RESISTIVITY OF ROCKS - the ARGeo
Transcript of RESISTIVITY OF ROCKS - the ARGeo
Sustainable Development Goals Short Course IV
on Exploration and Development of Geothermal Resources
Ásdís Benediktsdóttir, Geophysicist, ÍSOR
Knútur Árnason, Geophysicist, ÍSOR
RESISTIVITY OF ROCKS
November 20th 2019
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
Why do we care about resistivity?
It tells us something about the reservoir-conditions in the Earth, without drilling.
Direct relationship between temperature and resistivity.
In high-temperature areas of volcanic origin alteration minerals dominate the resistivity structure.
m a.s.l.400 -
200 -
- 200 -
- 400 -
- 600 -
0 -
0 500 1000 1500 2000 m
ResistivityTemperature°C
Alteration
Unaltered rocks
Smectite - zeolite zone
Mixed layered clay zone
Chlorite zone
Chlorite-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
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
What is resistivity?
Electrical resistivity and its inverse, electrical conductivity, is a fundamental
property of a material that quantifies how strongly it resists or conducts electrical
current.
Units
Resistivity: Ωm (ohm meter)
Conductivity: S/m (siemens per meter)
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
Ohm´s law:
U = R*I
U potential difference [V]R resistance [Ω]I current [A=C/s]
Electric field:E = U/L = dU/dx [V/m]Current density:J = I/S [A/m2]
E*L = R*S*J => E = R*S/L*J
E = ρ*J (Ohm’s law)
Resistivity:
ρ = R*S/L [Ωm]
Ohm’s law and resistivity
SDG Short Course IV on Exploration and Development of Geothermal Resources
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Conductance:
S = 1/R [Ω-1=S]
Conductivity:
σ = 1/ρ [S/m]
J = σ*E (Ohm’s law)
R and S depend on size and shapeρ and σ are intrinsic propertiesindependent of size and shape.
Serial resistors Parallel resistors
R = R1 +R2 S = S1 +S2
1/R = 1/R1 + 1/R2
Ohm’s law and resistivity
SDG Short Course IV on Exploration and Development of Geothermal Resources
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ρ = ρw*a*φ-n
ρ bulk resistivity, ρw pore fluid resistivity, φ fractional porosity, (0 < φ < 1)
a and n are empirical factors (a ~ 1, n ~ 2)
From the oil industry and works well for sandstone saturatedwith relatively saline water
Conductivity σw increases (resistivity decreases) with increasingtemperature and salinity
σw = 1/ ρw = 1/ρw0*(1. + 0.023*(T - 23)) for T < 200°C σw = k*C for moderate salinities C
Archie’s law
SDG Short Course IV on Exploration and Development of Geothermal Resources
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As a function of temperature As a function of salinity
Resistivity of electrolytes
SDG Short Course IV on Exploration and Development of Geothermal Resources
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Conclusion
The higher the temperature, the lower the resistivity!
But!!!!
OLD ASSUMPTIONS
− Archie’s law approximately valid
− Pore fluid conduction dominates
− Concentration of dissolved ions increases with temperature
− The resistivity of electrolytes decreases with temperature
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
Ap
pare
nt
resis
tivity [
m]
Ap
pare
nt
resis
tivity [
m]
Ö ´ t 1000
AB/2 [m]
Schlumberger soundingTEM sounding
Contradicting evidence
SDG Short Course IV on Exploration and Development of Geothermal Resources
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A PUZZLE
− Data show very low resistivity at a certain depth range in the geothermal systems.
− At greater depth and at higher temperatures the the resistivity increases considerably again.
− What is the explanation of this?
− What is controlling the resistivty?
− How can we distinguish between resistive cold rocks and resistive hot rocks?
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
m a.s.l.400 -
200 -
- 200 -
- 400 -
- 600 -
0 -
0 500 1000 1500 2000 m
ResistivityTemperature°C
Alteration
Unaltered rocks
Smectite - zeolite zone
Mixed layered clay zone
Chlorite zone
Chlorite-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
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
Conduction mechanisms
Negatively charged mineral surface
Ions in solution
Loosely bound cations
Host rock
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Alterationminerals
Four contributions:
1. Conduction in the rock matrix (normally negligible)
2. Conduction by dissolved ions in the pore fluid (Archie’s law)
3. Conduction by adsorbed ions on the pore surface
4. Conduction in alteration minerals
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
Which conductionmechanism dominatesdepends on salinity ofthe pore fluid, theamount and type ofalteration minerals.
Conduction mechanisms –solution to the puzzle
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
Resistivity as a function of pore fluid resistivity; φf is the fracture porosity
Conduction mechanisms
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
Resistivity as a function of temperature
Conduction mechanisms
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
The solution of the puzzle
− In the high-temperature geothermal systems water-rock interaction produces alteration (secondary) minerals.
− The type of minerals formed depends on the chemical composition of the host-rocks and pore fluid and temperature.
− The amount of alteration depends on time, permeability and the chemical composition and texture of the rocks.
− Some of the alteration minerals are highly conductive others are less conductive or resistive.
The role of alteration
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
-50°C
100°C
200°C
230°C
250°C
Thermal Alteration starts
Thermal Alteration prominent
Smectite Zeolites Dominant
Smectite Zeolites disappear
S - Ch Mixed layered clay
Chlorite
Chlorite Epidote Dominant
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Alteration mineralogy at different temperatures
The role of alteration
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
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SMECTITE CHLORITE
MOBILE
CATIONS
BRUCITE
LAYER
CONDUCTIVE RESISTIVE
CONDUCTIVITY OF ALTERATION MINERALS
The role of alteration
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
m a.s.l.400 -
200 -
- 200 -
- 400 -
- 600 -
0 -
0 500 1000 1500 2000 m
ResistivityTemperature°C
Alteration
Unaltered rocks
Smectite - zeolite zone
Mixed layered clay zone
Chlorite zone
Chlorite-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
The role of alteration
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
The role of alteration (cont.)
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> 260m
Dep
th(m
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Resistivity
Alteration
Smectite-zeolitezone
Mixedlayeredclayzone
Chloritezone
Chlorite-epidotezone
Temperature
> 260m1.5- 2.3m2.6m
250°C
200°C
150°C
100°C
100°C
ASAL-4 TEM-soundingDJ-11
1.5m
2.3mWATERTABLE
2.6m
Asal-RiftDjibouti
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
•
− Field and laboratory data show that the general resistivity structure is controlled by the degree and type of alteration.
− In unaltered rocks the conductivity is mainly in the pore fluid.
− In altered rocks the conductivity is dominated by mineral and/or surface conduction, except for cases of extremely saline pore fluid.
− Details depend on types of minerals and porosity ( ρ = b*φ-m ; with different b)
The role of alteration
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
The general resistivity structure
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
− If the host rocks are relatively homogenous and alteration mineralogy is in equilibrium with temperature YES.
− If the rocks are very inhomogeneous (inter-bedded sediments) or alteration not in equilibrium with temperature NO.
− If the system cools down, the alteration remains.
− The resistivIty is a sort of maximum thermometer.
Is the resistivity a thermometer?
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
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l
l l l l l
500
m a.s.l
0
0 1000 2000 3000 4000 5000m
- 500
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KG
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Caldera
rim
Alteration
Resistivity
> 100 m10 - 100 m
1 - 6 m low resistivity capHigh resistivity core
250
TEM sounding
Temperature °C
Unaltered rocks
Smectite - zeolite zone
Mixed layered clay zone
Chlorite zone
Chlorite-epidote zone
200
250
300
Krafla
An example of cooling
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
•
SUMMARY
− Electrical conduction in rocks is a complex phenomenon
− Different conduction mechanisms contribute:
− Rock matrix conduction (negligible)
− Pore fluid conduction
− Alteration mineral conduction
− Surface conduction
− When one conduction mechanism dominates:
ρ = k*φ-m
k decreases with temperature
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
SUMMARY (CONT.)
− Pore fluid conduction dominates in unaltered rocks.
− When alteration minerals are present, mineral and/or surface conduction dominate, except in the case of extremely saline fluids.
− Different alteration minerals have different conductivities.
− Different minerals formed at different temperatures.
− If equilibrium between temperature and alteration then the resistivity structure reflects the temperature.
− If cooling has occurred, the resistivity is a maximum thermometer.
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
Conduction mechanisms
Negatively charged mineral surface
Ions in solution
Loosely bound cations
Host rock
-
-
+
-
-
-
--
+
+
+
+
+ +
+ +
+
+
+
+
+
+ +
++
+
+
Alterationminerals
Four contributions:
1. Conduction in the rock matrix (normally negligible)
2. Conduction by dissolved ions in the pore fluid (Archie’s law)
3. Conduction by adsorbed ions on the pore surface
4. Conduction in alteration minerals
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
Which conductionmechanism dominatesdepends on salinity ofthe pore fluid, theamount and type ofalteration minerals.
Conduction mechanisms –solution to the puzzle
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
The general resistivity structure
SDG Short Course IV on Exploration and Development of Geothermal Resources
Organized by UNU-GTP and KenGen | Lake Bogoria and Lake Naivasha | Nov 13-Dec 3, 2019
m a.s.l.400 -
200 -
- 200 -
- 400 -
- 600 -
0 -
0 500 1000 1500 2000 m
ResistivityTemperature°C
Alteration
Unaltered rocks
Smectite - zeolite zone
Mixed layered clay zone
Chlorite zone
Chlorite-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
The role of alteration