SPE 121633

Spectroscopic Determination of Naphthenic Acid Composition from Various Calcium Naphthenates Field Deposits Mohammed Murtala Ahmed, SPE and K. S. Sorbie, SPE, Institute of Petroleum Engineering, Heriot-Watt University, Riccarton, Edinburgh, EH14 4AS, UK

Copyright 2009, Society of Petroleum Engineers This paper was prepared for presentation at the 2009 SPE International Symposium on Oilfield Chemistry held in The Woodlands, Texas, USA, 20–22 April 2009. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract Naphthenate formation in oil production systems has been recognised for some times as a troublesome and potentially

expensive flow assurance problem. The production of biodegraded or higher TAN (total acid number) crude oils together

with the changes in both chemical and physical properties during production operations may trigger naphthenate formation.

These naphthenate deposits may form as either a soft emulsion type sodium naphthenate or as a hard solid deposit of calcium

naphthenate.

Within the Flow Assurance and Scale Team (FAST) at Heriot-Watt, we have carried out an extensive series of naphthenic

acid extraction and characterization experiments on a range of field sodium and calcium naphthenates samples from around

the world. In particular, we have analysed these samples using the negative ion mass spectroscopic techniques, ESMS and

APCI-MS. In this paper, we present results of naphthenate extract analyses for calcium naphthenate field deposits from the

North Sea and Asia. Some of the samples analysed show the presence of both lower and higher molecular weight naphthenic

acids in the spectra, whilst others showed mostly the presence of higher molecular weight naphthenic acids with minor traces

of lower molecular weight species. Results from this work have now shed more light on the naphthenic acid composition in

different field calcium naphthenates, which has been a topic of debate for some time. Results are also presented from novel

re-precipitation experiments using the naphthenic acid extract and synthetic brine. Both APCI-MS and ESEM/EDAX data

are presented for these laboratory formed precipitates. These results lead to some clear conclusions on the final composition

of calcium naphthenate deposits.

Background and Introduction The increasing world demand for crude oil has stimulated the exploitation of higher TAN (total acid number) or

biodegraded crudes in various parts of the world. In some cases, this leads to the formation of either hard calcium

naphthenate deposits or soft sodium naphthenate emulsion formation during production operations1-11

. These crudes, which

are usually acidic in nature, tend to react with either the calcium or sodium ions in the formation water to form naphthenate

deposits/emulsions as a result of some physical and chemical changes that occur during production operations. The

naphthenate deposits/emulsions which are formed cause production problems such as blockage of the surface equipment,

shutdown periods of offshore installations, detrimental effect on BS&W and entrapped oil in sludges1,7-9,12-14

.

Extraction and spectroscopic characterization of different types of naphthenic acids responsible for the formation of either

calcium or sodium naphthenates is important for understanding the actual composition of these acids in the

deposits/emulsions. Naphthenic acids in crude oil are predominantly carboxylic acids, R-COOH, where R is often a saturated

cyclic structure15, 16

. However, it is also possible for naphthenate deposits to be formed from long-chain aliphatic carboxylic

acids, as observed in the soap industry17

. Formation of naphthenate deposits and emulsions during crude oil production is

usually relatively small – of order 10s mg of deposit per litre of crude. Although this appears to be an insignificant quantity,

the cumulative effect may lead to a very significant flow assurance problem1,18

. In early work, spectroscopic analyses of the

extract from calcium naphthenate field deposits often revealed the presence of predominantly higher molecular weight

naphthenic (tetra-carboxylic) acids, referred to as ARN, as the major acid component19,20,21,22

. However, this was not always

the case, as a combination of both lower and higher molecular weight naphthenic acids was also noticed in some calcium

naphthenate field samples23

. Analyses of naphthenic acid extract from field calcium naphthenate are performed using a soft

ionization source to avoid unnecessary fragmentation, and hence a less complex peak assignment of the mass-to-charge (m/z)

ratios from the spectra. Two of the commonly used soft ionization techniques are Electrospray Mass Spectrometry (ESMS)

2 SPE 121633

and Atmospheric Pressure Chemical Ionization Mass Spectrometry (APCI-MS)15,24,25,26,27,28,29,30,31

. However, fragmentation/

multiple ionisation of the higher molecular weight acids (ARN) from ESMS is now suspected, as reported in this paper. Each

of these spectroscopic techniques has a different sensitivity to the detection of both lower and higher molecular weight

naphthenic acids with the APCI-MS favouring the detection of ARN acid as compare to ESMS. A similar trend was

observed when commercial naphthenic acids were tested 32

. In addition to this inherent molecular weight sensitivity of the

soft ionisation MS techniques, solvent and equipment settings have been reported to influence the detection of these acids

from field naphthenate samples 33

. The formation of calcium naphthenate precipitates in the laboratory, using a combination

of commercial naphthenic acids and synthetic pH adjusted brine, has been extensively studied 5,7

. However, little previous

work has been carried out using re-precipitated naphthenic acid extracts from field naphthenate samples and this type of

experiment is described in this paper. This paper presents examples of the two different categories of naphthenic acid

distribution found in various calcium naphthenate field samples i.e. where both lower and higher molecular weight acids

occur together and where almost entirely higher molecular weight acids occur. In addition, novel re-precipitation experiments

using an extract from field deposits and pH adjusted brine are presented and discussed. For all cases, ESEX/EDAX analysis

was carried out on the field calcium naphthenate deposits and on the laboratory formed precipitates.

Experimental Details Three samples of calcium naphthenate field deposits were used for the naphthenic acid extraction, comprising of two samples

from the North Sea (UK and Norway sectors) and one from Asia. The field calcium naphthenates deposits used were treated

as received from the suppliers using the current standardize method of extraction employed within the Flow Assurance and

Scale Team (FAST). The extracts obtained from the extraction procedure were analysed using ESMS (electrospray mass

spectrometry) and APCI-MS (atmospheric pressure chemical ionisation mass spectrometry).

Calcium naphthenates deposits were rigorously clean using various solvents (acetone, methylene chloride, toluene and iso

octane). Naphthenic acid extraction from the cleaned deposits was carried out using 3M hydrochloric acid and toluene

(VWR Analar grade) at a room temperature for ~22 hours under magnetic stirring to achieve complete dissolution. The

organic fluid phase is then filtered using HY 579 hydrophobic silicone coated filter paper and the toluene solvent evaporated

using a rotary evaporator at a temperature between 50oC to 60

oC.

ESMS analysis of the extracts was performed using PE-Sciex API 150EX single quadrupole mass spectrometry in

negative ion mode with nitrogen both as nebuliser gas and curtain gas at a flow rate of 6 litre/min and a pressure of 60 psi.

Source temperature used was 350oC, needle voltage for ionspray -4.5 kV and the samples were introduced using a Harvard

pump with flow rate between 1 to 10μl/min and mass spectra were recorded from m/z 115 to m/z 2000. The APCI-MS

analysis was carried out using a PE-Sciex API 150EX mass spectrometer with a heated nebuliser source. Nitrogen was used

as both nebuliser and curtain gas with gas pressure of 10 to 12 psi, respectively. Nebulizer voltage and temperature were -6

kV and 400oC.

The re-precipitation experiments were carried out using naphthenic acid extracts from field calcium naphthenate deposits

dissolve in toluene (~1% acid concentration) and a pH adjusted synthetic brine (2.5% Na+ and 2.0% Ca

++) to mimic crude oil

and produced water phases respectively. The pH adjustment of the water phase was carried out using 10% wt NaOH (aq)

solution, prior to mixing of oil and water phases in the ratio of 1:1 (20ml of oil phase to 20ml of water phase) in a 100ml

Durrant flask. The mixture was shaken for 1 minute and left to settle before filtering the precipitates using 0.2μm filter paper

and subsequently re-extracting naphthenic acid from the laboratory formed precipitates using 3M HCl and toluene (VWR

Analar grade). Analysis of the extract was carried out using APCI-MS and the formed precipitate was also analysed using

ESEM/EDAX technique using SUTW-sapphire type detector.

Results and Discussion

Analytical: The calcium naphthenate samples studied in this paper are described in Table 1 below:

Table 1 – Brief description of the 3 calcium naphthenate samples used in this work

Sample Location of Naphthenate Deposit

Physical Description

Z1 North Sea (UK sector) Hard, dark brown, quite sticky deposit.

Z2 Asia Hard, dark brown, non-sticky deposit.

Z3 North Sea (Norway sector) Hard, shining dark brown, sticky deposit.

Analysis was carried out using ESMS and APCI-MS for each of the samples (Z1, Z2 and Z3) and two distinct distribution

patterns of the naphthenic acids were noticed. The spectra from extracts Z1 and Z2 showed minor traces of lower molecular

weight naphthenic acids (acyclic, monocyclic, bicyclic and alkylbenzoics) in the region of m/z 200 to m/z 650 and a very

pronounced ion peak at around m/z 1230 corresponding to the presence of ARN acid species. In contrast, the spectrum from

extract Z3 revealed a broad distribution of lower molecular naphthenic acids and a small but significant ion peak at around

m/z 1230 indicating the presence of ARN acid species.

SPE 121633 3

Electrospray mass spectrometry (ESMS): The negative ion ESMS spectra from samples Z1 and Z2 are presented in

Figures 1 and 2. Both of these spectra are dominated by essentially the same ion peaks at m/z 312.9 and m/z 313,

respectively. In both spectra this peak is strongly suspected to be z = 4 charged molecular ion fragment of the higher

molecular weight naphthenic acid referred as ‘ARN’ acids. Normally ARN acid (4-protic naphthenic acids) species are

detected at m/z ~1230 and upon fragmentation 4 charged ion fragments may be formed. Hence, these dominant ion peaks at

around m/z 313 from the spectra are interpreted and being z = 4 charged m = 1230 species i.e. ARN. In addition, the spectra

of both Z1 and Z2 show a number of much lower intensity naphthenic acid species as follows: (i) the naphthenic acid extract

spectrum of Z1 (Figure 1) shows the presence of acyclic (C20 to C20), monocyclic (C20 and C24), bicyclics; weaker evidence of

(C12, C13, C15, C16, C18 to C20 and C38 to C43 and C21) alkylbenzoics, and (ii) the Z2 spectrum (Figure 2) contains acyclic (C10,

C12 and C14 to C24), moncyclic (C11, C13, C17 to C19 and C23), bicyclic (C12, C13, C16 and C18) and alkylbenzoics (C11, C14, C16

to C18 and C21) naphthenic acid species.

The ESMS spectrum of the Z3 extract is presented in Figure 3. This spectrum revealed a significant abundance of each of

two classes of naphthenic acid families, as follows: (i) a broad distribution of lower molecular weight naphthenic acid from

m/z ~200 to m/z 650, and (ii) a much smaller but significant ion peak at m/z ~1230, which is clear evidence of higher

molecular weight naphthenic acid (ARN). The wide range of the lower molecular weight naphthenic acids present is

comprised of acyclic (C10 to C25, C27 to C29 and C36), monocyclic (C11, C13 to C28 and C30 to C32), bicyclic (C11 to C13, C15 to

C22, C24, C25 and C27) and alkylbenzoics (C8 to C21, C23, C25 to C27 and C31).

Atmospheric pressure chemical ionization mass spectrometry (APCI-MS): Spectra of the naphthenic acid

extract from samples Z1 and Z2 using the APCI-MS techniques are shown in Figures 4 and 5, respectively. The main feature

in both spectra is that they are totally dominated by a huge pronounced ion peak at m/z ~ 1230. Clearly, this is due to the

very significant presence of higher molecular weight naphthenic acid (ARN) species. The ARN acid species in both samples

appears to be the major component of the deposit, with other lower molecular weight acid species being a minor proportion

of the samples. It was also observed from the spectra (Figures 4 and 5) that there is weaker evidence of lower molecular

weight naphthenic acids in the range of m/z 200 to m/z 600 and m/z 200 to m/z 500 respectively, however, these are very

much in the minority in these extracts. In the Z1 spectrum, we observe the presence of trace amounts of acyclic (C19 to C21),

weaker evidence of monocyclic (C18 and C20 to C37), bicyclic (C19, C21 to C27, C29 to C37, C39 to C42) and alkylbenzoics (C22 to

C27, C29 to C35, C37, C38 and C41 to C43) naphthenic acid species. In the Z2 sample, we observe acyclic (C24, C26 to C29, C31 to

C37 and C39 to C47), monocyclic (C26 to C28, C30, C31, C33 to C41 and C44 to C47), bicyclic (C24 to C47) and alkylbenzoics (C24 to

C27, C29 to C40, C43 and C45 to C47) naphthenic acid species. Weak evidence of further naphthenic acids of mass greater than

m/z 690 was noticed in Figure 5 i.e. the Z2 extract sample; however, the responses are very weak relative to the background

noise of the sample. The spectrum of sample Z3 (Figure 6) revealed the presence of both lower and higher molecular weight

naphthenic acids in the region of m/z 200 to m/z 650 and m/z 1225 to m/z 1265, respectively. In addition, a cluster of ion

peaks at m/z 1430 to m/z 1470 of unknown origin from the spectrum was observed; however, the same ion cluster of this

mass range was seen in the ARN acid reference standard. Although this ARN acid peak is quite prominent in the Z3 extract

spectrum, it is not the major component of this deposit. The Z3 deposit is made up mainly of many lower molecular weight

naphthenic acids, as shown in Figure 6 unlike samples Z1 and Z2 which are made up mostly of higher molecular weight

naphthenic acids. The lower molecular weight naphthenic acids presents in the Z3 extract are as identified as acyclic (C19 to

C22), weak evidence of monocyclic (C18 and C20 to C37), bicyclic (C19, C21 to C27, C29 to C39 and C42) and alkylbenzoics (C22

to C27, C29 to C35, C37 to C38 and C41 to C43) acid species.

Re-precipitation experiment – extraction and APCI-MS analysis: The re-precipitation experiment was carried out

using the naphthenic acid extract from sample Z3 and a pH adjusted brine (pH 9 and pH 9.5). The reason for carrying out

such an experiment was to answer the question: if we fully digest a given naphthenic acid deposit (using the technique

described above) and then cause this to re-precipitate, do we get a deposit of the same composition or do we obtain selective

deposition/precipitation of only certain (probably higher molecular weight) naphthenic acid components? As the reader will

appreciate, this has some very important consequences for a range of issues concerned with naphthenate field management,

the experimental determination of deposit composition and naphthenate inhibitor testing. In this experiment, the Z3 fully

digested field deposit – for which the APCI-MS spectrum is shown in Figure 6 - was re-precipitated as shown in Figure 7.

Formation of abundant precipitates was observed at both pH values, with the pH 9.5 adjusted brine giving a slightly larger

quantity of the formed precipitates as observed in Figure 7. Naphthenic acid extraction from this re-precipitated deposit was

performed exactly as on the original cleaned field deposit and the APCI-MS spectrum was measured as before.

The negative ion mode APCI-MS spectrum on the re-precipitated Z3 deposit was recorded from m/z 150 to m/z 2000 and

is shown in Figure 8. The most important observation on this APCI spectrum in Figure 8 is that it is very similar to that of

the original Z3 deposit, shown in Figure 6. That is, the re-precipitation spectrum (Figure 8) shows mainly lower molecular

weight naphthenic acids and a lower but still significant amount of higher molecular weight (ARN) species, as does the

original spectrum (Figure 6). To emphasise this point, a direct comparison of the two APCI spectra of the original Z3 deposit

and the re-precipitated Z3 deposit is presented in Figure 9.

For completeness here, we give an analysis of the APCI spectrum of the re-precipitated Z3 deposit in Figure 8. It is clearly

seen that the spectrum contains both lower molecular weight (acyclic, monocyclic, bicyclic and alkylbenzoics) and higher

4 SPE 121633

molecular weight (ARN) naphthenic acids. The lower molecular weight naphthenic acids present in the spectrum comprised

of acyclic (C21 to C24 and C26 to C33), monocyclic (C24 to C34), bicyclic (C13 to C17 and C24 to C32) and alkylbenzoics (C15, C25,

C27 and C29 to C31) acid species in the range of m/z 200 to m/z 700. A cluster of ion peaks at m/z 1225 to m/z 1270 was

detected with a very strong ion peak at m/z 1230.5 corresponding to the presence of ARN acids. Furthermore, an observation

of cluster of ions at m/z 1430 to m/z 1465 of unknown origin was made; however, it should be noted that similar ions cluster

of this mass range was seen in the ARN reference standard under the same condition.

Environmental electron microscopy/energy dispersive x-ray analysis (ESEM/EDAX): An ESEM/EDAX

analysis was carried out on the naphthenate deposits Z2 and Z3 and on the laboratory formed precipitate from Z3 to examine

their morphologies and (approximately) establish the elemental composition of these deposits. The data from this study is

shown in Table 2. From this table it can be seen that both samples (Z2 and Z3) and the corresponding laboratory formed

precipitates contain calcium, carbon and oxygen in additions to some other minor elements. The elemental compositions of

calcium, carbon, oxygen are observed in the ratio 0.48: 90.55: 7.76 (atomic %) for Z2 deposit and 1.05: 81.49: 4.37 (atomic

%) in the case of the formed precipitate from Z2 extract whilst the Z3 deposit showed a composition in the ratio of 0.84:

92.59: 6.04 (atomic %) and 1.9: 87.55: 3.88 (atomic %) for formed precipitate from Z3 extract. The ESEM images from these

samples are presented in Figures 10 – 13 which shows the morphology of the various deposits.

Conclusions In this study, results on the analysis of calcium naphthenate deposits from 3 fields (Z1 – Z3) are presented, along with results

from a novel re-precipitation experiment on the Z3 deposit. An ESEM/EDAX analysis of the deposits and laboratory formed

precipitates is also presented. The main conclusions from this study are as follows:

(i) The ESMS analyses of the 2 naphthenic acid extracts (samples Z1 and Z2) showed the presence of traces of lower

molecular weight naphthenic acids (acyclic, monocyclic, bicyclic and alkylbenzoics) from the spectra, in addition, to the

dominant ion peaks at m/z 312.9 for Z2 and m/z 313 for Z2. These peaks are strongly suspected to be the z = 4 charged

molecular ion fragments of the higher molecular weight naphthenic acid (ARN) and are probably due to the ionisation

method used in the ESMS technique. This is essentially confirmed by the subsequent (milder ionisation) APCI-MS results

which show very clear peaks at m/z ~1230) for the same samples. The ESMS spectrum of the Z3 extract shows a distribution

of broad range of lower molecular weight naphthenic acids in the range of m/z 200 to m/z 650 and a small but significant ion

peak at around m/z 1230 which corresponds to ARN acid species. This ESMS result for Z3 is subsequently confirmed by

APCI-MS (see below).

(ii) The APCI-MS spectra of the Z1 and Z2 extract clearly shows a very strong pronounced ion peaks at around m/z 1228 to

m/z 1260 and m/z 1230 to m/z 1250 for each sample, respectively, which corresponds to ARN. In addition, these spectra

also show weaker evidence for the presence of lower molecular weight naphthenic acids, although these are at close to

background levels. In contrast, the APCI-MS spectrum of the Z3 extract very distinctively shows a proportionate distribution

of both lower and higher molecular weight naphthenic acids with quantitatively mainly a broad distribution of the lower

molecular weigh naphthenic acid in the range of m/z 200 to m/z 650, with a much lesser but still significant ion peak of ARN

acid in the range of m/z 1230 to m/z 1240.

(iii) The APCI-MS spectrum of the re-precipitated Z3 deposit clearly shows the presence of mainly lower molecular weight

naphthenic acids species (acyclic, monocyclic, bicyclic and alkylbenzoics) in the range of m/z 200 to m/z 650 and a lesser but

pronounced higher molecular weight naphthenic acid (ARN) specie in the range of m/z 1230 to m/z 1245. The APCI-MS

spectrum from the Z3 precipitate is very similar to the spectrum from the original deposit extract (Z3) and a direct

comparison of these is shown in Figure 9.

(iv) The ESEM/EDAX analyses of the naphthenate deposits (Z2 and Z3) and the corresponding laboratory formed

precipitates have shown that they contain calcium, carbon and oxygen in various ratios, indicating that they are calcium

naphthenates deposits and precipitates respectively.

Acknowledgements The authors would like to thank the sponsors of the Flow Assurance and Scale Team (FAST) at Heriot-Watt University:

FAST III sponsors – Baker Hughes, BP, BWA Water additives, Champion Technologies, Chevron, Clariant Oil Services,

ConocoPhillips, Halliburton, MI Swaco, Nalco, Petrobras, Petronas, REP, Rhodia, Saudi Aramco, Shell, Statoil-Hydro and

Total. We would also like to thank several of our sponsor companies for supplying naphthenate deposits. Specifically, we

acknowledge the very useful technical advice from Justin Debord, Jen-Emil Vinstad, Tom Baugh, Heidi Mediaas and Darrell

Gallup.

SPE 121633 5

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6 SPE 121633

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Sample Condition Magnification EDAX Atomic %

Ca : C : O

EDAX Atoms

Normalised

Ca : C : O

Z2 deposit Cleaned with acetone

and toluene

(x1300) 0.48 : 90.55 : 7.76 1 : 188 : 11.6

Precipitate from Z2 Cleaned with distilled

water

(x2409) 1.05 : 81.49 : 4.37 1 : 77.6 : 18.6

Z3 deposits Cleaned with acetone

and toluene

(x2258) 0.84 : 92.59 : 6.04 1: 110.2 : 15.3

Precipitate from Z3 Cleaned with distilled

water

(x1000) 1.90 : 87.55 : 3.88 1 : 46.10 : 22.5

Table 2: Summary of ESEM/EDAX Data.

Figure1: ESMS naphthenic acid spectrum from Z1 extract.

Figure 2: ESMS naphthenic acid spectrum from Z2 extract.

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Figure 3: ESMS naphthenic acid spectrum from Z3 extract.

Figure 4: APCI-MS naphthenic acid spectrum from Z1 extract.

Figure 5: APCI-MS naphthenic acid spectrum from Z2 extract.

8 SPE 121633

Figure 6: APCI-MS naphthenic acid spectrum from Z3 extract.

Figure 7: Re-precipitation experiment using Z3 extract and two pH adjusted brines (to pH 9 and pH 9.5).

Figure 8: APCI-MS spectrum of extract from laboratory formed precipitate (Z3 sample).

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Figure 9: APCI-MS spectra of Z3 extract and precipitate formed from Z3 extract (both spectra resemble each other very closely).

Figure 10: ESEM image of Z2 solvent cleaned deposit.

Figure 11: ESEM image of laboratory formed precipitate from Z2 extract.

10 SPE 121633

Figure 12: ESEM image of Z3 solvent cleaned deposit.

Figure 13: ESEM image of laboratory formed precipitate from Z3 extract.