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Detection and Impact of carboxylic acids at the crude oil-water interface Simon I Andersen, Mahavadi Sharath Chandra, John Chen, Ben Yanbin Zeng, Fenglou Zou, Mmilili M. Mapolelo, Wael A Abdallah, and Johannes Jan Buiting Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02930 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 14, 2016

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Detection and Impact of carboxylic acids at the crude oil-water interface Simon Andersen1*, Mahavadi Sharath Chandra1, John Chen1, Ben Y. Zeng1, Fenglou Zou1, Mmilili Mapolelo,1 Wael Abdallah2, Johannes Jan Buiting3 1

Schlumberger DBR Tech Center, Edmonton, Alberta, Canada, 2 Schlumberger Dhahran Carbonate Center, Dhahran, Saudi Arabia, 3Saudi Aramco, Dhahran, Saudi Arabia.

*: corresponding author: [email protected] ABSTRACT

The impact of surface active indigenous components on interfacial tension (IFT) of crude oil – water systems is an important parameter in many aspects of crude oil production such as emulsion stability, reservoir wettability and capillary number calculations. These components may affect productivity across the reservoir due to variations in concentrations. In most cases simulation of IFT is not taking interfacial activity into account and is purely based on oil bulk properties. In this paper we examine two crude oils and their subfractions such as maltenes, deacidified crude and natural acidic components. Films were prepared at the toluene – water interface with crude oil and its various fractions and studied for interfacial activity and chemical compositions. The chemical analysis of the interfacial active material indicated that carboxylic acids are preferentially adsorbed or concentrated at the oil/water interface. Infrared spectroscopic analysis of the interfacial films clearly demonstrates that carboxylic acids species (e.g. fatty acids, resins or asphaltenes with a -COOH functionality) are concentrated at the interface. The GC-MS analysis of the interfacial film revealed the presence of homologous series of linear chain carboxylic acids ranging from C10 – C25+. 2DGC-MS analysis showed that heteroacids are also present. Acid free crude was prepared and back mixed with the original crude oil in different proportions confirming the role of the acids in decreasing IFT. The removal of these relatively small amounts of acids leads to a decrease in IFT between 1.3 and 2.2 mN/m. The results also indicate that acids can be preferentially removed by ion exchange resins without affecting the overall composition of the oil as is shown by back-mixing de-acidified oil into the original oil. 1. INTRODUCTION The interfacial activity of crude oil components has been known for many years. This leads to lowering of the interfacial tension relative to hydrocarbons as well as increase emulsion stability due to interfacial film formation. The solid interfacial film or skin formation at the petroleum/water interface was reported initially by Strassner.1 The skin is seen as a solidified material surrounding droplets of crude oil in water. Seifert2 performed an extensive study of 1 ACS Paragon Plus Environment

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crude oil interfacial chemistry in the 1960s. We know from these studies that pH and salinity can activate the species bearing carboxylic acids which lead to a low IFT at high pH; this is also the concept of alkaline enhanced oil recovery (EOR). However, the role of basic nitrogen in lowering the IFT for some reservoir crudes at low pH is still unclear. Seifert also showed that IFT could be correlated with the relative change in infrared absorbance of the C=O vibration. Biodegradation is a common path to formation of carboxylic acids and is abundantly investigated in the literature 3-6. Interfacial films often are ascribed to asphaltenes which may be modified by resins7. Neumann et al. reported a qualitative correlation between resin content and the formation of the interfacial film8; The higher the resin content (polars) the higher the likelihood of skin formation based on 5 crude oils analyzed. Kilpatrick and co-workers made a significant contribution to the understanding of the effect of asphaltenes and resins as emulsion stabilizers in petroleum systems and film forming components9. Measuring petroleum/water interfacial tension is known to be time dependent. Depending on the chemistry of the crude, it may take an extended period of time (hours) to get equilibrated interfacial tension value. In addition, as a result of equilibration of two phases, this will also alter the physical properties viz., density of the individual phases, which will influence the IFT measurements10. However, if we look at the equilibration process at the reservoir scale, during the production life cycle of reservoir, the concentration of the naphthenic acids will gradually increases which suggest a gradual decrease in the IFT during the reservoir life cycle. In addition, apparent concentration of the polar species would change with the depth of the reservoir as various geochemical studies show11. Many studies have been dedicated effects of asphaltenes at interfaces showing the ability of this generic fraction to stabilize emulsions, lower interfacial tension and create interfacial films between a hydrocarbon phase and a water phase. However, none of these factors related to interfacial active species are considered in the traditional reservoir simulators while predicting the interfacial tension. A common approach used in reservoir simulation is the so-called Sutton equation12: ߛ௢௪

ସ

1.58ሺߩ௪ − ߩ௢ ሻ + 1.76 =ቈ ]቉ ܶ௥଴.ଷଵଶହ

Where γow is the IFT, ρ is the density of respectively (o) oil and (w) water and Tr is the reduced temperature of the hydrocarbon phase (Tr=T/Tc , where Tc is the critical temperature). As observed, this equation provides bulk property relations where the pressure and temperature effects on IFT are captured through both the critical temperature and the density of the oil. However, both of these values do not at all relate to surface activity. Neither it is capable of capturing any significant variations in content of natural surfactants which may not even manifest itself as a change in density or in gas chromatographic analysis. In reality, components

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present in the range as low as 100 -1000 ppm could dominate interfacial behavior as we know from oil field additive that may be effective down to less than 50 ppm. A common notion is that asphaltenes dominate the interface between oil and water. This is often reported in model studies of say asphaltene solutions in toluene.13, 9, Water-in-oil emulsion studies have be mainly addressing physical film properties and the relation with emulsion stability and content of various oil fractions14-20. Also the impact of naphthenic acids have been examined for interfacial properties 21,22. Carboxylic fatty acids are known to exist in oils due to biological activity and in biological systems these acids are powerful surface film forming components.23 Hence, the effect of chemical composition of the brine, the reservoir hydrocarbon fluid and the variation in the composition affects the interfacial tension (IFT). This also leads to practical problems in obtaining correct and accurate measurements of the interfacial tension for assessment of reservoir performance. Even relatively small variations in the IFT may propagate through simulations and contribute with large uncertainties in final results. The hypothesis of this study was therefore, to investigate if carboxylic acids were indeed found as part of the interfacial film formed between crude oil and water. We further aim at understanding the interaction between carboxylic acids and asphaltenic species at the interface which will be reported in an upcoming manuscript. The formation of rigid interfacial films between crude oil and water has been frequently reported and this obviously affects IFT measurements. In the present work, we examine the water-crude oil interfacial properties both in terms of interfacial tension and especially surface film composition. A technique is employed to capture concentrated interfacial films formed at a large surface area. These were analyzed with analytical techniques such as FTir and GC-MS to capture the chemical composition of the interface. Most of these techniques are not capable of analyzing asphaltenes chemistry due to limitations in the techniques with regards to molecular size and spectral data may also be very similar for different fractions such as maltenes and asphaltenes. Hence, we can only infer the presence of asphaltenes by observations of rigid films around pendant drops as reported in many works. The effect and role of natural occurring surface active carboxylic acid bearing components (fatty acids, resins and asphaltenes) can in part be measured directly and in this work this is examined further by removal of these acids using ion exchange resins and examining the impact on interface chemistry and properties. 2. EXPERIMENTAL 2.1.Materials. Whole oils were used as received from depressurization of high pressure bottom-hole samples. Table 1 provides a brief overview of central properties of the two oils investigated. The fractions viz., de-acidified oil, maltenes, asphaltenes and organic acids were extracted from the whole oil as described below. HPLC grade toluene was 3 ACS Paragon Plus Environment

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purchased from Fisher Scientific, Canada and used as received. The water used throughout the study was purified with a Millipore system and had a resistivity of 18.2 mΩ/m (Milli-Q water). 2.2.Sample Preparation. Crude oil was fractionated to study the influence of various components at the oil/water interface. Maltenes were prepared by removing the asphaltenes from crude oil using a modified ASTM D4124 method. De-acidified oil was prepared by extracting the acid components from crude oil using the following method based on ion exchange resins (IER): The procedure of acid extraction is based on Mediaas et al. with some modifications primarily being the lack of dilution of the oil before contact with the EIR agent24. The extraction procedure resulted in an acid sample and a de-acidified oil sample. 1 g acid IER is conditioned in 1M NaHCO3 water solution and dried before being added to 20 mL of oil. The oil-IER mixture was then stirred gently overnight at room temperature. Afterwards, the mixture was centrifuged at 10,000 RPM for 3 hours at room temperature. The top phase was removed and stored as the deacidified oil. The resin was stirred in a dichloromethane (DCM) – formic acid solution overnight. The DCM – formic acid mixture was then extracted and evaporated to obtain the acid sample. While the de-acidified oil did not experience any evaporative loss, the recovered acidic material from the IER had loss of light-ends due to the evaporation of the DCM and formic acid. Therefore an accurate mass balance was not possible also because of potential residual oil within the IER material. 2.3.Interfacial Film Preparation. Interfacial films were prepared to identify the chemical constituents present at the interface. Initially, a toluene/water interface was prepared by pouring known amount of deoxygenized water as the sub phase and toluene as the top phase in a glass trough. 2 ml of crude oil was injected in the top-phase (toluene) with a hypodermic needle. The resulting interface had a length and width of 23.3 cm and 17.7 cm, respectively, and a total area of 412.4 cm2. The trough was covered and allowed to equilibrate at room temperature for 24 hrs. The top phase was then removed without disturbing the interface and a new batch of 400 mL pure toluene was added to the trough. This procedure was repeated at least 3 times to remove any material that is not strongly adsorbed to the interface. The top phase solution was kept for further analysis. After the final removal of toluene the interfacial film was transferred to a Teflon strip by skimming the interface. The captured material was removed by dichloromethane which was evaporated under nitrogen. Interfacial films were also obtained by contacting maltene solutions and water, and de-acidified oil and water mainly for the GC-MS studies. 2.4. Infrared Spectroscopy. The oils, the extracted interfacial film samples from the experiments in which crude oil was contacted with water, as well as the extracted acid, the de-acidified oil, maltenes and asphaltene samples from the oil were analyzed by Fourier transform infrared spectroscopy (FTir) by attenuated total reflection (ATR) with a Varian 600-IR series spectrometer equipped with a Pike Technologies MIRacle ATR accessory. Each sample was first dissolved in DCM prior to measurement, and then 4 ACS Paragon Plus Environment

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several drops of the resulting solution were placed on ATR crystal platform, and the DCM was allowed to evaporate. Measurements were only taken after all DCM had evaporated from the platform. The ATR crystal stage was thoroughly cleaned with DCM between samples to avoid any cross contamination. 2.5. Gas Chromatography – Mass Spectroscopy (GC-MS). The saturated and the aromatic hydrocarbon fractions were analyzed using a gas chromatograph coupled to a mass spectrometer (GC–MS) from Thermo Scientific. Compound separation was performed on a Trace Ultra Series GC equipped with a low bleed dimethyl-polysiloxane capillary column (VF-1ms; 30 m length, i.d. = 0.25 mm, 10 film thickness = 0.25 lm). Helium was used as carrier gas and the temperature of the GC oven was programmed from 40°C (1 min) to 325°C at 3°C/min, followed by isothermal heating at 325°C for 15 min. For compound identification, the gas chromatographic system was linked to a TSQ Quantum XLS mass spectrometer operating in the electron impact mode (70 eV). MS detection mode started 11 min after GC injection to avoid detection of solvent peak with the MS source. MS scan range was 60 – 600 m/z using a scan time of 1 s. The temperature of the source was 250°C ± 5°C tolerance. The software Xcalibur 2.1 form Thermo Scientific was used to evaluate the GC-MS chromatograms. Identification and quantification of compounds was done using a mass to charge ratio of 73 (m/z) for carboxylic acids. Fluoren-9-one was used as external standard. A known concentration of fluorenone (2500 ppm) in hexane/toluene solution was run before and after the sample set to assure stability of the chromatographic procedure. Combining both gas chromatography and mass spectrometry into a single method allows for dramatically increased sensitivity and the ability to accurately identify specific compounds within each sample. Interfacial film samples were first dissolved in a 50/50 mixture of hexane and dichloromethane in order to prepare them for GC-MS analysis. 2.6. 2D - Gas Chromatography – Mass Spectroscopy (2D GC-MS). 2D GC-TOFMS analyses were carried out on Leco Pegasus GC TOFMS instrument coupled with a primary column of VF-1ms (50 m × 0.25 mm × 0.25 µm) and a secondary column of BPX-50 (1 m × 0.1 mm × 0.1 µm). Helium served as carrier gas with a flow rate of 1 ml/min. Primary GC oven was initially heated to 60oC and held for 5 minutes, and then further heated to 295oC (held for 20 minutes) at a rate of 2oC/min. The secondary oven was 20oC higher than the primary oven. A modulation period of 4 seconds was set up for the 2D GC-MS. Sample inlet temperature was 315oC and the split ratio was 60. MS scanning ranged from 35 to 450 m/z under electron energy of 70 eV. Mass spectral libraries of “mainlib”, “nist_ms”, “replib” and “wiley8” were used for compound identification. Carboxylic acids were methylated to enhance detection: The sample was redissolved in ca. 0.5 ml dichloromethane and 60 µl TMSH (trimethyl sulfonium hydroxide) solution (0.25 M in methanol solution) was added to methylate the acids.


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2.7.Density Measurements. The densities of the separated aqueous and organic phases were measured using Anton Paar mPDS 2000 v3 densitometer for use in the interfacial tension measurements 2.8. IFT Measurements. All the IFT measurements were performed at room temperatures viz. 24oC. Interfacial tension measurements between the aqueous and organic phases were carried out using rising drop (inverted pendant drop) technique. Crude oil (or modified crude oil) was used as the organic phase and de-ionized water as aqueous bulk phase in the cell. Using a screw type syringe a drop of the crude oil was made at the tip of the capillary in the aqueous bulk phase. Maximum drop size was prepared during all the experiments. The drop was illuminated with a light source and its projected image shape was captured using a CCD camera, which is directly connected to a dedicated PC. An image capturing software supplied by TECLIS was used to analyze drop shapes.

3. RESULTS AND DISCUSSION In literature, many studies have used the emulsion technique to generate interfacial films for studies of chemical compositions and properties of these10. The approach is as follows: An emulsion is generated and separated from free oil and water phases. The emulsion is then diluted with solvent to gently replace oil and to remove the non-surface active material without disturbing the interfacial film. Following this the emulsion is broken and the interfacial material is recovered in solution. The described procedure of solvent washing of the emulsion and the recovery of surface inactive species may indeed be difficult. Therefore, we employed a method in which a monolayer of adsorbed material is captured after extensive washing of a large surface. This in principle allows capturing only the strongly adsorbed interfacial active material and diminish the impact of other oil components. The equilibrium interface formed by placing crude oil at the toluene - water interface was extracted and analyzed below. FTir spectra of the original oil, the interfacial film recovered after contact of crude oil and water, and, as reference, the acidic material extracted by IER for both crude oil systems are shown in Figure 1. In this analysis it is important to remember that individual vibrations also have very different absorbance coefficients e.g. the peak related to CH2 stretch at 2927 cm-1 in n-alkanes has an absorbance coefficient 8 times larger than the related vibration at 1467 cm-1 25. The carboxylic acid C=O vibration at about 1720 cm-1 has a very high absorbance coefficient approximately 4 times larger than the methylene stretch peak improving quantification and relative analysis of concentration26. Infrared spectroscopic analysis of the interfacial films clearly demonstrates that carboxylic acids species (fatty acids, resins or asphaltenes with COOH functionality) are increased in concentration at the interface. Absorbance coefficient of the C=O vibration are commonly believed to be quite constant regardless of the hydrocarbon backbone at which the group is situated26. The absorbance coefficient will be affected by hydrogen bonding but not to an extent which can affect the relative conclusions of this work. The FTir analysis of 6 ACS Paragon Plus Environment

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the interfacial material clearly indicated the presence of carboxylic acid groups at 1718 cm-1. The concentration of acid in the neat oils is low and cannot be observed as a separate peak while we see fully resolved peaks in the interfacial material related to carbonyl (C=O) vibrations which we can relate to the carboxylic acid group COOH. This means that partitioning in the interfacial layer must be substantial for carboxylic acids to become apparent in the spectrum. The acids extracted by IER were examined by FTir in order to verify that acids were indeed removed using the methodology applied. While no discernible C=O peaks were seen in the oils they were observed in the IER extracted material. In the acid spectrum carboxylic group appeared at 1724 cm-1. IER extracted acids from crude oil 1 were seen to be more complex with a major peak situated at 1200 cm-1. This could in part be due to a contamination by formic acid used in the extraction of the acids from the ion exchange resin. However, this will not affect our prime target of investigating the actual crude oil interfacial film which never saw formic acid. From the comparison of spectra related to Crude oil 2, which has no indication of contamination, it is remarkable that the C=O peak at 1720 cm-1 is relatively smaller compared to the interfacial film indicating that the IER recovered acids might be contaminated by co-extracted non-acidic oil components. Again it should be remembered that the recovery of the acids only served as a check for the removal of acids in connection with the below IFT investigation. While many former ATR and DRIFT devices in the past were seen as semi-quantitative, the PIKE ATR stage is known to be quantitative in response and hence this allow us to assess relative differences between interfaces from the two oils. Often the comparison should be on a relative basis say to peaks in the same region of the spectrum. The results presented in Figure 1 indicates that the content of COOH groups relative to the methylene group vibration at 1467 cm-1 in Crude Oil 1 is higher than that of Crude Oil 2. The FTir spectra indicate the presence and increased adsorption of carboxylic acids at the interface; however, FTir cannot confirm the hydrocarbon structure of the acids. A gravimetric analysis of the recovered material could give some qualitative indications of the latter based on acid groups/mass but recovered amount were unfortunately too small for such analysis and could be subject to evaporative loss during the solvent removal step. Figure 2 compares spectra of the interfacial material, the IER recovered acids, the asphaltenes, the deacidified oil and the original crude oil 2 with an enlargement of the 1900 -1500 cm-1 region. Based on peak height the concentration of COOH in the interface from oil 2 increases at least 15 times relative to the original oil, while the relative content of aromatics to alkanes remain the same. The spectra also indicates that the content of C=O in carboxylic acids is relatively small in the asphaltenes, something we have observed in other works as well 27. Two peaks are seen in the carbonyl range for the interfacial material with the second contribution at slightly lower wavenumbers (1710 cm-1) indicating that COOH is hydrogen bonded in some molecules. This peak is seen to be the most important in the spectrum for the IER extracted acids indicating that a majority is in the hydrogen bonded stage. The spectrum of asphaltenes indicates the expected increased aromaticity of this fraction at about 3050, 1600 and 900-700 cm-1. This is 7 ACS Paragon Plus Environment

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not seen in the interfacial material and could indicate that relative composition of the interface is made up by molecules with more saturated acidic functionality than aromatic functionality. While the FTir analysis indicates that COOH containing species are concentrated relative to the bulk oil by adsorption to the water-solution interface the technique cannot tell details on the type of species. We proceeded to investigate if any detailed information could be obtained using gas chromatography. In order to identify specific components of the actual chemistry of the interfacial material GCMS as well as 2D GC-MS analysis was performed on all the samples. The GC analysis applied only reports molecules boiling below 400oC which corresponds to n-alkanes with 32 or less carbons. This limitation means that higher boiling compounds are not recorded by the technique. Also it should be remembered that asphaltenes cannot be subject to gas chromatographic analysis and hence no information can be gathered regarding the presence of these using any GC. Therefore the chromatograms only represent part of the composition of the samples investigated as is always the case. GC-MS Chromatograms are presented in Figures 3 & 6 of both original oils prior to contact with the interface, the recovered interfacial films and the remaining supernatant oils after contact with the water phase and therefore extraction of the interfacial material. We present analysis of contact with maltenes (deasphaltened with heptane) and the corresponding interfacial films and supernatant oil (Figures 4 and 7). As well as the de-acidified oils, their interfacial and supernatant material after water contact (Figures 5 and 8). Hence we represent different fractions of oils where both asphaltenes (interfacial active) and acids have been removed separately. GC-MS analysis was able to identify the presence of homologous series of carboxylic acids at the interface and in cases in the supernatant samples after equilibration. All the interfacial samples from crude oil samples analyzed indicate the presence at the interface of a relatively increased concentration of linear chain carboxylic acids with carbon numbers between 10 and 25 (figure 3 and 6).For Crude oil 2 the distribution starts at C15OOH. Though linear chain carboxylic acids were seen in the supernatant/ toluene phase after equilibration, their concentration was insignificant in comparison with the interfacial material. Hence the oil/water interface preferential adsorbs the fatty acids relative to other components in the range detected. Figures 3-5 present the GC-MS chromatograms for the different fractions of Crude oil 1. In the crude oil, the acids are not detected due to the dominance of the hydrocarbons; the n-alkane homologous series can be seen in the oil but cannot be seen in the interfacial material at any substantial level. The adsorption from maltenes of crude oil 1 (Figure 4) showed less linear carboxylic acid than the same experiment with crude oil 2 maltenes (Figure 7). The acids seen at the interface could not be observed in the spectrum of the maltenes before contact with the interface. For the contact of the interface with de-acidified oil we also observe remaining linear carboxylic acids in de-acidified crude 2 (Figure 8), while crude 1 (Figure 5) shows no signs of linear acids in the amenable range.


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Overall crude oil related films show the presence of higher concentration of linear chain organic acids followed by the interfacial films formed by maltenes. The presence of substantial amounts of linear carboxylic acids in bulk oil has been reported for many oils 24. The interfacial films prepared by the de-acidified crude oil indicate only trace amounts of carboxylic acids as expected. This also confirms that the ion exchange resin extraction procedure specifically target the acidic species. As presented in Figures 6-8, Crude oil 2 showed relatively lower amounts of fatty acids at the interface confirming the FTir observations. It is also observed that Crude oil 2 presents almost entirely linear chain acids while Crude oil 1 has more complex acids along with the linear acids in the GC resolved range. The observed ester compound at a retention time of 40.22 minutes seen in chromatograms of crude oil 1 is assumed to be a contaminant which is not seen in Crude 2. The maltenes contain a reduced amount of carboxylic acids in the interfacial film compared to the crude oil which indicates that these are lost in the deasphaltening either through direct precipitation with the asphaltenes or by adsorption to the glassware and filter material. The latter is a well-known issue in fatty acid analysis. Whole oil, maltenes and de-acidified oil show almost identical chromatograms at 73 m/z for Crude oil 1. Crude oil 2 which contains larger amounts of light ends only show identical chromatograms for whole oil and de-acidified oil while a substantial loss of light ends has taken place during the deasphalting and recovery of the maltenes by heptane evaporation. This is in agreement with the very close resemblance of the FTir spectra of these fractions. The 2D-GC-TOF-MS analysis was performed after methylation of the crude 1 interfacial material sample. The derivatization normally enhances detection and also the range of molecular size is increased as the boiling point of the methyl ester of the carboxylic acids is substantially less than the carboxylic acid itself. However, due to the very small sample size only major molecules could be observed in the interfacial material; traces of linear alkanes (m/z 57) from nC15 to nC29 were observed in the interfacial film. Interestingly, except for traces of pristane (2, 6, 10, 14-tetramethylpentadecane) and phytane (2, 6, 10, 14-Tetramethylhexadecane), no other branched alkanes were detected in the sample. Fragments of Alkyl mono aromatics (m/z 91) were mainly due to the toluene solvent used in the study. This basically indicates that no low molecular weight aromatics are found as interfacial active at the interface. Methylated linear carboxylic acids (m/z 74) detected in the material were from nC8COOH (nonanoic acid) to nC17COOH (octadecanoic acid, stearic acid). m/z 74 was used instead of 73 in this part to also observe n-alkanes present. The 74 fragment will be found both in the alkane and in the methylated carboxylic acid and hence we observe both in the same run using the second GC dimension to separate the two. Surprisingly when comparing with the GC-MS data, nC15COOH (palmitic acid) and nC17COOH are more prominent compared to the other acids using this technique. Both Palmitic and Stearic acid could be a result of bioactivity in the sample but are also the most abundant fatty acids in nature. Hence the dominance in this particular sample analysis could be due to contamination during the sample preparation after the extraction. Some di-acids and acids containing both the carbonyl functional group (-C=O) and sulphur (-S), 9 ACS Paragon Plus Environment

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were seen in the interfacial film at m/z 59 as well and identified using the available mass spectral libraries. Di-acids were identified as succinic acid (butanedioic acid) and glutaric acid (pentandioic acid). Branched monoacids were also observed. It is interesting to note that we have not observed any nitrogen based compounds at the interface. This is in agreements with analysis of emulsion interfacial material by Muller et al.22 who reported less nitrogen at the interface compared to the entire oil. The semi-quantitative ratio of linear alkanes, aromatics (exclude toluene and xylene), linear acids and hetero-acids are shown in Table 2 based on the peaks identified in the “detection window” below n-alkane C32. Therefore, this does not comprise larger molecules such as asphaltenic species. This was only done for oil 1 as the 2DGC-TOF-MS methodology was found to be too qualitative also compared to the GC-MS application. Interfacial Tension Measurements: The above analysis indicates that interfacial active carboxylic acids can be removed from crude oil using ion exchange resins (IER) without having a great impact on the apparent oil composition. Therefore it is interesting to investigate how removal of the acid affects the interfacial tension as IFT is an important parameter in reservoir simulation. Interfacial tension by pendant drop shows that for both crude oils the same trend is seen: the deacidified oil has a higher IFT compared to the original oil. Figure 9 shows the IFT of crude oil 1 and 2 along with their de-acidified fraction. This confirms that carboxylic acids (In any form) are active at the interface also in the presence of asphaltenes which we assume only is partially affected by the IER. Also, it is noteworthy that based on the FTir and GC-MS analysis crude 2 contains less acid than crude 1 but the effect could also be due to asphaltenes which cannot be confirmed by the analytical methods used. Despite this, the relative change when acids are removed is larger in crude oil 2 (2.2 mN/m) than in crude oil 1 (1.3 mN/m) which could be cause by differences in composition of the acids affecting the hydrophilic-lipophilic balance, the asphaltene content as well as the surface activities of the individual molecules. As noted in the GC-MS analysis linear carboxylic acids were observed in the interfacial film from deacidified crude oil 2. This indicates that the IER extraction process was not entirely quantitative for this oil. In order to further understand the impact of the acid on the interfacial tension with varying content of other species such as asphaltenic types, mixtures were prepared of the original oil and the de-acidified oil. By varying the ratio of the two, one can emulate changes in extracted acid concentration without back mixing say acids or asphaltenes into the oil as we assume that the ion exchange resin only target the carboxylic acid bearing compounds and no other composition was affected. This is a viable assumption as the mass ratios of all other species remains the same as they are not affected by the removal of acid types as the evaporative loss was kept at a minimum. The data presented above seems to support this hypothesis. As seen in Figure 10 there is steady 10 ACS Paragon Plus Environment

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decrease in IFT as more original oil containing the acid is added to the de-acidified oil. We assume the asphaltene content is the same in all samples and hence the figure indicates that the variations of molecules with carboxylic functionalities affects the IFT. This supports the hypothesis that carboxylic acid containing species affect IFT of oils. The difference between the two oils can be both due to acid content and asphaltene content that are lower in crude oil 2 compared to crude oil 1, as it should be remembered that the EIR process does not discriminate between where the COOH is located. We believe this approach might reflect better the true nature of the petroleum systems relative to the prevalent use of examinations of solutions of different oil fraction. In the latter one could be observing the behavior of compounds suffering from denaturation after lengthy steps of separation, drying, and redissolution. Qualitatively we also see the expected response to changes in concentration of surface active species.

4. CONCLUSIONS Interfacial equilibrium films were formed and extracted physically using a mechanical skimming of the surface after repeated washing of the interface to remove non-adsorbed material. By analyzing interfacial films formed using different analytical techniques, it was shown that species containing carboxylic acid functionality and especially fatty linear carboxylic acids preferentially adsorb at the water/oil interface. The carboxylic acid functionality is an important factor in anchoring molecules at the oil/water interface as also known from biological systems. The FTir only gives an average composition while the GC based methods have a limited range due to volatility and boiling point restrictions. None of the methods applied can directly observe the presence of asphaltenes. FTir analysis did on the other hand side not show an increase in aromatic hydrocarbons at the interface which could be an expected indication of asphaltenic material. The analysis however does not exclude the existence of fractions of the asphaltenes at the interface, but indicate that the interface also strongly attracts other species such as the carboxylic acids. It will be important to understand how these acids affects the film formation and properties of these films. Finally, the role in IFT of native carboxylic acids was demonstrated by performing interfacial tension measurements on both de-acidified oil and original crude oil. Removal of small amounts of acid results in a relatively significant change in IFT even when the removal is partial. To further support the approach, it was observed that acid removal did not alter the GC-MS chromatograms or the FTir spectra relative to the original oil. In conclusion, we have demonstrated that it is important during oil production (such as during reservoir life span) to be able to monitor changes in content of even smaller amounts of surface active substances such as carboxylic acids.

AUTHORS INFORMATION 11 ACS Paragon Plus Environment

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Corresponding author Sinon Ivar Andersen, Tel: +45 25135656, [email protected].

ACKNOWLEDGEMENT We are grateful for the permission to publish this work granted by Schlumberger and Saudi Aramco and the financial support from Saudi Aramco. Also thanks to Dr. Eric Lehne (SLBDBR) for executing GC-MS measurements. Thanks to Drs. Shawn Taylor and Ronald van Hal (Schlumberger) for fruitful suggestions. The input from reviewers is highly appreciated.

REFERENCES (1) Strassner, J.E. Effect of pH on Interfacial Films and Stability of Crude Oil-Water Emulsions. J. Pet. Tech. 1968, 20, 303-312. (2) Seifert, W.K. Effects of Phenols on the Interfacial Activity of Crude Oil. Anal.Chem. 1969, 41, 562-568. (3) Nascimento, L.R.; Reboucas, L.M.C.; Koike, L.; Reis, F.A.M.; Soldan, A.L.; Cerqueira, J.R.; Marsaioli, A.J. Acidic Biomarkers from Albacora oils, Campos Basin, Brazil. Organic Geochem. 1999, 30, 1175-1191. (4) Meredith, W.; Kelland, S.J., Jones, D.M. Influence of Biodegradation on Crude Oil Acidity and Carboxylic Acid Composition. Organic Geochem. 2000, 31, 1059-1073. (5) Barth, T.; Hoiland., S.; Fotland, P.; Askvik, K.M.; Pedersen, B.S.; Borglund, A.E. Acidic Compounds in Biodegraded Petroleum. Organic Geochem. 2004, 35, 1513-1525. (6) Skaare, B.B.; Wilkes, H.; Vieth, A.; Rein, E.; Barth, T. Alteration of Crude Oils from the Troll area by Biodegradation: Analysis of Oil and Water Samples. Organic Geochem. 2007, 38 (11) ,1865-1883 (7) Ese, M.-H.; Yang, X.; Sjoblom, J. Film Forming Properties of Asphaltenes and Resins. A Comparative Langmuir-Blodgett study of Crude Oils from North Sea, European Continent and Venezuela. Colloid Polym. Sci. 1998, 276, 800-809. (8) Neumann, H.J.; Kopsch, H.; Jury, D.; Samii, B. Beitraege zur Analytik Deutcher Rohoele, Forschungsberichte 180, DGMK 1980. (9) a) McLean, J. D.; Kilpatrick, P. K. Effects of Asphaltene Solvency on Stability of Water-in-Crude-Oil Emulsions. J. Colloid Interface Sci. 1997, 189, 242-253. b) Yang, X.;, 12 ACS Paragon Plus Environment

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Verruto, V.J.;, Kilpatrick, P.K. Dynamic Asphaltene-Resin Exchange at the Oil/Water Interface: Time-Dependent W/O Emulsion Stability for Asphaltene/Resin Model Oils. Energy Fuels 2007, 21, 1343-1349. (10) Chandra. M. S.; Zacharia, J.; Horvath-Szabo, G. Impact of Pre-equilibration on the Assessment Methodology of Interfacial Tension Measured between Aqueous and Heavy Oil Phases. Energy Fuels 2011, 25, 2542–2550 (11) Larter, S. Private communication; and Shafiee, N. S.; Jones, M.; Oldenburg, T.; Larter, S. Crude Oil Acidity, Measurement Development and application. Paper OM2-3 presented at IMOG 26, Tenerife, Spain, Sep. 15-20, 2013. (12) Sutton, R.P. An Improved Model for Water-Hydrocarbon Surface Tension at Reservoir Conditions, SPE Annual Technical Conference and Exhibition, 4-7 October, New Orleans, Louisiana, SPE 124968, 2009. (13) Rane, J.P.; Harbottle, D.; Pauchard, V.; Couzis, A.; Banerjee, S. Adsorption Kinetics of Asphaltenes at the Oil–Water Interface and Nanoaggregation in the Bulk. Langmuir 2012, 28, 9986-9995. (14) Gao, S.; Moran, K.; Xu, Z.; Masliyah, J. Role of Bitumen Components in Stabilizing. Water-in-Diluted Oil Emulsions. Energy Fuels 2009, 23 (5), 2606–2612. (15) Solovyev, A.; Zhang, L. Y.; Xu, Z.; Masliyah, J. H. Langmuir Films of. Bitumen at Oil/Water Interfaces. Energy Fuels 2006, 20 (4), 1572-1578. (16) (a) Zhang, L. Y.; Breen, P.; Xu, Z.; Masliyah, J. H. Asphaltene Films at a Toluene/Water Interface. Energy Fuels 2007, 21, 274-285; (b) Zhang, L. Y.; Xu, Z.; Masliyah, J. H. Characterization of Adsorbed Athabasca Asphaltene Films at Solvent−Water Interfaces Using a Langmuir Interfacial Trough. Ind. Eng. Chem. Res. 2005, 44, 1160-1174. (17) Lobato, M. D.; Pedrosa, J. M.; Hortal, A. R.; Martınez-Haya, B.; Lebron-Aguilar, R.; Lago, S. Characterization and Langmuir Film Properties of Asphaltenes Extracted from Arabian Light Crude Oil. Colloids and Surfaces A: Physicochem. Eng. Aspects 2007, 298, 72–79. (18) Wang, S.; Liu, J.; Zhang, L.; Masliyah, J.; Xu, Z. Interaction Forces between Asphaltene Surfaces in Organic Solvents. Langmuir 2010, 26 (1), 183-190. (19) Alvarez, G.; Poteau, S.; Argillier, J.-F.; Langevin, D.; Salager, J.-L. Heavy Oil/Water Interfacial Properties and Emulsion Stability: Influence of Dilution. Energy Fuels 2009, 23 (1), 294–299. (20) Chandra, M. S.; Xu, Z.; Masliyah, J. Interfacial Films Adsorbed from Bitumen in Toluene Solution at a Toluene‐Water Interface: A Langmuir and Langmuir‐Blodgett Film Approach. Energy Fuels 2008, 22, 1784 - 1791. 13 ACS Paragon Plus Environment

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(21) Gao, S.; Moran, K.; Xu, Z.; Masliyah, J. Role of Naphthenic Acids in Stabilizing Water‐inDiluted Bitumen Emulsions. J. Phys. Chem. B 2010, 114 (23), 7710–7718. (22) Muller, H.; Pauchard, V.O.; Hajji, A.A. Role of Naphthenic Acids in Emulsion Tightness for a Low Total Acid Number (TAN)/High Asphaltenes Oil: Characterization of the Interfacial Chemistry. Energy Fuels 2009, 23, 1280–1288 (23) Birdi, K.S. Self-Assembly Monolayer Structures of Lipids and Macromolecules at Interfaces, Kluwer Academic/Plenum Press NY, 1999. (24) Mediaas, H.; Grande K. V.; Hustad, B.M.; Rasch, A.; Rueslatten, H. G.; Vindstad, J. E. The Acid-IER Method - a Method for Selective Isolation of Carboxylic Acids from Crude Oils and Other Organic Solvents. Paper #80404 presented at SPE 5th int. Symp., Aberdeen UK, Jan. 2003. (25) Rao, C.N.R. Chemical Application of Infrared Spectroscopy, Academic Press Inc. New York, USA 1963. (26) Petersen, J.C. Quantitative Method using Differential Infrared Spectrometry for the Determination of Compound Types Absorbing in the Carbonyl Region in Asphalts. Anal. Chem. 1975, 17 (1), 112-117. (27) Andersen, S.I.; Cheng, Y.: Chandra Mahavadi, S.; Mapolelo, M.; Indo, K.; Memon, A.; Khan, R.; Schmidt, K.; Quinones- Cisneros, S.; Ratulowski, J. Relations and Correlations between Crude Oil Chemistry and Rheology in Commingling Operations. Oral presentation, Petrophase 2014, Galveston, TX. USA. 2014.


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Table 1. Crude Oil Properties of the samples analyzed. Crude oil Crude oil 2 1

Method

Density @ 25oC [g/cc]

0.85833

0.86190

Anton Paar

nC7Asphaltene content [%wt]

1.4

1.0

Modified D4142

27.2

Pendant drop

Interfacial Tension [mN/m] vs deionized 26.5 water

ASTM

Table 2: Relative content of GC-MS amenable linear alkanes, aromatics, linear acids and hetero acids present at crude oil 1/water interface. Compound Class

Normalized percentage (%) based on GC peak area* 4.0

Linear alkanes Aromatics (exclude toluene and xylene)

43.6

Linear acids

7.4 15 ACS Paragon Plus Environment

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Hetero-acids 45.0 * MS detection is only qualitative as applied here and only covers the GC-MS observation window which is limited by compound volatility.


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Figure 1. FTir spectra of original crude oil 1 and 2 (bottom) along with their respective extracted acid fraction and the interfacial film (top). Crude 1 acids may be contaminated with formic acid.


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Figure 2. FTir spectra related to Crude oil 2: Left panel entire range, right panel zoom of range between 1900 and 1500 cm-1. Peaks at 1750 to 1700 cm-1 relates to C=O vibrations while the peak around 1600 cm-1 is assigned to aromatic C=C stretch.


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Whole Oil

Supernatant

Interfacial Film


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Figure 3. Crude oil 1. GC-MS trace (73 m/z) for the interfacial film, supernatant obtained after injecting crude oil 1 into the toluene top phase. Interface highlights the presence of C10 – C22 carboxylic acids. The prominent peak at 40.22 min is assumed to be an ester contaminant in the sample. The homologous series observed in the whole oils chromatogram is due to n-alkanes which is also seen for Crude oil 2 in Figure 6.


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Figure 4. Maltenes of Crude oil 1. GC-MS trace (73 m/z) for the interfacial film, supernatant obtained after injecting maltenes into the toluene top phase. The supernatant did not contain specific amounts of carboxylic acids (dominant peaks are due to n-alkanes) while the interface had between C11 (23.77 min) and C19 (45.32 min). Again the ester peak at 40.22 min is observed.


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Figure 5. De-acidified Crude oil 1. GC-MS trace (73 m/z) for the interfacial film, supernatant obtained after injecting de-acidified oil into the toluene top phase. The Interfacial material shows no indication of acids beyond trace level.


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Figure 6. Crude oil 2. GC-MS trace (73 m/z) for the interfacial film, supernatant obtained after injecting crude oil 2 into the toluene top phase. C15 to C28 are observed. Trace amounts of acids can be seen in the supernatant phase. Whole oils chromatogram dominated by n-alkane fragments. C15oic acid was here eluting at 36.09 minutes and C19oic acid at 45.05 min.


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Figure 7. Maltenes of Crude oil 2. GC-MS trace (73 m/z) for the interfacial film, supernatant obtained after injecting maltenes from crude oil 2 into the toluene top phase. Acids are seen in both supernatant and interface. Interface highlights the presence of C10 – C20 carboxylic acids with indications of the series reaching at least C24. C22oic acid at 50.19, C15oic acid at 36.18 min and C10oic acid at 19.18 min. The relative difference in intensity is not necessarily an indication of a difference in content when comparing the supernatant and the interfacial material as amounts of sample injected was not controlled.


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Figure 8. De-acidified Crude oil 2. GC-MS trace (73 m/z) for the interfacial film, supernatant obtained after injecting de-acidified oil into the toluene top phase. While there are no acids present in the supernatant the full range of carboxylic acids (C10 to C24) can although at trace level still be seen in the interfacial material after the de-acidification. C22oic acid at 50.21, C15oic acid at 36.20 min and C10oic acid at 19.18 min.


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Figure 9. Interfacial tension vs de-ionized water of crude oil samples 1 and 2 before and after removal of the acid fraction by ion exchange resins.


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Figure 10. Interfacial tension measurements of crude 1 and 2 along with their various deacidified fractions. The standard deviation on IFT measurements is in the range of 0.3 mN/m.


Detection and Impact of Carboxylic Acids at the ... - ACS Publications

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