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



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)



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



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



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



As a function of temperature As a function of salinity

Resistivity of electrolytes



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



Ap

pare

nt

resis

tivity [

m]

Ap

pare

nt

resis

tivity [

m]

Ö ´ t 1000

AB/2 [m]

Schlumberger soundingTEM sounding

Contradicting evidence



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?



m a.s.l.400 -

200 -

- 200 -

- 400 -

- 600 -

0 -

0 500 1000 1500 2000 m


Alteration

Unaltered rocks



Chlorite zone


> 25 m

10 - 25 m



250

200

200

150

100

50

NJ-

11

NG

-7

NG

-10

Nesjavellir



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



Which conductionmechanism dominatesdepends on salinity ofthe pore fluid, theamount and type ofalteration minerals.

Conduction mechanisms –solution to the puzzle



Resistivity as a function of pore fluid resistivity; φf is the fracture porosity




Resistivity as a function of temperature




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



-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

-

-

--

Alteration mineralogy at different temperatures




+ + + + +

E

E

E

E

SMECTITE CHLORITE

MOBILE

CATIONS

BRUCITE

LAYER

CONDUCTIVE RESISTIVE

CONDUCTIVITY OF ALTERATION MINERALS




m a.s.l.400 -

200 -

- 200 -

- 400 -

- 600 -

0 -

0 500 1000 1500 2000 m


Alteration

Unaltered rocks



Chlorite zone


> 25 m

10 - 25 m



250

200

200

150

100

50

NJ-

11

NG

-7

NG

-10

Nesjavellir




The role of alteration (cont.)

0-

100-

200-

300-

400-

500-

600-

700-

800-

900-

1000-

1100-

1200-

1300-

> 260m

Dep

th(m

)

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



•

− 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 general resistivity structure



− 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?



_

_

_

_

_

l

l l l l l

500

m a.s.l

0

0 1000 2000 3000 4000 5000m

- 500

- 1000

- 1500

KG

-8

KG

-10

KJ-

15

KJ-

19

KJ-

20

KJ-

16

KJ-

18

Caldera

rim

Alteration

Resistivity

> 100 m10 - 100 m

1 - 6 m low resistivity capHigh resistivity core

250

TEM sounding

Temperature °C

Unaltered rocks



Chlorite zone


200

250

300

Krafla

An example of cooling



•

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



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.




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



Which conductionmechanism dominatesdepends on salinity ofthe pore fluid, theamount and type ofalteration minerals.

Conduction mechanisms –solution to the puzzle



The general resistivity structure



m a.s.l.400 -

200 -

- 200 -

- 400 -

- 600 -

0 -

0 500 1000 1500 2000 m


Alteration

Unaltered rocks



Chlorite zone


> 25 m

10 - 25 m



250

200

200

150

100

50

NJ-

11

NG

-7

NG

-10

Nesjavellir


resistivity of rocks - the argeo

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