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Magnetic susceptibility of petroleum reservoir fluids
Physics and Chemistry of the Earth 29 (2004) 899–907 www.elsevier.com/locate/pce
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 characterisation, petroleum exploration and production. However, magnetic information is currently scarce for the vast majority of reservoir fluids. For instance, there is little in the literature concerning basic magnetic susceptibility values of crude oils or formation waters. 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 magnitude between the different fluid types. In particular, vm for 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 concentration of the fluids. Light fractions of crude oil were the most diamagnetic. The magnetic measurements potentially allow physical 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 fluid–rock 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 complete studies are by Ergin et al. (1975), and Ergin and Yarulin (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-
*
Corresponding author. E-mail address: [email protected] (D.K. Potter).
1474-7065/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2004.06.001
ceptibility of the crude oils was diamagnetic (low and negative) and varied from 0.942 to 1.042 · 10 8 m3 kg 1, but mainly within the range 0.98 to 1.02 · 10 8 m3 kg 1. 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 hydrocarbon compounds were the alkanes, cyclopentanes and cyclohexanes. These ranged in value from about 1.00 to 1.13 · 10 8 m3 kg 1. In contrast, the oxygen and nitrogen compounds were significantly less diamagnetic. The Ergin and Yarulin (1979) study also found some correlations 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 exceptions. More significantly, they found that the mass
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Alkanes
-0.60 Hydrocarbons
-0.65
Mass Magnetic Susceptibility -8 3 (10 m /kg)
Oxygen compounds
Cyclopentanes and cyclohexanes
Nitrogen compounds
Benzol and its homologue series
-0.70
Naphtheno-aromatic hydrocarbons Polycyclic aromatic hydrocarbons
Sulphur compounds
-0.75 -0.80
Monocarboxylic acids
-0.85
Phenols
-0.90
Naphthenic acids
-0.95
Piradines
-1.00
Quinolines
-1.05
Thiols
-1.10
Sulphides
-1.15
Thiophenes and thiophanes
0
50
100
150
200
250
300
350
400
450
Crude Oil Compounds
Fig. 1. Mass magnetic susceptibility of crude oil compounds (based on the data of Ergin and Yarulin (1979)).
susceptibilities of the oil from individual oil provinces had distinctly different values. There were also small variations in the values between different tectonic areas in the same oil province, and even between different collectors 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 fluids. 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 reservoirs, 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 crude oil 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 contained light to heavier fractions including gasoline, kerosene, 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 formation waters 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 susceptibility of distilled water for comparison. All the fluid samples were clean from mechanical and other fluid contamination. All the samples were previously well characterised by the supplying companies, who assured us of the sample cleanliness. We are therefore confident that the results we observe are not due to some artefact of the extraction infrastructure (pipelines etc.).
Table 1 Formation waters and sea water solute composition Solute composition
NaCl CaCl2 Æ 6H2O MgCl2 Æ H2O KCl BaCl2 SrCl2 Na2SO4
Concentration Dunbar formation water (kg m 3)
Forties formation water (kg m 3)
Sea water (kg m 3)
34.13 7.74 1.25 0.43 0.43 0.47 0.00
79.5 10.93 6.18 1.25 0.48 2.35 0.00
24.41 2.34 11.44 0.88 0.00 0.00 3.98
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907
2.2. Experimental procedures
made throughout the measurement period and were within ±0.35% of the published value for water. The values of vm for 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 susceptibility measurements we analysed some samples using a Magnetic Properties Measuring System (MPMS 2) SQUID magnetometer. The measurements again depend on a dc field. The crude oil samples were measured in gelatine capsules, while the formation water was measured 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.
Oil North America
Oil Far East
Oil N. Sea Forties
Oil N. Sea Dunbar
Oil Middle East
-1.02
Oil N. Sea Marathon
Oil N. Sea
Oil North America
Oil N. Sea Clare
Oil N. Sea Dalia 3
Oil N. Sea
Oil N. Sea Moho
Fig. 2 details the measurements made on the Sherwood Scientific MSB Mark I, and shows that the mass magnetic susceptibilities of all the natural reservoir fluids studied were diamagnetic. There is a distinct difference 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 crude oil and formation water from the same oilfield. This demonstrates that there is a real difference between the
Oil N. Sea Dalia 2
Oil Middle East
3.1. Mass magnetic susceptibilities of crude oils and formation waters
Oil N. Sea Harding
-1.00
3. Results
Oil N. Sea Miller
Oil N. Sea
-0.98
Oil N. Sea Kuito
-0.96
Oil N. Sea Bilondo
-0.94
Oil N. Sea Orquidea 1
-0.92
Oil Russia
FW N. Sea Forties
-0.90
FW N. Sea Dunbar
Mass Magnetic Susceptibility (10
-8
-0.88
Oil N. Sea McGee
Sea water
3
m /kg)
-0.86
Distilled water
Since we expected that the fluids would have diamagnetic (low and negative) magnetic susceptibilities we required very sensitive measuring equipment. We therefore initially measured the mass magnetic susceptibility (vm) of the fluids using a Sherwood Scientific magnetic susceptibility balance (MSB) Mark I. The MSB Mark I or Evans magnetic balance is designed as a reverse traditional Gouy magnetic balance. The Evans method uses the same configuration as the Gouy method 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 positioned 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 movement is optically detected. A compensating force is applied by a coil between the other pair of magnets. The current required to bring the beam back into equilibrium is proportional to the force exerted by the sample, which in turn is proportional to the magnetic susceptibility. 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 (10 8 m3 kg 1) for the mass magnetic susceptibility of water at 20 C (Selwood, 1956) was used for the calibration. Repeat calibration measurements were regularly
-0.84
901
-1.04 -1.06 0
5
10
15
20
25
Formation Waters and Crude Oils 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 (10 8 m3 kg 1), close to the size of the symbols.
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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 compared 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 below ±0.004 · 10 8 m3 kg 1. Our values for crude oil are comparable to those determined by Ergin and Yarulin (1979). Most of their results were within the range 0.98 to 1.02 · 10 8 m3 kg 1, 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. 3 shows 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 considering 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 theoretically calculated the mass magnetic susceptibilities. The results are given in Table 2 and show that the theoretical values are very close to those determined exper-
0 Crude oil (Dunbar) Formation water (Dunbar)
-20
-5
2
Mass Magnetisation (10 Am /kg)
-10
-30 -40 -50 -60 -70 -80 0
10
20
30
40
50
-3
60
70
80
90
Magnetic Field (10 A/m)
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 (10 8 m3 kg 1) Measured
Formation water (Forties) Formation water (Dunbar) Sea water
0.873 0.886 0.897
Calculated 0.878 0.893 0.892
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 susceptibility. It seems clear from Fig. 2 that there are variations between the different crude oil samples, and these may be related to their physical and chemical properties as detailed below. 3.2. Relation between mass magnetic susceptibility and physical properties The main purpose of the following analyses was to determine whether magnetic susceptibility measurements 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. 4 shows a plot of density versus mass magnetic susceptibility 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 magnetic susceptibility increases from the lighter to the heavier 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 compounds 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. 5 shows 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 hydrocarbon components. It is evident that the higher the residue content the higher is the mass magnetic susceptibility. The samples with higher residue content are also
-8
Mass Magnetic Susceptibility (10 m3/kg)
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907 -0.86 -0.88 -0.90 -0.92 -0.94 -0.96 -0.98 -1.00 -1.02 -1.04 -1.06 -1.08
903
FW Forties FW Dunbar
Crude oil Oil fraction Distilled water Sea water Formation water
Vacuum gas oil Kerosine Heavy gas oil Light gas oil Oil Dunbar Oil Forties
Gasoline
0
200
400
600
800
1000
1200
Density (kg/m3 )
Fig. 4. Density versus mass magnetic susceptibility of crude oils, refined oil fractions and formation waters.
-0.95 -0.96
-8
3
Mass Magnetic Susceptibility (10 m /kg)
-0.94
-0.97
2
R = 0.75
-0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 -1.05 0
10
20
30
40
50
60
70
80
90
o
Residue Content above 342 C (wt %)
Fig. 5. Residue content above 342 C versus mass magnetic susceptibility of crude oil samples.
the samples with higher density, so the trend given in Fig. 5 is consistent with the density versus susceptibility results of Fig. 4. The stock tank oil gravity versus mass magnetic susceptibility is given in Fig. 6 for 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 temperatures and pressures). The gravity is expressed in API degrees 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
3
-0.95
-8
Mass Magnetic Susceptibility (10 m /kg)
-0.94
-0.96 -0.97 -0.98 -0.99 -1.00 -1.01 2
-1.02
R = 0.72
-1.03 -1.04 -1.05 0
5
10
15
20
25
30
35
40
45
50
Gravity (API degrees)
Fig. 6. Stock tank oil gravity versus mass magnetic susceptibility of crude oil samples.
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907
3
-0.94 -0.95
-8
Mass Magnetic Susceptibility (10 m /kg)
904
-0.96 -0.97 -0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 -1.05 0
20
40
60
80
100
120
140
160
o
Viscosity at 40 C (cSt)
Fig. 7. Viscosity at 40 C versus mass magnetic susceptibility of crude oil samples.
Petroleum Institute. For a value of 10 API, So is 1.0, the specific gravity of water. Fig. 6 shows that there is a distinct trend of decreasing mass magnetic susceptibility with increasing gravity, consistent with the expected trend on the basis of the density versus susceptibility results. Fig. 7 shows 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 suggestion that the higher the magnetic susceptibility, the higher the viscosity. We have omitted the linear regression line, where R2 = 0.73, since it is fairly meaningless given that there appear to be two clusters, and the correlation may be non-linear. The broad trend we observe might be expected since the samples with higher viscosity 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 constrained, 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 viscosity. 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 sulphur 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 susceptibility. 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 Middle 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
-0.95
-8
3
Mass Magnetic Susceptibility (10 m /kg)
-0.94
-0.96
Russia
-0.97 -0.98 North Sea
-0.99 -1.00
Middle East
-1.01 North Sea North Sea
-1.02
North America
Middle East North America
-1.03 -1.04
Far East
-1.05 0
0.5
1
1.5
2
2.5
3
Sulphur Content (%wt)
Fig. 8. Sulphur content versus mass magnetic susceptibility of crude oil samples.
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907
-8 3
-0.96
-0.94
Mass Magnetic Susceptibility (10 m /kg)
3
-0.95
-8
Mass Magnetic Susceptibility (10 m /kg)
-0.94
2
R = 0.78
-0.97 -0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 -1.05 10
20
30
40
50
60
70
80
Vanadium Content (ppm wt)
2 R = 0.63
-0.97 -0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 0
2
4
6
8
10
12
14
16
18
Cadmium Content (ppb wt)
-0.94
Mass Magnetic Susceptibility (10 m /kg)
-0.95
-8 3
-8 3
Mass Magnetic Susceptibility (10 m /kg)
-0.96
(b)
-0.94
2 R = 0.69
-0.96 -0.97 -0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 -1.05
-0.95 -0.96 -0.97 2 R = 0.37
-0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 -1.05
0
(c)
-0.95
-1.05 0
(a)
905
5
10
15
20
25
30
Nickel Content (ppm wt)
35
0
2
4
(d)
6
8
10
12
Iron Content (ppm wt)
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.
3.4. Differences between oil provinces
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 Russian sample and the Far East sample appear to be quite distinct. This may reflect specific features of the geological 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 provinces might be distinguished on the basis of their magnetic susceptibility. Clearly, however, more samples need to be measured in order to confirm any broad consistent differences between the various oil provinces.
-0.95
Mass Magnetic Susceptibility (10-8m 3/kg)
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 corresponds 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 compounds. 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 0.6–0.7 · 10 8 m3 kg 1, using values of mass susceptibility for iron given by Potter and Stephenson (1988, Table 1). In reality the metals are likely to be components in organometallic 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.
-0.96
N. Sea
-0.98
N. Sea N. Sea N. Sea
-0.99
Middle East
N. Sea N. Sea N. Sea N. Sea N. Sea N. Sea N. Sea N. Sea
-1.00 -1.01 -1.02
North America Middle East
-1.03
North America Far East
-1.04 -1.05
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
Russia
-0.97
0
1
2
3
4
5
6
Location
Fig. 10. Mass magnetic susceptibility in relation to specific oil provinces.
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907
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 to Fig. 11. This might be a factor in influencing the changes in wettability that one often observes between clean sandstones (quartz rich with little clay) and muddy sandstones (containing 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 wettability may also be a possibility.
4. Discussion of possible applications for petroleum reservoirs Magnetic susceptibility measurements might find a use in passive sensors in reservoirs for distinguishing between formation waters and crude oils. For example, such a sensor could potentially help to monitor the onset of 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 possible to further miniaturize such a system and employ it downhole. Such sensors would provide an environmentally friendly alternative to radioactive tracers. Although viscosity meters might also distinguish between formation waters and crude oils, magnetic sensors would have a further advantage in being able to also rapidly detect small concentrations of ferrimagnetic or antiferrimagnetic minerals, or migrating fines from important paramagnetic clays such as illite or chlorite (small concentrations of which can dramatically affect fluid permeability). The magnetic susceptibility sensors might thus also be used to monitor formation damage, or any anomalous effects arising from the hydrocarbon extraction infrastructure. It is also worth noting the values of the mass susceptibility of crude oils and formation waters in relation to some typical petroleum reservoir minerals, such as the diamagnetic matrix minerals and the paramagnetic permeability 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 diamagnetic matrix reservoir minerals such as quartz, feldspar, and calcite. However, the values are significantly less diamagnetic than the clay kaolinite. Magnetic properties may possibly play some role in rock–fluid interactions. 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
5. Conclusions The following conclusions can be drawn from the present study:
130
Paramagnetic
20
-30
Calcite
Magnesite
Feldspar
Anhydrite
Halite
Quartz
Gypsum
Formation water
-20
Dolomite
0 -10
Crude oil
10
Illite
30
Vermiculite
Muscovite
40
Montmorillonite
50
Chlorite BVS
60
Lepidocrocite
70
Glauconite
80
Chlorite CFS
90
Chamosite
Diamagnetic
100
Nontronite
110
Kaolinite
Mass Magnetic Susceptibility (10-8 m 3/kg)
120
Siderite
There were distinct differences between the mass magnetic susceptibilities (vm) of crude oils and formation waters. All the samples studied were diamagnetic, but the values for the crude oils were more negative. Two independent pieces of sensitive equipment 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 fractions and formation waters correlated with their densities. The values for crude oil also correlated with other physical properties, namely residue content, stock tank oil gravity, and viscosity. The results suggest that the magnetic measurements could potentially be used to rapidly characterise the physical differences between various petroleum reservoir fluids.
Ilmenite
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Minerals and Fluids
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 from Hunt et al. (1995), Borradaile et al. (1990), and Thompson and Oldfield (1986).
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907
The values of vm for the crude oils also showed correlations with trace amounts of chemical components, namely the contents of sulphur, vanadium, cadmium, nickel, and iron. The results, however, should be treated with some caution, since the samples with higher contents of these elements also generally have higher density, which also correlates with vm. It appears that crude oils with higher density have higher residue content, and also contain higher concentrations of the above components. The relative contributions of intrinsic fluid density and these trace components to the total magnetic susceptibility signal is presently unclear. The values of vm for the formation waters was related to their solute composition. The experimental measurements were within 1% of the theoretically calculated 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 consistent 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 Norman Lang, of the Institute of Petroleum Engineering
907
at Heriot-Watt for providing us with crude oil and formation 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 University) 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 Eduard Petrovsky, for their constructive comments, which helped to improve the manuscript.
References Borradaile, G.J., MacKenzie, A., Jensen, E., 1990. Silicate versus trace mineral susceptibility in metamorphic rocks. Journal of Geophysical Research––Solid Earth 95, 8447–8451. Ergin, Y.V., Kostrova, L.I., Subaev, I.Kh., Yarulin, K.S., 1975. Magnetic Properties of Oils (in Russian). Depositor of VINITI, N3265-75. Ergin, Y.V., Yarulin, K.S., 1979. Magnetic Properties of Oils (in Russian). Nauka Publishers, Moscow p. 200. Guan, H., Brougham, D., Sorbie, K.S., Packer, K.J., 2002. Wettability effects in a sandstone reservoir and outcrop cores from NMR relaxation time distributions. Journal of Petroleum Science and Engineering 34, 33–52. Hunt, C.P., Moskowitz, B.M., Banerjee, S.K., 1995. Magnetic properties of rocks and minerals. In: Ahrens, T.J. (Ed.), Rock Physics and Phase Relations: a Handbook of Physical Constants. American Geophysical Union Reference Shelf 3, pp. 189– 204. Potter, D.K., Stephenson, A., 1988. Gyroremanent magnetization in magnetic tape and in iron and iron alloy particles. IEEE Transactions on Magnetics MAG-24, 1805–1807. Selwood, P.W., 1956. Magnetochemistry, 2nd edition. Interscience Publishers, New York p. 435. Thompson, R., Oldfield, F., 1986. Environmental Magnetism. Allen and Unwin, London p. 277.
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 characterisation, petroleum exploration and production. However, magnetic information is currently scarce for the vast majority of reservoir fluids. For instance, there is little in the literature concerning basic magnetic susceptibility values of crude oils or formation waters. 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 magnitude between the different fluid types. In particular, vm for 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 concentration of the fluids. Light fractions of crude oil were the most diamagnetic. The magnetic measurements potentially allow physical 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 fluid–rock 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 complete studies are by Ergin et al. (1975), and Ergin and Yarulin (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-
*
Corresponding author. E-mail address: [email protected] (D.K. Potter).
1474-7065/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2004.06.001
ceptibility of the crude oils was diamagnetic (low and negative) and varied from 0.942 to 1.042 · 10 8 m3 kg 1, but mainly within the range 0.98 to 1.02 · 10 8 m3 kg 1. 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 hydrocarbon compounds were the alkanes, cyclopentanes and cyclohexanes. These ranged in value from about 1.00 to 1.13 · 10 8 m3 kg 1. In contrast, the oxygen and nitrogen compounds were significantly less diamagnetic. The Ergin and Yarulin (1979) study also found some correlations 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 exceptions. More significantly, they found that the mass
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Alkanes
-0.60 Hydrocarbons
-0.65
Mass Magnetic Susceptibility -8 3 (10 m /kg)
Oxygen compounds
Cyclopentanes and cyclohexanes
Nitrogen compounds
Benzol and its homologue series
-0.70
Naphtheno-aromatic hydrocarbons Polycyclic aromatic hydrocarbons
Sulphur compounds
-0.75 -0.80
Monocarboxylic acids
-0.85
Phenols
-0.90
Naphthenic acids
-0.95
Piradines
-1.00
Quinolines
-1.05
Thiols
-1.10
Sulphides
-1.15
Thiophenes and thiophanes
0
50
100
150
200
250
300
350
400
450
Crude Oil Compounds
Fig. 1. Mass magnetic susceptibility of crude oil compounds (based on the data of Ergin and Yarulin (1979)).
susceptibilities of the oil from individual oil provinces had distinctly different values. There were also small variations in the values between different tectonic areas in the same oil province, and even between different collectors 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 fluids. 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 reservoirs, 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 crude oil 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 contained light to heavier fractions including gasoline, kerosene, 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 formation waters 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 susceptibility of distilled water for comparison. All the fluid samples were clean from mechanical and other fluid contamination. All the samples were previously well characterised by the supplying companies, who assured us of the sample cleanliness. We are therefore confident that the results we observe are not due to some artefact of the extraction infrastructure (pipelines etc.).
Table 1 Formation waters and sea water solute composition Solute composition
NaCl CaCl2 Æ 6H2O MgCl2 Æ H2O KCl BaCl2 SrCl2 Na2SO4
Concentration Dunbar formation water (kg m 3)
Forties formation water (kg m 3)
Sea water (kg m 3)
34.13 7.74 1.25 0.43 0.43 0.47 0.00
79.5 10.93 6.18 1.25 0.48 2.35 0.00
24.41 2.34 11.44 0.88 0.00 0.00 3.98
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907
2.2. Experimental procedures
made throughout the measurement period and were within ±0.35% of the published value for water. The values of vm for 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 susceptibility measurements we analysed some samples using a Magnetic Properties Measuring System (MPMS 2) SQUID magnetometer. The measurements again depend on a dc field. The crude oil samples were measured in gelatine capsules, while the formation water was measured 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.
Oil North America
Oil Far East
Oil N. Sea Forties
Oil N. Sea Dunbar
Oil Middle East
-1.02
Oil N. Sea Marathon
Oil N. Sea
Oil North America
Oil N. Sea Clare
Oil N. Sea Dalia 3
Oil N. Sea
Oil N. Sea Moho
Fig. 2 details the measurements made on the Sherwood Scientific MSB Mark I, and shows that the mass magnetic susceptibilities of all the natural reservoir fluids studied were diamagnetic. There is a distinct difference 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 crude oil and formation water from the same oilfield. This demonstrates that there is a real difference between the
Oil N. Sea Dalia 2
Oil Middle East
3.1. Mass magnetic susceptibilities of crude oils and formation waters
Oil N. Sea Harding
-1.00
3. Results
Oil N. Sea Miller
Oil N. Sea
-0.98
Oil N. Sea Kuito
-0.96
Oil N. Sea Bilondo
-0.94
Oil N. Sea Orquidea 1
-0.92
Oil Russia
FW N. Sea Forties
-0.90
FW N. Sea Dunbar
Mass Magnetic Susceptibility (10
-8
-0.88
Oil N. Sea McGee
Sea water
3
m /kg)
-0.86
Distilled water
Since we expected that the fluids would have diamagnetic (low and negative) magnetic susceptibilities we required very sensitive measuring equipment. We therefore initially measured the mass magnetic susceptibility (vm) of the fluids using a Sherwood Scientific magnetic susceptibility balance (MSB) Mark I. The MSB Mark I or Evans magnetic balance is designed as a reverse traditional Gouy magnetic balance. The Evans method uses the same configuration as the Gouy method 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 positioned 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 movement is optically detected. A compensating force is applied by a coil between the other pair of magnets. The current required to bring the beam back into equilibrium is proportional to the force exerted by the sample, which in turn is proportional to the magnetic susceptibility. 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 (10 8 m3 kg 1) for the mass magnetic susceptibility of water at 20 C (Selwood, 1956) was used for the calibration. Repeat calibration measurements were regularly
-0.84
901
-1.04 -1.06 0
5
10
15
20
25
Formation Waters and Crude Oils 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 (10 8 m3 kg 1), close to the size of the symbols.
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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 compared 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 below ±0.004 · 10 8 m3 kg 1. Our values for crude oil are comparable to those determined by Ergin and Yarulin (1979). Most of their results were within the range 0.98 to 1.02 · 10 8 m3 kg 1, 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. 3 shows 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 considering 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 theoretically calculated the mass magnetic susceptibilities. The results are given in Table 2 and show that the theoretical values are very close to those determined exper-
0 Crude oil (Dunbar) Formation water (Dunbar)
-20
-5
2
Mass Magnetisation (10 Am /kg)
-10
-30 -40 -50 -60 -70 -80 0
10
20
30
40
50
-3
60
70
80
90
Magnetic Field (10 A/m)
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 (10 8 m3 kg 1) Measured
Formation water (Forties) Formation water (Dunbar) Sea water
0.873 0.886 0.897
Calculated 0.878 0.893 0.892
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 susceptibility. It seems clear from Fig. 2 that there are variations between the different crude oil samples, and these may be related to their physical and chemical properties as detailed below. 3.2. Relation between mass magnetic susceptibility and physical properties The main purpose of the following analyses was to determine whether magnetic susceptibility measurements 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. 4 shows a plot of density versus mass magnetic susceptibility 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 magnetic susceptibility increases from the lighter to the heavier 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 compounds 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. 5 shows 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 hydrocarbon components. It is evident that the higher the residue content the higher is the mass magnetic susceptibility. The samples with higher residue content are also
-8
Mass Magnetic Susceptibility (10 m3/kg)
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907 -0.86 -0.88 -0.90 -0.92 -0.94 -0.96 -0.98 -1.00 -1.02 -1.04 -1.06 -1.08
903
FW Forties FW Dunbar
Crude oil Oil fraction Distilled water Sea water Formation water
Vacuum gas oil Kerosine Heavy gas oil Light gas oil Oil Dunbar Oil Forties
Gasoline
0
200
400
600
800
1000
1200
Density (kg/m3 )
Fig. 4. Density versus mass magnetic susceptibility of crude oils, refined oil fractions and formation waters.
-0.95 -0.96
-8
3
Mass Magnetic Susceptibility (10 m /kg)
-0.94
-0.97
2
R = 0.75
-0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 -1.05 0
10
20
30
40
50
60
70
80
90
o
Residue Content above 342 C (wt %)
Fig. 5. Residue content above 342 C versus mass magnetic susceptibility of crude oil samples.
the samples with higher density, so the trend given in Fig. 5 is consistent with the density versus susceptibility results of Fig. 4. The stock tank oil gravity versus mass magnetic susceptibility is given in Fig. 6 for 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 temperatures and pressures). The gravity is expressed in API degrees 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
3
-0.95
-8
Mass Magnetic Susceptibility (10 m /kg)
-0.94
-0.96 -0.97 -0.98 -0.99 -1.00 -1.01 2
-1.02
R = 0.72
-1.03 -1.04 -1.05 0
5
10
15
20
25
30
35
40
45
50
Gravity (API degrees)
Fig. 6. Stock tank oil gravity versus mass magnetic susceptibility of crude oil samples.
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907
3
-0.94 -0.95
-8
Mass Magnetic Susceptibility (10 m /kg)
904
-0.96 -0.97 -0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 -1.05 0
20
40
60
80
100
120
140
160
o
Viscosity at 40 C (cSt)
Fig. 7. Viscosity at 40 C versus mass magnetic susceptibility of crude oil samples.
Petroleum Institute. For a value of 10 API, So is 1.0, the specific gravity of water. Fig. 6 shows that there is a distinct trend of decreasing mass magnetic susceptibility with increasing gravity, consistent with the expected trend on the basis of the density versus susceptibility results. Fig. 7 shows 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 suggestion that the higher the magnetic susceptibility, the higher the viscosity. We have omitted the linear regression line, where R2 = 0.73, since it is fairly meaningless given that there appear to be two clusters, and the correlation may be non-linear. The broad trend we observe might be expected since the samples with higher viscosity 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 constrained, 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 viscosity. 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 sulphur 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 susceptibility. 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 Middle 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
-0.95
-8
3
Mass Magnetic Susceptibility (10 m /kg)
-0.94
-0.96
Russia
-0.97 -0.98 North Sea
-0.99 -1.00
Middle East
-1.01 North Sea North Sea
-1.02
North America
Middle East North America
-1.03 -1.04
Far East
-1.05 0
0.5
1
1.5
2
2.5
3
Sulphur Content (%wt)
Fig. 8. Sulphur content versus mass magnetic susceptibility of crude oil samples.
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907
-8 3
-0.96
-0.94
Mass Magnetic Susceptibility (10 m /kg)
3
-0.95
-8
Mass Magnetic Susceptibility (10 m /kg)
-0.94
2
R = 0.78
-0.97 -0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 -1.05 10
20
30
40
50
60
70
80
Vanadium Content (ppm wt)
2 R = 0.63
-0.97 -0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 0
2
4
6
8
10
12
14
16
18
Cadmium Content (ppb wt)
-0.94
Mass Magnetic Susceptibility (10 m /kg)
-0.95
-8 3
-8 3
Mass Magnetic Susceptibility (10 m /kg)
-0.96
(b)
-0.94
2 R = 0.69
-0.96 -0.97 -0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 -1.05
-0.95 -0.96 -0.97 2 R = 0.37
-0.98 -0.99 -1.00 -1.01 -1.02 -1.03 -1.04 -1.05
0
(c)
-0.95
-1.05 0
(a)
905
5
10
15
20
25
30
Nickel Content (ppm wt)
35
0
2
4
(d)
6
8
10
12
Iron Content (ppm wt)
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.
3.4. Differences between oil provinces
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 Russian sample and the Far East sample appear to be quite distinct. This may reflect specific features of the geological 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 provinces might be distinguished on the basis of their magnetic susceptibility. Clearly, however, more samples need to be measured in order to confirm any broad consistent differences between the various oil provinces.
-0.95
Mass Magnetic Susceptibility (10-8m 3/kg)
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 corresponds 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 compounds. 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 0.6–0.7 · 10 8 m3 kg 1, using values of mass susceptibility for iron given by Potter and Stephenson (1988, Table 1). In reality the metals are likely to be components in organometallic 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.
-0.96
N. Sea
-0.98
N. Sea N. Sea N. Sea
-0.99
Middle East
N. Sea N. Sea N. Sea N. Sea N. Sea N. Sea N. Sea N. Sea
-1.00 -1.01 -1.02
North America Middle East
-1.03
North America Far East
-1.04 -1.05
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
Russia
-0.97
0
1
2
3
4
5
6
Location
Fig. 10. Mass magnetic susceptibility in relation to specific oil provinces.
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907
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 to Fig. 11. This might be a factor in influencing the changes in wettability that one often observes between clean sandstones (quartz rich with little clay) and muddy sandstones (containing 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 wettability may also be a possibility.
4. Discussion of possible applications for petroleum reservoirs Magnetic susceptibility measurements might find a use in passive sensors in reservoirs for distinguishing between formation waters and crude oils. For example, such a sensor could potentially help to monitor the onset of 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 possible to further miniaturize such a system and employ it downhole. Such sensors would provide an environmentally friendly alternative to radioactive tracers. Although viscosity meters might also distinguish between formation waters and crude oils, magnetic sensors would have a further advantage in being able to also rapidly detect small concentrations of ferrimagnetic or antiferrimagnetic minerals, or migrating fines from important paramagnetic clays such as illite or chlorite (small concentrations of which can dramatically affect fluid permeability). The magnetic susceptibility sensors might thus also be used to monitor formation damage, or any anomalous effects arising from the hydrocarbon extraction infrastructure. It is also worth noting the values of the mass susceptibility of crude oils and formation waters in relation to some typical petroleum reservoir minerals, such as the diamagnetic matrix minerals and the paramagnetic permeability 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 diamagnetic matrix reservoir minerals such as quartz, feldspar, and calcite. However, the values are significantly less diamagnetic than the clay kaolinite. Magnetic properties may possibly play some role in rock–fluid interactions. 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
5. Conclusions The following conclusions can be drawn from the present study:
130
Paramagnetic
20
-30
Calcite
Magnesite
Feldspar
Anhydrite
Halite
Quartz
Gypsum
Formation water
-20
Dolomite
0 -10
Crude oil
10
Illite
30
Vermiculite
Muscovite
40
Montmorillonite
50
Chlorite BVS
60
Lepidocrocite
70
Glauconite
80
Chlorite CFS
90
Chamosite
Diamagnetic
100
Nontronite
110
Kaolinite
Mass Magnetic Susceptibility (10-8 m 3/kg)
120
Siderite
There were distinct differences between the mass magnetic susceptibilities (vm) of crude oils and formation waters. All the samples studied were diamagnetic, but the values for the crude oils were more negative. Two independent pieces of sensitive equipment 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 fractions and formation waters correlated with their densities. The values for crude oil also correlated with other physical properties, namely residue content, stock tank oil gravity, and viscosity. The results suggest that the magnetic measurements could potentially be used to rapidly characterise the physical differences between various petroleum reservoir fluids.
Ilmenite
906
Minerals and Fluids
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 from Hunt et al. (1995), Borradaile et al. (1990), and Thompson and Oldfield (1986).
O.P. Ivakhnenko, D.K. Potter / Physics and Chemistry of the Earth 29 (2004) 899–907
The values of vm for the crude oils also showed correlations with trace amounts of chemical components, namely the contents of sulphur, vanadium, cadmium, nickel, and iron. The results, however, should be treated with some caution, since the samples with higher contents of these elements also generally have higher density, which also correlates with vm. It appears that crude oils with higher density have higher residue content, and also contain higher concentrations of the above components. The relative contributions of intrinsic fluid density and these trace components to the total magnetic susceptibility signal is presently unclear. The values of vm for the formation waters was related to their solute composition. The experimental measurements were within 1% of the theoretically calculated 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 consistent 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 Norman Lang, of the Institute of Petroleum Engineering
907
at Heriot-Watt for providing us with crude oil and formation 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 University) 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 Eduard Petrovsky, for their constructive comments, which helped to improve the manuscript.
References Borradaile, G.J., MacKenzie, A., Jensen, E., 1990. Silicate versus trace mineral susceptibility in metamorphic rocks. Journal of Geophysical Research––Solid Earth 95, 8447–8451. Ergin, Y.V., Kostrova, L.I., Subaev, I.Kh., Yarulin, K.S., 1975. Magnetic Properties of Oils (in Russian). Depositor of VINITI, N3265-75. Ergin, Y.V., Yarulin, K.S., 1979. Magnetic Properties of Oils (in Russian). Nauka Publishers, Moscow p. 200. Guan, H., Brougham, D., Sorbie, K.S., Packer, K.J., 2002. Wettability effects in a sandstone reservoir and outcrop cores from NMR relaxation time distributions. Journal of Petroleum Science and Engineering 34, 33–52. Hunt, C.P., Moskowitz, B.M., Banerjee, S.K., 1995. Magnetic properties of rocks and minerals. In: Ahrens, T.J. (Ed.), Rock Physics and Phase Relations: a Handbook of Physical Constants. American Geophysical Union Reference Shelf 3, pp. 189– 204. Potter, D.K., Stephenson, A., 1988. Gyroremanent magnetization in magnetic tape and in iron and iron alloy particles. IEEE Transactions on Magnetics MAG-24, 1805–1807. Selwood, P.W., 1956. Magnetochemistry, 2nd edition. Interscience Publishers, New York p. 435. Thompson, R., Oldfield, F., 1986. Environmental Magnetism. Allen and Unwin, London p. 277.