) X X

Porosity X X X

Permea-

bilityX X X

Pressure (x)Fluid Che-

mistryX

Mineralogy X X X

Grain

DensityX X

Texture X X


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Evaluation of Surface Geophysical Methods

Resolut.

at depth

Iterpret.

ambigu.

Equipm.

cost

Labour Diagnost. Cost

efficiency

Surface thermal

mapping

Low Low Low Low High

High

Low

Med.Low

Low

MedHigh

Med

High

Med

High

Electrical methods

(resistiv ity, IP)

Med. Low Med.

High

Low

Med.

High

Magnetics Low

Med.

High Low

Med.

Low Med.

Gravity Low

Med.

High High High Low

Active seismics

(reflect./refract.)

High Low High High Low

Self-Potential Low High Low Low Med.

Passive seismics

(micro earthq.)

High Low High High Med.-

High


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Resistivity is the most diagnostic parameter

Main Electrical MethodsDC-methods(Schlumberger, Profiling Dipole-Dipole)

TEM-methods(Central-Loop TEM, LOTEM)

MT(Natural source, Controlled source)


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-50C

100C

200C

230C

250C

Thermal Alteration starts

Thermal Alteration prominent

Smectite Zeolites Dominant

Smectite Zeolites disappear

S - Ch Mixed layered clay

Chlorite

Chlorite Epidote Dominant

-

-

-

-

Alteration mineralogy at different temperatures


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+ + + + +

E

E

E

E

SMECTITECHLORITE

MOBILECATIONS

BRUCITELAYER

CONDUCTIVE RESISTIVE

CONDUCTIVITY OF ALTERATION MINERALS


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m a.s.l.400 -

200 -

- 200 -

- 400 -

- 600 -

0 -

0 500 1000 1500 2000 m

ResistivityTemperatureC

Alteration

Unaltered rocks

Smectite - zeolite zone

Mixed layered clay zone

Chlorite zoneChlorite-epidote zone

> 25 m

10 - 25 m

2 - 10 m low resistivity cap

High resistivity core

250

200

200

150

100

50

NJ

-11

NG

-7

NG

-10

Nesjavellir


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Objectives of geochemical surveys

To obtain information about: Temperature in the reservoir Chemical composition of fluids

Source of fluids

Active upflow zones

Etc.


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Geochemical methods

Water chemistry Steam chemistry

Stable isotope methods Soil chemical anomalies

Soil diffuse degassing


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Water classification

0 25 50 75 100

25

50

75

100

0

0

25

50

100

75

Cl

SO4 HCO3

Cl

SO4 HCO3

Legend Title

Katwe, cold water, dilute

Katwe, cold water, saline

Katwe, cold water, brackish

Katwe, hot spring water

Buranga, cold water, dilute

Buranga, hot spring water

Kibiro, cold water, dilute

Kibiro, cold water, brackish

Kibiro, hot spring water

VOLC

ANIC

WAT

ERS

STEAM HEATED WATERS

MATUREWATERS

PERIPHERA

LWATERS

.


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Chemical geothermometers

Temperature sensitive reactions controlconcentrations and concentration ratios ofchemical components in water solutions andgases

This allows evaluation of subsurfacetemperatures based on fluid and gascompositions

Chemical geothermometers implicitly assumethat equilibrium was attained in the reservoirand no reactions occurred during the upflow


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Univariant: e.g. SiO2, CO2, H2S, H2 etc. Simple Sensitive to secondary changes such as

dilution, steam loss and condensation. Ratios: e.g. Na/K, CO2/H2, CO2/Ar etc.

Not as susceptible to dilution orcondensation

Equilibrium and rate conditions limiting

C


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Chemical geothermometers:equilibrium controlling CO2 concentration

in fluids at Reykjanes, SW Iceland

CO2 buffer reaction2 clinozosite + 2 calcite + 3 quarz + 2 H2O

= 3 prehnite + 2 CO2

aczo=1

apre=1

aczo=0.17

apre=0.80


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Best to use as many geothermometersas possible (complete analyses)

Discrepancies between results ofdifferent geothermometers may provideimportant information about the nature

of the system e.g. extent and distribution of condensation

and interactions with cold groundwater


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Geochemical field work


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CO2-temperature map of Torfajkullgeothermal system, Central Iceland


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Multiple equilibria geothermometry


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Soil diffuse degassingSoil diffuse degassing


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Soil diffuse degassing studies

Identify upflow zones and active faults Allow evaluation of natural heat lossfrom the system

Soil diffuse CO2 flux at Reykjanes SW


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Soil diffuse CO2 flux at Reykjanes, SWIceland

Results of soildiffuse degassingsurvey were used to

site a directionallydrilled well.

The well, RN-23, isnow the best, by far,

in the area


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Soil diffuse degassing: Krafla, N Iceland

Active faults?


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CONCLUSIONS

Geothermal exploration is a multidisciplinary

task (geology, geochemistry, geophysics) No single method universally superior, but

electrical methods and chemical

geothermometry usually important Integrated multi-method and dynamicapproach important

Cost-efficiency should be considered Successful surface exploration will save bigmoney when project enters development

phase


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Thank you

for the attention!


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DC-method(Schlumberger) TEM-method (central-loop)


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EXPLORING HIGH TEMPERATURE FIELDS

Electrical soundings with TEM method are at present the most

effective exploration method for high temperature fields.

In winter on snow scooters: In summer by helicopter:

Science-Geothermal Surface Exploration

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