A New Rig for Measuring Electrical Properties of Fluid Saturated Crustal Rocks:Impedance Spectroscopy, AC Voltammetry & Electro-Kinetic Behaviour

Paul W.J. Glover, David Mainprice, Phillippe Pezard, Benoît Ildefonse & Didier LoggiaLaboratoire de Tectonophysique, Université Montpellier II, Place Eugene Bataillon, 34095 Montpellier cédex 05, France

INTRODUCTION MEASUREMENT CELLPerspex Pressure

VesselPerspexSpacers

CurrentConnexion 1

TemperatureLead-through

Swagelock Fittings(Drilled-out)

SMA Gold-PlatedCoaxial Connectors

BNC CoaxialConnectors

Perspex EndClosure

Pt100Sensor

Sample

ConfiningPressure

Inlet

Fluid InletFluid Outlet

VoltageConnexion 1

CurrentConnexion2

VoltageConnexion2

Voltage ElectrodeBlacked Pt Gauze

PTFEEnd-piece

PolyolefinSleeving

FilterPaper

Current ElectrodeBlacked Pt Gauze

PorousDisc

Pump1(Syringe)

Heater

Pump2(Peristaltic)

Fans(x2)

Stirrers

Measurement Cell

Plexiglass Cabinet

Return ToReservoir

Reservoirs

PressureTransducer

PressureTransducer

10 Bar GasConfiningPressure

Pt100

Temperature Controller

FluidPressureLogger

Solartron1260FRA

Solartron1294Impedance

Interface

50 TerminationsΩ

NEW EXPERIMENTAL APPARATUS INITIAL ROCK MEASUREMENTS

MEASUREMENT CELL DESCRIPTION

The cell is constructed entirely from non-conducting materials, and is capable ofmeasuring the electrical and electro-kinetic properties of rocks to a high degree ofprecision using either a 2 or 4 electrode system.

The cell consists of (i) two end-piece assemblies, which are placed at each end ofthe sample, and which support a choice of two or four electrodes, (ii) a samplesleeve, (iii) a pressure vessel, and (iv) a supporting cage.

REFERENCES

Initial tests were carried out on a well indurated sample of feldspathic sandstonewith a porosity of 9% and a permeability of 5.7 mD that was saturated with0.001M NaCl solution.

FURTHER DEVELOPMENTS

Each flat end of the cylindrical sample is in direct contact with an end-pieceassembly, which are available in two core plug diameters (2.54 mm & 3.81mm).They consist of a PTFE cylinder housing a drilled-out1/8" swagelock-1/4" taperedNPT polymer fitting with a hole to enable fluids to pass through the sample. Theother end is machined with radial grooves to facilitate the flow of fluids.

The PTFE also houses connections for two miniature coaxial leads. Insulatedelectrical leads pass through the PTFE cylinder and terminate at each of theelectrode assemblies, which consist of a filter paper-electrode-filter paper sandwich.For a four electrode arrangement, the two electrode assemblies are separated by adisc of porous ceramic 5 mm thick.

50 Ω

The electrodes themselves are discs of platinum gauze (99.995 % purity, 52 mesh,0.1 mm wire thickness from Johnson Matthey) cut from a larger sheet and stabilisedby spot-welding. Each platinum disc was coated with a very high surface area layer

of amorphous black platinum byelectro-deposition from HydrogenHexachloroplatinate solution atapproximately100mA/cm DC.3

The sample and end-pieces aresleeved in transparent polyethyleneheat shrinkable sleeving.

DUMMY TEST MEASUREMENTSThe electrical measurement arrangement has been tested with a range of dummycells such as that shown below from 10 Hz to 32 MHz to ensure that the measuringdevices are operating correctly, and to ensure that electrical noise from the fans andheater elements are negligible.

µ

The results of these tests (examplegiven to the right) shows that highquality noise-free measurementscan be made.

1 k Ω 4.7 k Ω100 Ω

2.2 nF 2.2 nF501 nF

CurrentInput fromGenerator

VoltageSense

1

VoltageSense

2

CurrentMeasurement

0

20

40

60

80

100

120

-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9

log 10 (Freq uen cy (Hz))

InPh

ase

Impe

dan

ce( Ω

)

-60

-50

-40

-30

-20

-10

0

Out

ofPh

ase

Impe

dan

ce( Ω

)

= log 10 (1/2πRC )= 3.18 kHz

PHOTOGRAPH OF THE RIG

The new rig has the capability to make the following MEASUREMENTS andto provide data for APPLICATION to the following scientific problems.

Revil, A. & Glover , P .W. J.

Revil, A. & Glover, P.W. J.

Revil, A., Pezard, P. & Glover, P.W. J.,

Revil, A., Hermitte, D., Spangenberg, E. & Cocheme, J.J.

Jouniaux, L., Bernard, M.-L., Zamora, M. & Pozzi, J.-P

,Theory of ionic surface electrical conduction in porous media, 55(3),1757-1773, 1997.

, Nature of surface electrical conductivity in natural sands, sandstones, and clays,. 25(5), 691-694, 1998.

Streaming potential in porous media. I. Theory of the zeta-potential,.104 (B9), 20021-20031, 1999.

,Electrical properties of zeolitized volcaniclasticmaterials, submitted to

Streaming potential in volcanic rocks from MountPelée, . 105, 8391-8401, 2000.

Phys. Rev. B

Geophys. Res. Lett

J. Geophys. Res

J. Geophys. Res.

J. Geophys. Res

MEASUREMENTSComplex electrical conductivity/admittanceComplex electrical resistivity/impedanceComplex electrical permittivity

Surface ConductionBulk ConductionStreaming Potential & Zeta PotentialElectro-kinetic coupling coefficients

All at multiple frequencies from10 Hz to 32 MHzFluid saturation from pure waterto 2M NaCl equiv. Variable pH.Temperature 20∞C to 60∞CConfining Pressure, 1MPaFluid Pressure +/-1 kPa

µ

APPLICA TIONSFundamental theory of bulk and surface conduction in saturated porous media.The generalisation of this theory to multi-frequency space.Improvement of interpretation and analysis of MT data.Understanding of electrical precursor signals associated with earthquakes.Understanding of varying electrical signals associated with volcanic activity.Fluid flow mapping in the crust using remote electrical tomography.Improved borehole and remote tools for the oil and water industries.Characterisation of sites for nuclear waste storage.

For additional information, please contact: Laboratoire de Tectonophysique, Université Montpellier II, Place Eugène Bataillon, 34095 Montpellier cédex 05, FRANCE

Dr. PaulW.J. Glover (glover@dstu.univ-montp2.fr) Dr. David Mainprice (david@dstu.univ-montp2.fr)Dr. Benoît Ildefonse (benoit@dstu.univ-montp2.fr) Dr. Didier Loggia (loggia@dstu.univ-montp2.fr)

A strong, electrically insulating andwater repellant supporting cagemade from varnished recycledmahogany holds the endcaps ofthe pressure vessel securelyremoving the need for large screw-threads in the polymer.

The experimental rig is attached to a logging PC (not shown in the photo above).

(A) Bode Impedance Frequency Response

(D) Bode Permittivity and Phase Response(C) Complex Impedance Plane

(B) Bode Admittance Frequency Response

0

100

200

300

400

500

600

1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Fre que nc y (Hz)

InPh

ase

Res

istiv

ity( Ω

m)

-300

-250

-200

-150

-100

-50

0

Out

ofPh

ase

Res

istiv

ity( Ω

m)

In P h a s e Re s is tivityTo ta l Re s is tivityOu t o f P h a s e Re s is tivity

0.00

0.02

0.04

0.06

0.08

0.10

0.12

1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Fre que nc y (Hz)

InP

hase

Con

duct

ivity

(S/m

)

0 .00

0.01

0.02

0.03

0.04

0.05

0.06

Out

ofPh

ase

Co

nduc

tivity

(S/m

)

In P h a s e Co n d u c tivity

Ou t o f P h a s e Co n d u c tivity

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01 1.E+0 0 1.E+0 1 1.E+0 2 1.E+0 3 1.E+0 4 1.E+0 5 1.E+0 6 1.E+0 7

Fre que nc y (Hz)Pe

rmitt

ivity

-80

-60

-40

-20

0

20

Phas

eA

ngle

(deg

rees

)

In P h a s e P e rmittivityOu t o f P h a s e P e rmittivityP h a s e An g le

-250

-200

-150

-100

-50

00 100 200 300 400 500

In P h a s e Re s is itiv ity (Ωm )

Out

ofPh

ase

Res

istiv

ity( Ω

m)

The data shows a distributed dispersion curve, centred at 69 kHz and containingdispersion mechanisms covering the range 45 kHz to 106 kHz. There is negligibleelectrode polarisation that is confined to frequencies below 1 Hz, exemplified by thevalues of the out of phase resistivity which are -14.452, -1.458 and -0.230 m at 0.1,1.0 and 10 Hz, respectively, compared to that at the peak dispersion (about 216 m).Hence the measurement is of good quality. Curve-fitting provides a resistivity of therock of 460 m(of which 450 m is dispersive), and a mean capacitance of 50 nF.

ΩΩ

Ω Ω

It is known that the bulk and surface properties of rocks saturatedwith saline pore fluids are extremely sensitive to porosity and themicro-structural distribution of that porosity.

Measurements of the complex electrical properties of rocks as a function offrequency are attibutable directly to the micro-structure of the porositydistribution. However, the detailed theory behind the link is currently stillunknown. Recently the theoretical studies (Revil & Glover, 1997; Revilet al., 1998; 1999) have provided the basic theory for the bulk & surfaceconduction mechanism in a non-frequency dependent form. Generalisation of this theory to take account of frequency dependence is difficult without access to high quality measurements of the complex electrical propertiesof saturated crustal rocks.

Furthermore, the electrical properties of rocks are fundamentally linkedwith their hydraulic properties, i.e. there is a relationship between the permeability of a rock and its conductivity. Such a relationship is calledan electro-kinetic link, and ensures that fluid flows in a rock when anelectric potential is applied OR an electrical current flows when a fluidpressure gradient is applied. The link occurs through the electricaldouble/triple layer that is developed at the matrix/fluid interface, the sameregion that gives rise to surface conduction. The electro-kinetic link isimplicit in the theory of Revil & Glover, and hence would also be in anyfrequency-dependent generalisation of it. Again, there is a need for moreexperimental data to develop the theory, but also to apply our growingneed to understand the electrical pre-cursor signals at the sitesof earthquakes and volcanoes that may be due to fluid flow (Revil et al., in press).

There are only a few laboratories that are well equipped to carry out highquality impedance spectroscopy, AC voltammetry and electro-kinetic studieson saturated rocks (e.g. Jouniaux et al., 2000; Revil et al., in press). The aimof the developments decribed in this poster is to provide such a facility atthe Laboratoire de Tectonophysique, Université Montpellier II.

The entire fluid flow system is housed in a temperature controlled enclosure whosetemperature is stabilised by a system of fans and heaters controlled by a speciallyconstructed control box which houses a PID controller, a fan controller andassociated electronics.Electrical measurements are made for both real and imaginary parts of the signalfrom 10 µHz to 32 MHz using a Solartron 1260 Frequency Response Analyser anda Solartron 1294 Impedance Interface with the required terminations.Fluid pressure measurements are made with two matched pressure transducers,which are logged every second by a dedicated logger connected to a PC.

MONTPELLIER II

UNIVERSITÉ

MONTPELLIER II

UNIVERSITÉ