Science-geothermal Surface Exploration
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Geothermal surface exploration
Thráinn Fridriksson ÍSOR – Iceland GeoSurvey
Introduction • The objective is to obtain as much information about the properties of the geothermal system as possible, prior to drilling • Successful surface exploration will reduce the cost of later stages in the development and thus 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
Surface exploration can provide information 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
Components of geothermal surface exploration • Geological mapping • Geophysical exploration • Geochemical exploration
Geological mapping • Volcanic history • Defines the active and extinct geothermal manifestations • Structural control over fluid flow in the subsurface • Risk assessment
Geological mapping
Objectives of geophysical surveys • To obtain information about: – Temperature – Permeability – Porosity – Structure and size of the geothermal system – Etc.
Fundamental/Obtainable Parameters
Temperature
Temperature
Resis -tivity
Magnet ization
X
X
(x)
Porosity
X
Permeability
X
Density
X
Pressure
X
Mineralogy
X
Texture
Streaming Pot.
Seismicity
(x)
X
X
X
X
X
(x)
Fluid Chemistry
Grain Density
Seismic Velocity
X
X X
X X
X
Evaluation of Surface Geophysical Methods Resolut. at depth
Iterpret. ambigu.
Equipm. cost
Labour
Diagnost.
Cost efficiency
Surface thermal mapping
Low
Low
Low
Low
High
Med – High
Electrical methods (resistivity, IP)
Med.
Low
Med. – High
Low – Med.
High
High
Magnetics
Low – Med.
High
Low – Med.
Low
Low – Med.
Med.
Gravity
Low – Med.
High
High
High
Low
Low
Active seismics (reflect./refract.)
High
Low
High
High
Low – Med
Low
Self-Potential
Low
High
Low
Low
High
Med.
Passive seismics (micro earthq.)
High
Low
High
High
Med – High
Med.High
Resistivity is the most diagnostic parameter
Main Electrical Methods •DC-methods (Schlumberger, Profiling Dipole-Dipole)
•TEM-methods (Central-Loop TEM, LOTEM)
•MT (Natural source, Controlled source)
Alteration mineralogy at different temperatures 50°C
-
100°C
-
230°C 250°C 200°C
Thermal Alteration starts Thermal Alteration prominent
Smectite
Zeolites
Smectite
Zeolites disappear
Dominant
S - Ch Mixed layered clay Chlorite Chlorite
Epidote
Dominant
CONDUCTIVITY OF ALTERATION MINERALS CHLORITE
SMECTITE
E +
+
+
+
+
E
CONDUCTIVE
MOBILE CATIONS
E E
RESISTIVE
BRUCITE LAYER
NG-7
NJ-11
400 m a.s.l.
NG-10
Nesjavellir 200 -
50 0-
100 - 200 -
150 200
- 400 -
250
- 600 -
0
200
500 Temperature°C
1000 Resistivity > 25 Ωm 10 - 25 Ωm 2 - 10 Ωm low resistivity cap High resistivity core
1500 Alteration
2000 m
Unaltered rocks Smectite - zeolite zone Mixed layered clay zone Chlorite zone Chlorite-epidote zone
Objectives of geochemical surveys • To obtain information about: – Temperature in the reservoir – Chemical composition of fluids – Source of fluids – Active upflow zones – Etc.
Geochemical methods • • • • •
Water chemistry Steam chemistry Stable isotope methods Soil chemical anomalies Soil diffuse degassing
Water classification Legend Title
Cl 0 10
0
Katwe, cold water, dilute Katwe, cold water, saline Katwe, cold water, brackish Katwe, hot spring water Buranga, cold water, dilute
25
.
Kibiro, cold water, dilute
TE
RS
Kibiro, cold water, brackish
MATURE WATERS
WA
Kibiro, hot spring water
75
Buranga, hot spring water
AN LC VO
75
LW
25
RA
HCO3
HE
SO4
RI P
IC
50
50
PE
Cl
AT ER
25
0
10 0
S
SO4 0
STEAM HEATED WATERS 50
75
100
HCO3
Chemical geothermometers • Temperature sensitive reactions control concentrations and concentration ratios of chemical components in water solutions and gases • This allows evaluation of subsurface temperatures based on fluid and gas compositions • Chemical geothermometers implicitly assume that equilibrium was attained in the reservoir and no reactions occurred during the upflow
Chemical geothermometers • 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 or condensation – Equilibrium and rate conditions limiting
Chemical geothermometers: equilibrium controlling CO2 concentration in fluids at Reykjanes, SW Iceland
1 a czo = 1 a re = p
0.17 a czo = 0.80 a re = p
CO2 buffer reaction 2 clinozosite + 2 calcite + 3 quarz + 2 H2O = 3 prehnite + 2 CO2
Chemical geothermometers • Best to use as many geothermometers as possible (complete analyses) • Discrepancies between results of different geothermometers may provide important information about the nature of the system – e.g. extent and distribution of condensation and interactions with cold groundwater
Geochemical field work
CO2-temperature map of Torfajökull geothermal system, Central Iceland
Multiple equilibria geothermometry
Soil diffuse degassing
Soil diffuse degassing studies • Identify upflow zones and active faults • Allow evaluation of natural heat loss from the system
Soil diffuse CO2 flux at Reykjanes, SW Iceland Results of soil diffuse degassing survey were used to site a directionally drilled well.
The well, RN-23, is now the best, by far, in the area
Soil diffuse degassing: Krafla, N Iceland Active faults?
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 dynamic approach important • Cost-efficiency should be considered • Successful surface exploration will save big money when project enters development phase
Thank you for the attention!
DC-method (Schlumberger)
TEM-method (central-loop)
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:
Thráinn Fridriksson ÍSOR – Iceland GeoSurvey
Introduction • The objective is to obtain as much information about the properties of the geothermal system as possible, prior to drilling • Successful surface exploration will reduce the cost of later stages in the development and thus 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
Surface exploration can provide information 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
Components of geothermal surface exploration • Geological mapping • Geophysical exploration • Geochemical exploration
Geological mapping • Volcanic history • Defines the active and extinct geothermal manifestations • Structural control over fluid flow in the subsurface • Risk assessment
Geological mapping
Objectives of geophysical surveys • To obtain information about: – Temperature – Permeability – Porosity – Structure and size of the geothermal system – Etc.
Fundamental/Obtainable Parameters
Temperature
Temperature
Resis -tivity
Magnet ization
X
X
(x)
Porosity
X
Permeability
X
Density
X
Pressure
X
Mineralogy
X
Texture
Streaming Pot.
Seismicity
(x)
X
X
X
X
X
(x)
Fluid Chemistry
Grain Density
Seismic Velocity
X
X X
X X
X
Evaluation of Surface Geophysical Methods Resolut. at depth
Iterpret. ambigu.
Equipm. cost
Labour
Diagnost.
Cost efficiency
Surface thermal mapping
Low
Low
Low
Low
High
Med – High
Electrical methods (resistivity, IP)
Med.
Low
Med. – High
Low – Med.
High
High
Magnetics
Low – Med.
High
Low – Med.
Low
Low – Med.
Med.
Gravity
Low – Med.
High
High
High
Low
Low
Active seismics (reflect./refract.)
High
Low
High
High
Low – Med
Low
Self-Potential
Low
High
Low
Low
High
Med.
Passive seismics (micro earthq.)
High
Low
High
High
Med – High
Med.High
Resistivity is the most diagnostic parameter
Main Electrical Methods •DC-methods (Schlumberger, Profiling Dipole-Dipole)
•TEM-methods (Central-Loop TEM, LOTEM)
•MT (Natural source, Controlled source)
Alteration mineralogy at different temperatures 50°C
-
100°C
-
230°C 250°C 200°C
Thermal Alteration starts Thermal Alteration prominent
Smectite
Zeolites
Smectite
Zeolites disappear
Dominant
S - Ch Mixed layered clay Chlorite Chlorite
Epidote
Dominant
CONDUCTIVITY OF ALTERATION MINERALS CHLORITE
SMECTITE
E +
+
+
+
+
E
CONDUCTIVE
MOBILE CATIONS
E E
RESISTIVE
BRUCITE LAYER
NG-7
NJ-11
400 m a.s.l.
NG-10
Nesjavellir 200 -
50 0-
100 - 200 -
150 200
- 400 -
250
- 600 -
0
200
500 Temperature°C
1000 Resistivity > 25 Ωm 10 - 25 Ωm 2 - 10 Ωm low resistivity cap High resistivity core
1500 Alteration
2000 m
Unaltered rocks Smectite - zeolite zone Mixed layered clay zone Chlorite zone Chlorite-epidote zone
Objectives of geochemical surveys • To obtain information about: – Temperature in the reservoir – Chemical composition of fluids – Source of fluids – Active upflow zones – Etc.
Geochemical methods • • • • •
Water chemistry Steam chemistry Stable isotope methods Soil chemical anomalies Soil diffuse degassing
Water classification Legend Title
Cl 0 10
0
Katwe, cold water, dilute Katwe, cold water, saline Katwe, cold water, brackish Katwe, hot spring water Buranga, cold water, dilute
25
.
Kibiro, cold water, dilute
TE
RS
Kibiro, cold water, brackish
MATURE WATERS
WA
Kibiro, hot spring water
75
Buranga, hot spring water
AN LC VO
75
LW
25
RA
HCO3
HE
SO4
RI P
IC
50
50
PE
Cl
AT ER
25
0
10 0
S
SO4 0
STEAM HEATED WATERS 50
75
100
HCO3
Chemical geothermometers • Temperature sensitive reactions control concentrations and concentration ratios of chemical components in water solutions and gases • This allows evaluation of subsurface temperatures based on fluid and gas compositions • Chemical geothermometers implicitly assume that equilibrium was attained in the reservoir and no reactions occurred during the upflow
Chemical geothermometers • 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 or condensation – Equilibrium and rate conditions limiting
Chemical geothermometers: equilibrium controlling CO2 concentration in fluids at Reykjanes, SW Iceland
1 a czo = 1 a re = p
0.17 a czo = 0.80 a re = p
CO2 buffer reaction 2 clinozosite + 2 calcite + 3 quarz + 2 H2O = 3 prehnite + 2 CO2
Chemical geothermometers • Best to use as many geothermometers as possible (complete analyses) • Discrepancies between results of different geothermometers may provide important information about the nature of the system – e.g. extent and distribution of condensation and interactions with cold groundwater
Geochemical field work
CO2-temperature map of Torfajökull geothermal system, Central Iceland
Multiple equilibria geothermometry
Soil diffuse degassing
Soil diffuse degassing studies • Identify upflow zones and active faults • Allow evaluation of natural heat loss from the system
Soil diffuse CO2 flux at Reykjanes, SW Iceland Results of soil diffuse degassing survey were used to site a directionally drilled well.
The well, RN-23, is now the best, by far, in the area
Soil diffuse degassing: Krafla, N Iceland Active faults?
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 dynamic approach important • Cost-efficiency should be considered • Successful surface exploration will save big money when project enters development phase
Thank you for the attention!
DC-method (Schlumberger)
TEM-method (central-loop)
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:
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