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Magnetic susceptibility of petroleum reservoir fluids

Oleksandr P. Ivakhnenko, David K. Potter *

Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, UK

Abstract

A knowledge of the magnetic properties of petroleum reservoir fluids may provide new techniques for improved reservoir char-

acterisation, petroleum exploration and production. However, magnetic information is currently scarce for the vast majority of res-

ervoir fluids. For instance, there is little in the literature concerning basic magnetic susceptibility values of crude oils or formationwaters. We have therefore measured the mass magnetic susceptibility (vm) of several crude oils, refined oil fractions, and formation

waters from local and world-wide sites. All the fluids measured were diamagnetic, however there were distinct differences in mag-

nitude between the different fluid types. In particular, vmfor the crude oils was more negative than for the formation waters of the

same locality. The magnetic susceptibility of the oils appears to be related to their main physical and chemical properties. The results

correlated with the density, residue content, API (American Petroleum Institute) gravity, viscosity, sulphur content and metal con-

centration of the fluids. Light fractions of crude oil were the most diamagnetic. The magnetic measurements potentially allow phys-

ical and chemical differences between the fluids to be rapidly characterised. The results suggest other possible applications, such as

passive in situ magnetic susceptibility sensors for fluid monitoring (for example, the onset of water breakthrough, or the detection of

migrating fines) in reservoirs, which would provide an environmentally friendly alternative to radioactive tracers. The mass magnetic

susceptibilities of the fluids in relation to typical reservoir minerals may also play a role in fluidrock interactions, such as studies of

wettability. The vm of crude oil from the various world-wide oil provinces that were tested also showed some differences, possibly

reflecting broad physical and chemical features of the geological history of each province.

2004 Elsevier Ltd. All rights reserved.

Keywords: Magnetic susceptibility; Crude oil; Formation water; Petroleum reservoir

1. Introduction

Magnetic methods and techniques are prominent in

the area of geoscience. However, there is little widely

available data concerning the magnetic susceptibility of

the majority of natural reservoir fluids. The most com-

plete studies are by Ergin et al. (1975), and Ergin andYarulin (1979), which are written in Russian and are

not well known to worldwide researchers. These studies

determined the mass magnetic susceptibility of crude oils

in some of the oil provinces of the former USSR. Ergin

and Yarulin (1979)showed that the mass magnetic sus-

ceptibility of the crude oils was diamagnetic (low and neg-

ative) and varied from 0.942 to 1.042108 m3kg1,

but mainly within the range 0.98 to 1.02108

m3kg1. They also analysed many of the components

of crude oil, which we have compiled and plotted in

Fig. 1, and showed that the most diamagnetic hydrocar-

bon compounds were the alkanes, cyclopentanes and cy-clohexanes. These ranged in value from about 1.00 to

1.13108 m3kg1. In contrast, the oxygen and nitro-

gen compounds were significantly less diamagnetic. The

Ergin and Yarulin (1979)study also found some correla-

tions between the mass susceptibility and certain other

physical and chemical properties of the oils. In general,

the authors found that the magnetic susceptibility of

the oils increased with depth, although there were excep-

tions. More significantly, they found that the mass

1474-7065/$ - see front matter 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.pce.2004.06.001

* Corresponding author.

E-mail address: [email protected] (D.K. Potter).

www.elsevier.com/locate/pce

Physics and Chemistry of the Earth 29 (2004) 899907
mailto:[email protected]:[email protected]


2/9

susceptibilities of the oil from individual oil provinces had

distinctly different values. There were also small varia-tions in the values between different tectonic areas in

the same oil province, and even between different collec-

tors of the same oil deposit. On the basis of these results

the authors suggested the possibility of distinguishing oils

from different provinces and stratigraphic intervals by

comparing their average magnetic susceptibility values.

In our present paper we detail a systematic study of

the mass magnetic susceptibility of natural reservoir flu-

ids. These included crude oils from various oil provinces

worldwide, and also refined oil fractions. In addition we

analysed the magnetic susceptibility of formation

waters, which represent the first such measurements as

far as we are aware.

2. Experimental measurements

2.1. Description of fluid samples

Three types of reservoir fluids were involved in the

current study: crude oil from active petroleum reser-

voirs, refined oil fractions, and formation waters. A suite

of 22 samples of fresh crude oil were collected mainly

from sites in the North Sea and other representative

world oil provinces such as the Middle East, North

America, the Far East and Russia. The samples of crudeoil from the other world provinces were chosen with a

range of distinctive physical and chemical differences.

The fluids were kept in their sealed containers until a

few days before the measurements when they were

poured into glass sealed tubes. The refined petroleum

fluids came from the Forties field crude oil and con-

tained light to heavier fractions including gasoline, ker-

osene, light gas oil, heavy gas oil and vacuum gas oil.

The formation waters came from the Dunbar and

Forties fields in the North Sea oil province. This allowed

magnetic susceptibility results to be directly compared

with those for crude oil samples from the same oilfields.

The composition of the solutes in these two formationwaters is shown in Table 1. We also studied a sample

of sea water, which was pumped through the injection

wells into the reservoirs, and measured the magnetic sus-

ceptibility of distilled water for comparison.

All the fluid samples were clean from mechanical and

other fluid contamination. All the samples were previ-

ously well characterised by the supplying companies,

who assured us of the sample cleanliness. We are there-

fore confident that the results we observe are not due to

some artefact of the extraction infrastructure (pipelines

etc.).

-1.15

-1.10

-1.05

-1.00

-0.95

-0.90

-0.85

-0.80

-0.75

-0.70

-0.65

-0.60

-0.55

0 50 100 150 200 250 300 350 400 450

Crude Oil Compounds

MassMagneticSusceptibility

(10-8m

3/kg)

Alkanes

Cyclopentanes and cyclohexanes

Benzol and its homologue series

Naphtheno-aromatic

hydrocarbonsPolycyclic aromatic hydrocarbons

Monocarboxylic acids

Phenols

Naphthenic acids

Piradines

Quinolines

Thiols

Sulphides

Thiophenes and thiophanes

Hydrocarbons

Sulphur compounds

Nitrogen

compounds

Oxygen

compounds

Fig. 1. Mass magnetic susceptibility of crude oil compounds (based on the data ofErgin and Yarulin (1979)).

Table 1

Formation waters and sea water solute composition

Solute composition Conc entration

Dunbar formation water (kgm3) Forties formation water (kg m3) Sea water (kg m3)

NaCl 34.13 79.5 24.41

CaCl2 6H2O 7.74 10.93 2.34

MgCl2 H2O 1.25 6.18 11.44

KCl 0.43 1.25 0.88

BaCl2 0.43 0.48 0.00

SrCl2 0.47 2.35 0.00

Na2SO4 0.00 0.00 3.98

900 O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899907


3/9

2.2. Experimental procedures

Since we expected that the fluids would have diamag-

netic (low and negative) magnetic susceptibilities we

required very sensitive measuring equipment. We there-

fore initially measured the mass magnetic susceptibility

(vm) of the fluids using a Sherwood Scientific mag-netic susceptibility balance (MSB) Mark I. The MSB

Mark I or Evans magnetic balance is designed as a re-

verse traditional Gouy magnetic balance. The Evans

method uses the same configuration as the Gouy meth-

od except that instead of measuring the force with which

a magnet exerts on the sample, the equal and opposite

force which the sample exerts on moving permanent

magnets is measured. Two pairs of magnets are posi-

tioned at opposite ends of a beam making a balanced

system. When the sample is placed in the susceptibility

balance between one pair of magnets, the beam is no

longer in equilibrium and is deflected, and the move-

ment is optically detected. A compensating force is ap-

plied by a coil between the other pair of magnets. The

current required to bring the beam back into equilib-

rium is proportional to the force exerted by the sample,

which in turn is proportional to the magnetic suscepti-

bility. Note that the measurements in this case depend

on a dc field and not the more common ac susceptibility

bridge method.

The calibration of the MSB was made using distilled

water, produced in the presence of air. The presence of

dissolved atmospheric oxygen in the calibrating sample

and fluid samples was ignored. A value of 0.9043

(108

m3

kg1

) for the mass magnetic susceptibility ofwater at 20 C (Selwood, 1956) was used for the calibra-

tion. Repeat calibration measurements were regularly

made throughout the measurement period and were

within 0.35% of the published value for water. The val-

ues ofvmfor the studied fluids were determined at room

temperature (normally about 18 C), and corrected for

the displaced air in the measuring tube.

In order to gain sensitive independent magnetic sus-

ceptibility measurements we analysed some samplesusing a Magnetic Properties Measuring System (MPMS

2) SQUID magnetometer. The measurements again de-

pend on a dc field. The crude oil samples were measured

in gelatine capsules, while the formation water was mea-

sured in glass capsules. The effect of the fluid containers

was subtracted from the results. The measurements in

these cases were made at a temperature of 20 C. The

measurement time using the SQUID was significantly

longer than for the MSB.

3. Results

3.1. Mass magnetic susceptibilities of crude oils and

formation waters

Fig. 2 details the measurements made on the Sher-

wood Scientific MSB Mark I, and shows that the mass

magnetic susceptibilities of all the natural reservoir flu-

ids studied were diamagnetic. There is a distinct differ-

ence between the values for the crude oils and all the

water samples. The crude oils all have more negative

mass magnetic susceptibilities than the waters. This is

exemplified by the Dunbar and Forties results, where

there are clear differences between the values for crudeoil and formation water from the same oilfield. This

demonstrates that there is a real difference between the

OilRussia

OilN.S

eaMcGee

OilN.SeaBilondo

OilN.Se

aOrquidea1

OilN.Se

a

OilN.SeaKuito

OilMiddle

East

OilN.SeaM

iller

OilN.SeaDalia2

OilN.SeaHa

rding

OilN.Sea

OilN.SeaMoho

OilN.SeaClare

OilN.SeaDalia3

OilN.Sea

OilNorthAmerica

OilN.SeaDunbar

OilN.SeaMarat

hon

OilN.SeaForties

OilMiddleEast

OilNorthAmerica

OilFarEast

Distilledwater

FWN.SeaForties

FWN

.SeaDunbar

Seawater

-1.06

-1.04

-1.02

-1.00

-0.98

-0.96

-0.94

-0.92

-0.90

-0.88

-0.86

-0.84

0 5 10 15 20 25

Formation Waters and Crude Oils

MassMagneticSusce

ptibility(10-8m

3/kg)

Fig. 2. Mass magnetic susceptibility of formation waters (FW) and crude oils determined using a Sherwood MSB Mark I. The measurement errors

are of the order of 0.004 (108 m3kg1), close to the size of the symbols.

O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899907 901


4/9

mass magnetic susceptibility of the crude oils and the

formation waters, which may have been less clear had

we only measured crude oil from one locality and com-

pared it with formation water from another locality. The

reproducibility of the readings was tested by subjecting

the samples to five repeat measurements, and was found

to be very high, with the standard deviation being below0.004108 m3kg1. Our values for crude oil are com-

parable to those determined by Ergin and Yarulin

(1979). Most of their results were within the range

0.98 to 1.02108 m3kg1, which is within the range

of the majority of our crude oil samples, and is distinct

from the formation waters measured here.

As an independent check on the differences between

the formation waters and the crude oils we measured

the Dunbar samples in a Magnetic Properties Measuring

System (MPMS 2) SQUID magnetometer.Fig. 3shows

the results of the mass magnetisation versus the applied

field. The slope of the lines represents the mass magnetic

susceptibility. The results show that the susceptibility of

the Dunbar crude oil is lower than that of the Dunbar

formation water, consistent with the results shown in

Fig. 2 derived from the Sherwood MSB. The absolute

values of magnetic susceptibility are within about 4%

of the Sherwood MSB measurements for the crude oil

and under 1% for the formation water. These appear

to be satisfactory independent measurements consider-

ing the different operating principles of the two sets of

equipment.

Small differences in the values for the different water

samples may be related to the solutes they contain. Since

the compositions are relatively straightforward, we the-oretically calculated the mass magnetic susceptibilities.

The results are given inTable 2and show that the the-

oretical values are very close to those determined exper-

imentally, the difference between them being less than

1%.

Since the composition of the crude oils is much more

complex, and we do not have detailed compositional

information for many of the samples, we have not as

yet attempted to theoretically calculate the susceptibil-

ity. It seems clear from Fig. 2that there are variations

between the different crude oil samples, and these may

be related to their physical and chemical properties asdetailed below.

3.2. Relation between mass magnetic susceptibility and

physical properties

The main purpose of the following analyses was to

determine whether magnetic susceptibility measure-

ments correlated with various physical properties of

the reservoir fluids, and to establish whether magnetic

measurements might provide a rapid alternative means

of characterising different petroleum reservoir fluids.

Fig. 4shows a plot of density versus mass magnetic sus-ceptibility for the crude oils, refined fractions, formation

waters, and other water samples. There is a trend of

higher density corresponding to higher mass magnetic

susceptibilities, with a clear difference between the oils

and the formation waters. The same general trend is also

shown for the refined oil fractions, where the mass mag-

netic susceptibility increases from the lighter to the hea-

vier fractions (from gasoline to light gas oil, heavy gas

oil and vacuum gas oil). The exception is kerosene, the

fraction extracted after gasoline. The oxygen com-

pounds of crude oil, usually naphthenic acids, are highly

represented in the kerosene fraction. These compounds

have relatively higher (less negative) mass susceptibilities

than many of the other components of the oil (see Fig.

1), and this may contribute towards the higher value

of vm for kerosene. Light fractions of crude oil, such

as gasoline, are the most diamagnetic.

Fig. 5shows the residue content above 342 C versus

mass magnetic susceptibility for the crude oils for which

we had some compositional data. The residue is what

remains after fractional distillation of the lighter hydro-

carbon components. It is evident that the higher the

residue content the higher is the mass magnetic suscep-

tibility. The samples with higher residue content are also

-80

-70

-60

-50

-40

-30

-20

-10

0

0 10 20 30 40 50 60 70 80 90

Magnetic Field (10-3

A/m)

MassMagnetisation(10-5Am

2/kg)

Crude oil (Dunbar)

Formation water (Dunbar)

Fig. 3. Mass magnetisation as a function of applied magnetic field for

North Sea Dunbar crude oil and formation water using the Magnetic

Properties Measuring System (MPMS-2) SQUID magnetometer. The

slope of the lines gives the mass susceptibility.

Table 2

Experimentally measured and theoretically calculated mass magnetic

susceptibility of waters

Waters Mass magnetic susceptibility

(108 m3kg1)

Measured Calculated

Formation water (Forties)

0.873

0.878Formation water (Dunbar) 0.886 0.893

Sea water 0.897 0.892



5/9

the samples with higher density, so the trend given in

Fig. 5is consistent with the density versus susceptibility

results ofFig. 4.

The stock tank oil gravity versus mass magnetic sus-

ceptibility is given inFig. 6for the crude oil samples for

which we had data. Stock stank oil is oil as it exists at

atmospheric conditions in a stock tank (it tends to lack

much of the dissolved gas present at reservoir tempera-

tures and pressures). The gravity is expressed in API de-

grees as follows: API = [141.5/So]131.5, where So is

the stock tank oil specific gravity, or relative density, to

water at 288 K, and API is an acronym for American

Oil DunbarOil Forties

KerosineHeavy gas oil

Vacuum gas oil

Light gas oil

Gasoline

FW Forties

FW Dunbar

-1.08

-1.06

-1.04

-1.02

-1.00

-0.98

-0.96

-0.94

-0.92

-0.90

-0.88

-0.86

0 200 400 600 800 1000 1200

Density (kg/m3 )

MassMagneticS

usceptibility(10-

8m3/kg)

Crude oil

Oil fraction

Distilled water

Sea water

Formation water

Fig. 4. Density versus mass magnetic susceptibility of crude oils, refined oil fractions and formation waters.

R2= 0.75

-1.05

-1.04

-1.03

-1.02

-1.01

-1.00

-0.99

-0.98

-0.97

-0.96

-0.95

-0.94

0 10 20 30 40 50 60 70 80 90

Residue Content above 342oC (wt %)

MassMagneticSusceptibility(10

-8m

3/kg)

Fig. 5. Residue content above 342C versus mass magnetic susceptibility of crude oil samples.

R2= 0.72

-1.05

-1.04

-1.03

-1.02

-1.01

-1.00

-0.99

-0.98

-0.97

-0.96

-0.95

-0.94

0 5 10 15 20 25 30 35 40 45 50

Gravity (API degrees)

MassMagneticSusceptibility(10-8m

3/kg)

Fig. 6. Stock tank oil gravity versus mass magnetic susceptibility of crude oil samples.



6/9

Petroleum Institute. For a value of 10 API,Sois 1.0, the

specific gravity of water.Fig. 6shows that there is a dis-

tinct trend of decreasing mass magnetic susceptibilitywith increasing gravity, consistent with the expected trend

on the basis of the density versus susceptibility results.

Fig. 7shows results for the viscosity at a temperature

of 40 C versus the mass magnetic susceptibility for

those crude oils for which we had data. There is a sug-

gestion that the higher the magnetic susceptibility, the

higher the viscosity. We have omitted the linear regre-

ssion line, where R2=0.73, since it is fairly meaningless

given that there appear to be two clusters, and the cor-

relation may be non-linear. The broad trend we observe

might be expected since the samples with higher viscos-

ity are also the ones with higher density, which gave

higher (less negative) values of magnetic susceptibility.Whilst our data does not appear to be very well con-

strained, we include it because Ergin and Yarulin

(1979, Fig. 4.1, p. 159)also found a similar broad trend

of higher magnetic susceptibility with increasing visco-

sity. Their relationship was non-linear and slightly better

constrained.

3.3. Relation between mass magnetic susceptibility of

crude oils and concentration of sulphur and metals

The mass magnetic susceptibility of crude oils may

also reflect their chemical composition, such as the sul-

phur content and the concentration of organometallic

compounds. Fig. 8 shows the sulphur content versus

mass magnetic susceptibility for the crude oils for which

we had compositional data. In general, a higher sulphur

content corresponds to a higher (less negative) mass sus-

ceptibility. There is a suggestion of possibly two trends:

one including the Russian and North Sea samples, and

the other containing the North American and Middle

East samples. The Russian and uppermost North Sea

sample have higher residue concentrations and higher

densities than the uppermost North American and Mid-dle East samples. Higher sulphur content also generally

corresponds to higher residue content and density within

each of the two trending groups.

Fig. 9(a)(d) show results for the content of trace

amounts of vanadium, cadmium, nickel and iron versus

mass magnetic susceptibility. In each case there appears

-1.05

-1.04

-1.03

-1.02

-1.01-1.00

-0.99

-0.98

-0.97

-0.96

-0.95

-0.94

0 20 40 60 80 100 120 140 160

Viscosity at 40oC (cSt)


3/kg)

Fig. 7. Viscosity at 40 C versus mass magnetic susceptibility of crude oil samples.

North Sea

Far East

North America

Middle East

North America

Middle East

North Sea

North Sea

Russia

-1.05

-1.04

-1.03

-1.02

-1.01

-1.00

-0.99

-0.98

-0.97

-0.96

-0.95

-0.94

0 0.5 1 1.5 2 2.5 3

Sulphur Content (%wt)

MassMagneticSusceptibility(

10-8m

3/kg)

Fig. 8. Sulphur content versus mass magnetic susceptibility of crude oil samples.



7/9

to be a trend of higher mass magnetic susceptibility with

increasing metal content. This trend might ordinarily be

expected. However, the results should be treated with

some caution as we noticed that samples with higher

metal content also had higher density, which also corre-sponds to higher mass susceptibility. The relative roles

of the metal content versus the intrinsic fluid density

are presently unclear. It seems that crude oil samples

with higher density have higher residue content and that

these contain greater amounts of organometallic com-

pounds. If the metal content was due to elemental metal,

then trace amounts would have a significant effect on the

susceptibility. For instance, just 10 ppm by weight of

ferromagnetic elemental iron would increase the mass

susceptibility of the sample by about 0.60.7108

m3kg1, using values of mass susceptibility for iron

given byPotter and Stephenson (1988, Table 1). In real-

ity the metals are likely to be components in organome-

tallic compounds (which would have substantially lower

intrinsic values of magnetic susceptibility), and without

knowing the exact composition of these compounds

their precise influence on the magnetic susceptibility of

the crude oils remains uncertain.

3.4. Differences between oil provinces

Our preliminary data on the vm of the crude oils we

studied (from the Far East, North America, North

Sea, the Middle East and Russia) seems to show some

differences between the various oil provinces (Fig. 10).

Whilst there is quite a large range between the various

North Sea samples, and a fair degree of overlap with

the North American and Middle East samples, the Rus-

sian sample and the Far East sample appear to be quitedistinct. This may reflect specific features of the geolog-

ical and geochemical history of the oil provinces, and

might lend some support to the suggestion by Ergin

and Yarulin (1979) that crude oils from different prov-

inces might be distinguished on the basis of their mag-

netic susceptibility. Clearly, however, more samples

need to be measured in order to confirm any broad con-

sistent differences between the various oil provinces.

R2= 0.78

-1.05

-1.04

-1.03

-1.02

-1.01

-1.00

-0.99

-0.98

-0.97

-0.96

-0.95

-0.94

0 10 20 30 40 50 60 70 80

Vanadium Content (ppm wt)


3/kg)

R2= 0.63

-1.05

-1.04

-1.03

-1.02

-1.01

-1.00

-0.99

-0.98

-0.97

-0.96

-0.95

-0.94

0 2 4 6 8 10 12 14 16 18

Cadmium Content (ppb wt)

MassMagn

eticSusceptibility(10-8m

3/kg)

R2= 0.69

-1.05

-1.04

-1.03

-1.02

-1.01

-1.00

-0.99

-0.98

-0.97

-0.96

-0.95

-0.94

0 5 10 15 20 25 30 35

Nickel Content (ppm wt)

Mass

MagneticSusceptibility(10-8m

3/kg)

R2= 0.37

-1.05

-1.04

-1.03

-1.02

-1.01

-1.00

-0.99

-0.98

-0.97

-0.96

-0.95

-0.94

0 2 4 6 8 10 12

Iron Content (ppm wt)

Mass

MagneticSusceptibility(10-8m

3/kg)

(a) (b)

(c) (d)

Fig. 9. Mass magnetic susceptibility of crude oils as a function of (a) vanadium content, (b) cadmium content, (c) nickel content, and (d) iron

content.

Russia

N. SeaN. SeaN. Sea

N. SeaN. Sea

Middle East

N. Sea

N. Sea

Middle EastNorth America

Far East

N. Sea

N. SeaN. Sea

N. Sea North America

N. Sea

-1.05

-1.04

-1.03

-1.02

-1.01

-1.00

-0.99

-0.98

-0.97

-0.96

-0.95

0 1 2 3 4 5 6

Location

MassMagneticSusceptibility(10-8 m3/kg)

Fig. 10. Mass magnetic susceptibility in relation to specific oil

provinces.



8/9

4. Discussion of possible applications for petroleum

reservoirs

Magnetic susceptibility measurements might find a

use in passive sensors in reservoirs for distinguishing be-

tween formation waters and crude oils. For example,

such a sensor could potentially help to monitor the onsetof water breakthrough. Current automated versions of

the MSB system are capable of being used as a detector

in conjunction with a flow cell, and it ought to be possi-

ble to further miniaturize such a system and employ it

downhole. Such sensors would provide an environmen-

tally friendly alternative to radioactive tracers. Although

viscosity meters might also distinguish between forma-

tion waters and crude oils, magnetic sensors would

have a further advantage in being able to also rapidly

detect small concentrations of ferrimagnetic or antiferri-

magnetic minerals, or migrating fines from important

paramagnetic clays such as illite or chlorite (small con-

centrations of which can dramatically affect fluid perme-

ability). The magnetic susceptibility sensors might thus

also be used to monitor formation damage, or any

anomalous effects arising from the hydrocarbon extrac-

tion infrastructure.

It is also worth noting the values of the mass suscep-

tibility of crude oils and formation waters in relation to

some typical petroleum reservoir minerals, such as the

diamagnetic matrix minerals and the paramagnetic per-

meability controlling clays. The differences are shown in

Fig. 11. The mass susceptibilities of the natural reservoir

fluids are more negative than the majority of the dia-

magnetic matrix reservoir minerals such as quartz, feld-spar, and calcite. However, the values are significantly

less diamagnetic than the clay kaolinite. Magnetic prop-

erties may possibly play some role in rockfluid interac-

tions. The relative magnetic forces between quartz and

formation water and between quartz and crude oil, in

the Earths field, might be a factor in determining the

wettability (water wet or oil wet) of the reservoir rock.

For reservoir rocks containing significant amounts of

paramagnetic clays, such as illite, the relative magnetic

roles of formation water and crude oil could be reversed

(compared to the quartz case) according toFig. 11. This

might be a factor in influencing the changes in wettabil-

ity that one often observes between clean sandstones(quartz rich with little clay) and muddy sandstones (con-

taining higher concentrations of paramagnetic clays).

Recent work has shown links between nuclear magnetic

resonance (NMR) and wettability (Guan et al., 2002),

and so a link between magnetic susceptibility and wetta-

bility may also be a possibility.

5. Conclusions

The following conclusions can be drawn from the

present study:

There were distinct differences between the mass

magnetic susceptibilities (vm) of crude oils and forma-

tion waters. All the samples studied were diamag-

netic, but the values for the crude oils were more

negative. Two independent pieces of sensitive equip-

ment confirmed the differences between samples of

formation water and crude oil from the same oilfield,

and each measurement system yielded very similar

results.

The values of vm for the crude oils, refined oil frac-

tions and formation waters correlated with their den-

sities. The values for crude oil also correlated withother physical properties, namely residue content,

stock tank oil gravity, and viscosity. The results sug-

gest that the magnetic measurements could poten-

tially be used to rapidly characterise the physical

differences between various petroleum reservoir

fluids.

Dolomite

Lepidocrocite

Calcite

Vermiculite

Chamosite

N

ontronite

Kaolinite

Ilmenite

Magnesite

Crudeoil

Formation

water

Halite

Quartz

Montmorillonite

Glauconite

Muscovite

Illite

Siderite

Gypsum

Feldspar

Anhydrite

ChloriteBVS

ChloriteCF

S

-30

-20

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

130

Minerals and Fluids

MassMagneticSusceptibility(1

0-8m

3/kg)

Diamagnetic Paramagnetic

Fig. 11. A comparison of the mass magnetic susceptibility of typical reservoir diamagnetic and paramagnetic minerals in relation to average crude oil

and formation water values from the present study. The values for the minerals were taken fromHunt et al. (1995),Borradaile et al. (1990), and

Thompson and Oldfield (1986).



9/9

The values ofvm for the crude oils also showed correla-

tions with trace amounts of chemical components,

namely the contents of sulphur, vanadium, cadmium,

nickel, andiron.The results, however, shouldbe treated

with some caution, since the samples with higher con-

tents of these elements also generally have higher den-

sity, which also correlates with vm. It appears thatcrude oils with higher density have higher residue con-

tent, and also contain higher concentrations of the

above components. The relative contributions of intrin-

sic fluid density and these trace components to the total

magnetic susceptibility signal is presently unclear.

The values ofvmfor the formation waters was related

to their solute composition. The experimental mea-

surements were within 1% of the theoretically calcu-

lated values based on the water compositions.

There is some suggestion from the results that crude

oils from different world oil provinces might be

broadly distinguished on the basis of their magnetic

susceptibility. However, there are significant ranges

and overlaps between the results for some provinces,

and more samples need to be measured before consis-

tent differences can be confirmed.

Acknowledgments

We are grateful to B. Woods and J. Gordon of BP for

providing us with some of the crude oils and refined oil

fractions, and also for associated data on those samples.

We thank the oilfield scale group, and especially Nor-man Lang, of the Institute of Petroleum Engineering

at Heriot-Watt for providing us with crude oil and for-

mation water samples and data. We thank Dr. A. Powell

(Heriot-Watt University) for the use of the Sherwood

MSB Mark I, and Prof. A. Harrison (Edinburgh Uni-

versity) for useful discussions and the use of the MPMS

2 SQUID magnetometer. We are grateful to reviewers

Brooks Ellwood and Bill Morris, and guest editor Edu-ard Petrovsky, for their constructive comments, which

helped to improve the manuscript.

References

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Hunt, C.P., Moskowitz, B.M., Banerjee, S.K., 1995. Magnetic

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Publishers, New York p. 435.

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