Behavior of Asphaltenes in Crude Oil at High-Pressure CO2

May 13, 2016 - Department of Chemical Engineering, Imperial College London, South Kensington Campus, .... Golden Gate ATR accessory around the diamond...
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Behavior of Asphaltenes in Crude Oil at High-Pressure CO2 Conditions: In Situ Attenuated Total Reflection−Fourier Transform Infrared Spectroscopic Imaging Study Anton A. Gabrienko,†,‡ Oleg N. Martyanov,‡ and Sergei G. Kazarian*,† †

Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia

‡

ABSTRACT: An attenuated total reflection−Fourier transform infrared (ATR−FTIR) spectroscopic imaging approach has been used in situ to study CO2-induced precipitation of asphaltenes from crude oil on a molecular level. The behavior of model and real oils subjected to high-pressure CO2 has been monitored, and precipitation has been detected. The precipitated species have been chemically analyzed in situ at high-pressure conditions. It has been observed that only asphaltenes with oxygencontaining functional groups that are capable of specific Lewis acid−base interactions with CO2 precipitated from crude oil at such conditions. It is supposed that CO2 is able to interact with precipitated asphaltenes. On the basis of the obtained spectroscopic data, the mechanism of CO2-induced precipitation of asphaltenes has been proposed.

1. INTRODUCTION The petroleum upstream industry applies different approaches to enhance oil recovery, for instance, a gas injection or miscible flooding method, which is commonly used nowadays. Carbon dioxide, natural gas, or nitrogen are widely used gases for crude oil extraction. Among other advantages, the low cost of carbon dioxide (CO2) attracts much attention as a result of its ability to reduce crude oil viscosity. The effectiveness of crude oil displacement driven by CO2 flooding is strongly determined by the reservoir temperature, pressure, crude oil composition, and phase behavior of the formed CO2/crude oil mixture. Moreover, the changes of petroleum fluid behavior as a result of CO2 injection into the reservoir can disturb the existing equilibrium of various crude oil components and cause undesired precipitation of different constituents. The general perception on the behavior and structure of the asphaltene/crude oil system is based on the colloidal model, which proposes asphaltenes to be in a dynamic equilibrium and stabilized by the association with resins that act as a natural dispersant.1−5 It should be mentioned, however, that another model (Yen−Mullins model) has been recently suggested to explain asphaltene properties and behavior in petroleum.6−8 This dynamic equilibrium can be disturbed by the alteration of the pressure and temperature, change of the chemical composition, and nature of injected gases and fluids.9−14 All of these factors affect the solubility of asphaltenes in crude oil, causing their flocculation, followed by precipitation, at certain conditions. It is particularly interesting that CO2 can induce asphaltene precipitation when it is dissolved in crude oil. This phenomenon results in severe technological problems, such as formation damage and wellbore plugging, and has been reported in many studies.9,13−28 Thus, dissolving CO2 in crude oil changes its chemical composition and can potentially act as an anti-solvent with respect to asphaltenes, similar to the effect of heptane.29 However, there have been no reported data on © 2016 American Chemical Society

the possible interactions of CO2 with asphaltene molecules, and thus, the mechanism of asphaltene destabilization is not fully understood. Therefore, there is a great interest in studying asphaltene behavior at high-pressure CO2 conditions with in situ spectroscopic techniques that could provide insight on a molecular level. Crude oil subjected to high-pressure CO2 can be considered as a perfect candidate to be studied with attenuated total reflection−Fourier transform infrared (ATR−FTIR) spectroscopy, particularly with the spectroscopic imaging approach.30 This is due to the ability of this method to measure highly infrared (IR)-absorbing materials, such as crude oil, providing chemically specific information about not only crude oil constituents31−38 but also the amount of CO2 and its interaction with chemical species in the oil. The latter was successfully demonstrated in the studies of CO2 sorbed by different polymers, solids, and solutes.39−44 Moreover, one of the main advantages of the spectroscopic imaging method is the combination of the analytical power of FTIR spectroscopy and the possibility to monitor dynamic processes in situ.30,45−47 Thus, any phase changes, for instance, precipitation, occurring in crude oil can be detected and measured with the use of this experimental technique. Recent works have demonstrated the feasibility of the ATR−FTIR spectroscopic imaging approach to study asphaltenes48 and monitor the changes of crude oil phase composition in situ.49−52 It was demonstrated that, by applying ATR−FTIR spectroscopic imaging, one can obtain unique information about the specifics of asphaltene and crude oil behavior at various conditions, such as different temperatures, the addition of heptane, or change of oil composition via blending. Received: March 28, 2016 Revised: May 11, 2016 Published: May 13, 2016 4750

DOI: 10.1021/acs.energyfuels.6b00718 Energy Fuels 2016, 30, 4750−4757

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the sample to be analyzed. The analysis of the data sets is performed by plotting the distribution of the integrated absorbance of the selected spectral bands, which correspond to a particular chemical component, as two-dimensional (2D) false-color images (or chemical images), where one pixel represents a full mid-IR spectrum and the color scale relates to the absorbance. Hence, the amount of the component within the measuring area of the sample and its spatial distribution are represented by color (red color, high concentration; blue color, low concentration). With a relatively high spatial resolution (ca. 10−15 μm),30 the macro ATR−FTIR spectroscopic approach allows spectral information on spatially localized individual components of the heterogeneous sample to be obtained, including trace material detection.55,56 Normally, ATR−FTIR spectroscopy probes a relatively thin layer of a sample (up to a few micrometers). It is determined by the refractive index of the material of the ATR crystal (diamond, germanium, silicon, etc.), wavenumber of the spectral band, and refractive index of a sample. For crude oil/asphaltene or benzene/asphaltene samples, this approach detects any changes occurring only in the vicinity of the diamond ATR crystal surface, i.e., at the bottom of the high-pressure cell. This implies that any phase changes that occur on the measuring surface can be observed and that the agglomerated asphaltenes should sink from a bulk volume of the sample to the diamond surface to be monitored with the ATR−FTIR spectroscopic imaging approach. In the latter case, the precipitation of asphaltenes can be detected as well (Figure 1). A TENSOR 27 continuous-scan FTIR spectrometer (Bruker Optics) with an IMAC macro chamber extension and a FPA (64 × 64 pixels) detector was used for the FTIR spectroscopic imaging experiments in this work. For the chemical imaging measurements, an Imaging Golden Gate diamond ATR accessory (Specac, U.K.) was placed and used in the macro chamber extension. A total of 128 coadded scans were accumulated for each mid-IR spectrum (3800−900 cm−1 region) with a spectral resolution of 8 cm−1. For the in situ experiments with the ATR−FTIR spectroscopic imaging approach, a high-pressure cell with an internal volume of 200 μL was used (shown schematically in Figure 1). The cell consists of two parts: the bottom part can be separately fixed on the Imaging Golden Gate ATR accessory around the diamond ATR crystal allowing the injection of the liquid sample of required volume to be performed, and the top part of the cell with attached metal tubes for CO2 injection can be positioned on the top of the bottom part and held by the clamp attached to the ATR accessory. The sealing of both parts of the cell is achieved by the use of two Teflon O-rings placed between the bottom part and diamond ATR crystal, the bottom and top parts. High-pressure CO2 was introduced via metal tubes using a syringe pump (HiP), and the pressure was regulated using a pressure gauge. The temperature of the system was set up and controlled using the temperature top-plate attachment to the ATR−FTIR spectroscopic accessory. Each experiment was performed as follows: first, the diamond ATR crystal and attached high-pressure cell were maintained at the required temperatures (25 or 50 °C) for a few hours, followed by the recording of the background spectra. Second, 100 μL of crude oil or freshly prepared asphaltene solution (model oil) was injected into the high-

In this work, the original results of asphaltene precipitation from benzene solution and crude oil induced with highpressure CO2 (50 bar and 25 and 50 °C) are presented. Relatively fast dissolution of CO2 in the hydrocarbon medium of crude oil has been observed. Further, the precipitation has been detected and monitored in situ with the ATR−FTIR spectroscopic imaging approach for the first time, which has allowed chemical analysis of precipitated asphaltenes to be performed. The presence of particular functional groups in precipitated asphaltene molecules has been revealed, resulting in an interesting observation of the possible CO2 interaction with such functional groups.

2. EXPERIMENTAL SECTION 2.1. Sample and Material Characterization. The properties and chemical composition of the crude oil sample studied are shown in Table 1. The asphaltene fraction (heptane insolubles) of the crude oil

Table 1. Composition and Physical Properties of the Crude Oil Sample properties of crude oil saturates (wt %) aromatics (wt %) resins (wt %) asphaltenes (wt %) API gravity (deg) kinematic viscosity (mm2/s) density at 20 °C (g/cm3)

21.0 37.2 35.1 6.7 16.4 3868.7 0.957

was extracted by the standard procedure using benzene as a solvent.53 The samples of the crude oil and extracted asphaltenes were analyzed by chemical analysis, FTIR, and proton nuclear magnetic resonance (1H NMR) spectroscopy methods.54 The content of sulfur in crude oil and asphaltene samples was obtained to be 4.1 and 8.2 wt %, respectively. A benzene solution of extracted deposits, further referred to as model oil, was prepared by dissolving 20 mg of extracted asphaltenes in 0.5 mL of benzene, which gives ca. 4.5 wt % asphaltene solution. The mixture was intensively shaken for 10 min and then left overnight for better dissolution. Only freshly prepared solution was used for the ATR−FTIR spectroscopic experiments. Benzene (99.9% purity) was purchased from VWR International, Ltd. and used without further purification. 2.2. ATR−FTIR Spectroscopic Imaging Approach. The main benefit of the ATR−FTIR spectroscopic imaging approach is the capability to measure several thousands of individual IR spectra simultaneously from the different areas of the sample. This is possible due to the use of a focal plane array (FPA) detector [a multi-channel mercury cadmium telluride (MCT) detector]. Obtained imaging data sets, which include all IR spectra measured simultaneously, allow the presence and spatial distribution of various chemical components of

Figure 1. Schematic of the ATR−FTIR spectroscopic imaging approach of crude oil/CO2 blends and precipitated asphaltenes using a high-pressure cell. (Left) Crude oil in the cell at ambient conditions, with asphaltenes (indicated as dark purple circles) dissolved in crude oil. (Middle) CO2 injection into the cell (indicated as white circles), with crude oil swelling as a result of CO2 dissolution. (Right) Asphaltene precipitation as a result of the interaction with CO2. 4751

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Energy & Fuels pressure cell. Further, the cell was sealed, and the sample was measured at ambient conditions with the ATR−FTIR spectroscopic imaging approach. Third, CO2 of required pressure (50 bar; pressure was maintained constant during the experiment by the injection of small additional amounts of CO2) was introduced into the cell, and the monitoring of the sample/CO2 blend behavior was conducted with the ATR−FTIR spectroscopic imaging approach. Three repetitions were performed to validate the result of each experiment. Only reproducible data are presented in this work.

swelling of initial model oil, which is reflected in ATR−FTIR spectra as the decrease of the absorbance of benzene bands. Using eq 1 and absorbance of different benzene spectral bands from the spectra measured before (A0) and after (A) CO2 dissolution, the degree of model oil swelling was estimated to be 38 ± 5%, assuming that the refractive index of benzene remains the same. S=

3. RESULTS AND DISCUSSION 3.1. Asphaltene Solution in Benzene (Model Oil) Subjected to High-Pressure CO2. Benzene solution of extracted asphaltenes was selected to be analyzed first with the ATR−FTIR spectroscopic imaging approach of crude oil and high-pressure CO2. Such solution can be considered as a model oil but of much simpler composition compared to real crude oil because it contains only asphaltenes and benzene as a solvent. The apparent simplicity of the asphaltene/benzene system excludes some factors that affect asphaltene stability in a real crude oil. This means that any results about the behavior of asphaltenes dissolved in benzene at high-pressure CO2 conditions should be used as model systems. However, such results, if any, can be used along with the results on crude oil behavior at similar conditions to reveal any differences or similarities between two systems because this comparison can potentially help to understand fairly more complex behavior of real crude oil. Figure 2 shows the ATR−FTIR spectra of the model oil measured at ambient pressure and 50 bar of CO2. These spectra

A0 −1 A

(1)

The behavior of CO2 dissolved in the model oil can also be analyzed using spectroscopic imaging results. According to the images obtained by plotting the distribution of the absorbance of the band at 2333 cm−1 (line A of Figure 3), CO2 dissolves in the model oil uniformly and its amount reaches an equilibrium value at about 60 min after injection of CO2. Careful comparison of the ATR−FTIR spectra measured before and after CO2 injection (Figure 2) helped to reveal the appearance of the spectral band at 1146 cm−1, which partly overlaps with the bands of benzene. With information from the spectrum that some change of model oil composition occurred after CO2 injection and how this change is reflected in the ATR−FTIR spectra, the obtained imaging data sets were further analyzed. Because the compared ATR−FTIR spectra were obtained by averaging corresponding imaging data sets, the band at 1146 cm−1 was used to create the chemical images of the model oil by plotting the absorbance distribution of this band as a function of all pixels. To do that, the spectral area of 1160−1070 cm−1 was integrated and the corresponding chemical images are shown in line B of Figure 3. As seen, the initial asphaltene solution shows no presence of the band at 1160−1070 cm−1, and it remains like this up to 60 min after CO2 was injected into the cell with the model oil. However, the appearance and non-uniform distribution of the band within chemical images can be easily observed when the sample was monitored further. Because the spectral band at 1160−1070 cm−1 corresponds to a particular component of studied model oil, it can be concluded that the change of the phase composition of asphaltene solution has occurred after CO2 dissolution in the model oil. Particularly, it can be stated that CO2 induced precipitation of some component from the model oil. One can reasonably suppose that the observed changes of the model oil composition have happened as a result of the sinking of non-dissolved asphaltene particles that were initially present in the sample because the model oil was prepared by dissolution of solid extracted asphaltenes in benzene. To check this, the model oil was monitored with the ATR−FTIR spectroscopic imaging approach under similar conditions but at ambient pressure. The analysis of obtained imaging data sets by the same method of 1160−1070 cm−1 band integrated absorbance plotting was performed, and the result is shown in Figure 4. Hence, it is clear that the model oil is stable under ambient conditions and shows no precipitation, unless being subjected to high-pressure CO2. The chemical images obtained (Figure 3) help to detect the location of the greatest amount of precipitated species within the measured area of the sample. Hence, the ATR−FTIR spectrum with the highest absorbance of the bands corresponding to the precipitated species can be selected from this location and analyzed to reveal specific spectral information about the chemical composition of the species observed to precipitate at high-pressure CO2 conditions. Figure

Figure 2. Averaged ATR−FTIR spectra of asphaltene solution in benzene (the model oil) measured at 25 °C and ambient pressure (blue), in 5 min (green) and 75 min (red) after CO2 injection at 50 bar. Each spectrum was obtained by averaging all ATR−FTIR spectra of the corresponding imaging data set.

were obtained by averaging all ATR−FTIR spectra of the corresponding imaging data sets (each averaged ATR−FTIR spectrum relates to one particular imaging data set shown in Figure 3). The first spectrum measured for the model oil at ambient pressure and 25 °C shows mainly the spectral bands of benzene, but additionally, some minor bands corresponding to the dissolved asphaltenes (at around 2900 cm−1) can be detected. The appearance of the strong spectral band at 2333 cm−1 corresponding to the ν3 band of dissolved CO2 as well as the decrease of the absorbance of benzene spectral bands can be observed almost immediately after CO2 was injected into the high-pressure cell containing the model oil. These observations suggest that CO2 is dissolving in benzene, resulting in the 4752

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Figure 3. In situ macro ATR−FTIR spectroscopic images of asphaltene solution in benzene (the model oil) measured at 25 °C and ambient pressure (the first images of A and B lines), after CO2 injection at 50 bar. The images were obtained on the basis of the distribution of the absorbance of the spectral band at 2333 cm−1 (line A), which corresponds to the antisymmetric stretching (ν3) band of CO2, and the integrated absorbance of the spectral band at 1160−1070 cm−1 (line B), which corresponds to precipitated asphaltenes. The measuring area is ca. 610 × 530 μm.

Figure 4. In situ macro ATR−FTIR spectroscopic images of asphaltene solution in benzene (the model oil) measured at 25 °C and ambient pressure. The images were obtained on the basis of the distribution of the integrated absorbance of the spectral band at 1160−1070 cm−1, which would correspond to precipitated asphaltenes that did not occur. The measuring area is ca. 610 × 530 μm.

Figure 5. In situ macro ATR−FTIR spectroscopic image of asphaltene solution in benzene (the model oil) measured at 25 °C in 75 min after CO2 injection at 50 bar. The image was obtained on the basis of the distribution of the integrated absorbance of the spectral band at 1160−1070 cm−1, which corresponds to precipitated asphaltenes. Two representative ATR−FTIR spectra selected from the regions showing high (A, red) presence and (B, blue) absence of precipitated asphaltenes. Spectrum C was obtained by spectral subtraction of spectra A and B.

spectra confirms the presence of the band at 1160−1070 cm−1 in spectrum A and the absence of this band in spectrum B, as follows from the false color image. However, no extra spectral information apart from the band at 1160−1070 cm−1 can be obtained as a result of the presence of strong spectral bands of

5 shows that two ATR−FTIR spectra A and B were selected from the specific locations in one of the imaging data sets represented by the corresponding chemical image, which was created on the basis of the distribution of the absorbance of the band at 1160−1070 cm−1. The comparison of these selected 4753

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Similar to the previous case, the band at 2334 cm −1 corresponding to the ν3 band of CO2 (which is slightly different compared to its position at 2333 cm−1 in model oil spectra, but this difference is negligible considering the effect of the environment and spectral resolution used in these experiments) and decrease of the spectral bands of crude oil were observed, showing the dissolution of CO2 in crude oil and crude oil swelling upon the dissolution of CO2. The distribution of CO2 within crude oil medium is uniform, and the saturation of crude oil with dissolved CO2 is achieved within a 15−60 min time frame according to the images obtained that were generated using the absorbance of CO2 stretching band at 2334 cm−1 (line A of Figure 7). The degree of crude oil swelling is 9 ± 1% (eq 1). The experiments with the model oil show that precipitation induced by CO2, if any, can be detected using chemical images created on the basis of the distribution of the integrated absorbance of the band at 1160−1070 cm−1. Therefore, this method was applied to imaging data sets measured for the crude oil subjected to a high-pressure CO2 environment. The corresponding chemical images (line B of Figure 7) show the appearance of the band at 1160−1070 cm−1, which indicates that the precipitate is observed in 60 min after CO2 injection. The local area where the precipitate was detected is relatively small, and this explains why the appearance of this band was not detected in the averaged ATR−FTIR spectra because the contribution of the spectra containing the bands corresponding to the precipitate is negligible. However, the ATR−FTIR spectroscopic imaging approach allows such minor changes in the composition of the sample at localized areas to be detected as a result of its high spatial resolution and sensitivity. With the knowledge of the exact location within the measured area of the sample, where the precipitate was formed, it is possible to select the ATR−FTIR spectra that can be further analyzed. Figure 8 shows two ATR−FTIR spectra selected from the imaging data set. The difference between spectra A and B can be clearly seen in the spectral range of 1180−1070 cm−1, which is reflected in the corresponding chemical image created on the basis of the distribution of integrated absorbance of the band 1160−1070 cm−1. Moreover, the spectral subtraction, similar to the subtraction performed with the spectra of the model oil, results in spectrum C, which

benzene and other components (CO 2 and dissolved asphaltenes) that are not related to the precipitated species. To remove these bands, the method of spectral subtraction was applied, and spectrum C was obtained as the result of spectral subtraction of spectra A and B, with the latter being used as the reference spectrum, which shows exclusively the bands of benzene, dissolved asphaltenes, and CO2. Clearly, the ATR− FTIR spectrum C can provide specific spectral information about the chemical composition of the precipitates because other bands corresponding to these species are now revealed by spectral subtraction. Now, the description of the results on the crude oil behavior at high-pressure CO2 conditions will be presented below, while the detailed analysis of spectrum C shown in Figure 5 will be performed further. 3.2. Crude Oil Subjected to High-Pressure CO2. CO2induced precipitation observed for the model oil is an interesting result in itself, but it should be considered along with the results on the crude oil behavior at high-pressure CO2 conditions. Therefore, high-pressure CO2 experiments were performed with real crude oil containing the same asphaltenes that were used for model oil preparation. Figure 6 shows averaged ATR−FTIR spectra of the crude oil measured at ambient pressure and 50 bar of CO2 at 50 °C.

Figure 6. Averaged ATR−FTIR spectra of the crude oil measured at 50 °C and ambient pressure (blue), in 5 min (green) and 240 min (red) after CO2 injection at 50 bar. Each spectrum was obtained by averaging all ATR−FTIR spectra of the corresponding imaging data set.

Figure 7. In situ macro ATR−FTIR spectroscopic images of the crude oil measured at 50 °C and ambient pressure (the first images of A and B lines), after CO2 injection at 50 bar. The images were obtained on the basis of the distribution of the integrated absorbance of the spectral band at 2334 cm−1 (line A), which corresponds to the antisymmetric stretching (ν3) band of CO2, and the integrated absorbance of the spectral band at 1160−1070 cm−1 (line B), which corresponds to precipitated asphaltenes. The measuring area is ca. 610 × 530 μm. 4754

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Figure 8. In situ macro ATR−FTIR spectroscopic image of the crude oil measured at 50 °C in 240 min after CO2 injection at 50 bar. The image was obtained on the basis of the distribution of the integrated absorbance of the spectral band at 1160−1070 cm−1, which corresponds to precipitated asphaltenes. Two representative ATR−FTIR spectra selected from the regions showing high (A, red) presence and (B, blue) absence of precipitated asphaltenes. Spectrum C was obtained by the spectral subtraction of spectra A and B.

containing C−O−R, ether or hydroxy, and SO sulfoxide functional groups. The structure of precipitated asphaltenes, particularly the presence of oxygen-containing functional groups, gives the opportunity to propose the mechanism of CO2-induced precipitation. Indeed, such functional groups have been previously demonstrated capable of forming specific interactions with CO2 molecules (Figure 9).41,62−65 This is a result

can be considered as a spectrum of the species precipitated from the crude oil as a result of CO2 presence. The comparison of two ATR−FTIR spectra of precipitated species for the case of the model oil (spectrum C of Figure 5) and real crude oil (spectrum C of Figure 8) shows that these spectra are very similar. This implies that the chemical compositions of both precipitates are similar too. Hence, it is reasonable to analyze and compare both spectra. It can be concluded that the precipitation of asphaltenes was observed from both the model and crude oil. Indeed, the model oil is a solution of extracted asphaltenes in benzene; therefore, only asphaltenes can precipitate by the injection of high-pressure CO2, and the evidence for that can be found in the spectra. The presence of the bands at 2950−2855 cm−1 (C−H stretching), 1464, 1455, 1379, and 1374 cm−1 (CH2 bending and CH3 deformation), and 1612 and 1646 cm−1 (aromatic CC stretching) points to the fact that the observed precipitates have the structure of an alkyl-substituted complex (polycyclic or condensed) aromatic hydrocarbon,57 which is generally considered to be the main component of asphaltenes.7,58,59 Additionally, other bands can be observed in the spectra of the precipitate. First, the broad band centered at ca. 1140 cm−1, which is actually the blend of a few overlapping bands in the region of 1230−1100 cm−1, can be assigned to ν(C−O) stretching of various oxygen-containing species, such as C−O− R functional groups, where R stands for H (hydroxy C−OH), Ar (aromatic ether C−O−R), and C (ether C−O−C).60 This assignment is supported by the detection of the bands at ca. 1440−1420 cm−1 (observed as the shoulder of the bands at 1464 and 1455 cm−1) that can be attributed to δ(C−O−R) deformation of the same species.60 Second, the band at 1030 cm−1 from ν(SO) stretching indicates the presence of sulfoxide groups.61 Hence, the chemical composition of the species precipitated from the model and crude oils at highpressure CO2 conditions can be described as the asphaltenes

Figure 9. Possible configuration of intermolecular interactions between the different functional groups in asphaltenes and CO2.

of Lewis-acid−base-type interactions, resulting in non-covalent intermolecular complex formation between CO2 and the substrate molecule. Thus, it can be proposed that the interaction between CO2 and asphaltenes can be important for asphaltene destabilization and precipitation. Nevertheless, the specific interactions between CO2 and oxygen-containing functional groups of asphaltenes can be plausibly considered as a driving force of CO2-induced precipitation. It can be suggested that CO2 can reduce the amount of resins, adsorbed on the asphaltene surface and acting as a peptizing agent, by competing with resin molecules for the interaction with asphaltenes. Adsorbed resin molecules are supposed to be in dynamic equilibrium with resins dissolved in crude oil.3 When the number of CO2 molecules interacting with the asphaltene molecule becomes large enough to change the asphaltene/resin ratio to a critical value, the asphaltene molecule can be destabilized, followed by flocculation and aggregation with other asphaltene molecules. This process 4755

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happens as a result of the presence of CO2 and results in asphaltene precipitation and deposition. The proposed mechanism of CO2-induced asphaltene precipitation is supported by the reported findings that CO2 is a more effective asphaltene precipitator than heptane or other alkanes, which can be explained by the ability of CO2 specifically interact with asphaltene molecules, making its destabilizing effect more efficient.66

4. CONCLUSION In summary, the main objective of this work was to investigate the interaction of CO2 with crude oil components, particularly asphaltenes, on a molecular level with the ATR−FTIR spectroscopic imaging approach. For the first time, this technique was used in situ to detect asphaltene precipitation from model and real oils induced by high-pressure CO2 and chemically analyze the composition of precipitated species, which is the main achievement of this work. Interestingly, only asphaltenes containing C−O−R and SO functional groups capable of specific Lewis acid−base interactions with CO2 were observed to precipitate after CO2 dissolution in crude oil. Additionally, the presence of CO2 likely interacting with these functional groups was detected within precipitated particulates. This finding allowed us to propose the mechanism of CO2induced precipitation of asphaltenes. It is suggested that CO2 can adsorb on the surface of asphlatene molecules and reduce the resin/asphaltene ratio by competing with resin molecules, which is essential for asphaltene stabilization in crude oil. These results demonstrate that the in situ ATR−FTIR spectroscopic imaging approach may be used in laboratory conditions for the assessment of CO2-induced precipitation of asphaltenes from crude oils of different compositions and at different temperatures and CO2 pressures. This may impact the understanding of CO2-induced viscosity reduction of crude oil and crude oil fouling phenomena, which are important for processes of enhanced oil recovery.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was performed within the Project 15-19-00119 of the Russian Science Foundation. REFERENCES

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