Ionic liquid in stabilizing asphaltenes during miscible CO2 injection in high pressure oil reservoir

Journal of Petroleum Science and Engineering 180 (2019) 1046–1057

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Ionic liquid in stabilizing asphaltenes during miscible CO2 injection in high pressure oil reservoir

T

Bisweswar Ghosha,∗, Nuhu Sulemanaa, Fawzi Banatb, Nevin Mathewb a b

Petroleum Engineering Department, Khalifa University, SAN Campus, P.O. Box 2533, Abu Dhabi, United Arab Emirates Chemical Engineering Department, Khalifa University, SAN Campus, P.O. Box 2533, Abu Dhabi, United Arab Emirates

ARTICLE INFO

ABSTRACT

Keywords: Ionic liquids Asphaltene inhibition CO2 injection Asphaltene onset pressure (AOP)

The potential of ionic liquids to prevent asphaltene precipitation during miscible Carbon dioxide (CO2) flooding was investigated. Asphaltene dispersion tests were performed to compare the effectiveness of commercial inhibitors and the ionic liquids at ambient conditions. Based on the results 1-Butyl-3-methyl-imidazolium bromide [BMIM][Br], Ionic Liquid was selected and asphaltene onset pressure (AOP) was determined at high-temperature high-pressure conditions through isothermal depressurization experiments in PVT cell. The effect of CO2, organic solvents and [BMIM][Br] on AOP were documented and correlated with nodal pressure analysis of the reservoir/ wells. The results show a 21% decrease in AOP compared to the untreated oil and about 46% decrease when 35 mol % of CO2 is solubilized in oil. [BMIM][Br] is seen to have higher activity when injected in solution with isopropanol-n-heptane compared to direct addition. The application of [BMIM][Br]in miscible CO2 flooded reservoir is found to have good potential in inhibiting asphaltene deposition within the reservoir and near wellbore nodal conditions.

1. Introduction Asphaltenes are the heaviest and most complex components of crude oil which coexist as a colloidal suspension under stabilized reservoir conditions. They are composed of poly-aromatic nuclei associated with aliphatic side chains and polar heteroatom functional groups, which provide them the polarity. Resins are structurally similar polar molecules and possess similar properties but are however much smaller than asphaltenes, which enable them to interact with the polar groups of asphaltenes. They provide a steric-stabilization or protective layer around the asphaltenes, thereby stabilizing asphaltenes in crude oil (Hammami et al., 2000). Resins are believed to be natural inhibitors and play an important role in keeping asphaltenes stable in the crude oil system. In the reservoir, asphaltenes remain at thermodynamic stability in crude oil, however, when this stability is disturbed due to changes in either of the CTP (compositional, temperature and pressure) properties, asphaltenes may destabilize and precipitate (Yonebayashi et al., 2018). During crude oil production, the reservoir pressure gradually drops towards the asphaltene onset pressure (AOP). This induces asphaltenes to precipitate out of the crude oil, thus initiating the process of asphaltene deposition. The deposition of asphaltenes on the surfaces of flow paths causes formation damage in the near-

wellbore region and flow restriction in flowlines, leading to partial or complete loss of well production. The injection or flooding of carbon dioxide under miscible conditions is a well-established and economic method for Enhanced Oil Recovery (EOR) for nearly half a century because of its higher efficiency as well as environmental benefit in combating greenhouse effects. However, CO2 injection under miscible conditions enhances asphaltene precipitation significantly as observed through X-ray CAT-scan supported Core flood experiments (K. Srivastava et al., 1999). ZanganehAyatollahi et al. (2012) have quantified asphaltene precipitation using a high-pressure visual cell with and without CO2 injection. Asphaltene precipitation increases with the concentration of injected CO2 under similar pressure depletion rate and cell temperature conditions. Behbahani et al. (2012) studied the effect of CO2 injection and subsequent pressure depletion on asphaltene destabilization and deposition in long cores at near-wellbore conditions. Severe pore plugging was observed due to multilayered asphaltene adsorption on the rock-pores. The precipitated asphaltenes not only cause formation plugging but also deposit on the flowlines and poses moderate to severe challenges in crude oil production, processing, and transportation facilities (Kokal and Sayegh, 1995).

Corresponding author. E-mail addresses: [email protected] (B. Ghosh), [email protected] (N. Sulemana), [email protected] (F. Banat), [email protected] (N. Mathew). ∗

https://doi.org/10.1016/j.petrol.2019.06.017 Received 24 January 2019; Received in revised form 26 May 2019; Accepted 5 June 2019 Available online 06 June 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic diagram of an average oil well of the target reservoir, showing the nodal pressure ranges, correlated with asphaltene onset pressure.

Recently, some of the Middle East (ME) Exploration and Production (E&P) giants started injecting CO2 to boost oil production in some of the high pressure carbonate reservoirs (above minimum miscibility pressure of CO2 at reservoir temperature) where the asphaltene problem already prevails. However, before CO2 injection the asphaltene precipitation and subsequent deposition occurred in these wells towards the downstream (production string and surface flow lines) as the AOP is low at normal production conditions. It is well established that AOP is boosted in proportion to the mole fraction of CO2 present in crude oil; therefore asphaltene precipitation is expected to occur further upstream when CO2 EOR is implemented. This may have dangerous consequences if the deposition happens at or near wellbore reservoir where the skin due to formation damage is most sensitive in terms of well productivity. Fig. 1 represents the range of nodal pressure in the wells of a ME oil reservoir which is considered for this study. The objective is to locate the node where nodal pressure will fall below the AOP at a given condition, thus initiating asphaltene precipitation and also a possible solution to depress the AOP so that the asphaltene deposition can be shifted to a safer node on the downstream. One of the possible solutions to the problem of asphaltene precipitation is the injection of chemical additives; asphaltene inhibitors or dispersants (Firoozinia et al., 2016). The activities of asphaltene inhibitors are similar to the crude oil resins. They interact with asphaltenes and suppress the phase separation process while the dispersants stabilize the asphaltene moieties in the crude oil by restricting the aggregate sizes and hindering further aggregate growth (Marcano et al., 2015). The purpose of inhibitor application is, therefore to suppress the asphaltenes onset pressure, thereby moving the deposition closer to the surface where the deposits can be easily managed. González and Middea (1991) studied the activity of a number of oil-soluble amphiphiles as asphaltene peptizing agents and found that the effectiveness of these amphiphiles was greatly influenced by the interaction between the polar group of the amphiphiles and asphaltenes. Others (Chang and Fogler, 1993, 1994a; 1994b; Permsukarome et al., 1997) investigated the effectiveness of various alkyl benzene derived amphiphiles to stabilize asphaltenes in aliphatic solvents. Their results indicated that the best amphiphiles for stabilizing asphaltenes in alkane solvents were

alkyl benzene sulfonic acid and alkyl phenols. The results also suggested that the interaction between asphaltenes and the amphiphiles was acid-base or hydrogen bonding. In recent years Ionic Liquids (ILs) have gained the attention of the petroleum scientists because of some of their unique properties and encouraging results in improving oil recovery and asphaltene inhibition properties. Ionic liquids are salts in the liquid state, whose melting point must be below 100 °C (212 °F). They possess unique properties such as extremely low vapour pressure, low melting points, thermal stability, non-flammability and good solubility in a wide range of solvents. Ionic liquids have found several potential applications in process and manufacturing industries and have recently been used in the petroleum industry for desulfurization and denitrogenation of diesel oil and as a possible asphaltene inhibitor (Murillo and Aburto, 2011; Sulemana and Ghosh, 2017). Hu and Guo (2005) studied the ability of ionic liquids with alkyl pyridinium [Cnpy]+ and alkyl iso-quinolinium [Cniql]+ cations with [PF6]-, [BF4]- and [Cl]- anions having varying alkyl chain length. The inhibition study was conducted on crude oils obtained from high pressure and high-temperature CO2 injected reservoirs. From the results, they interpreted that due to the low density of the cation charge delocalized on the whole of aromatic rings, the cations cannot bond with asphaltene moieties to form stable complexes. The enhanced inhibitive effect of [Cniql][Cl] and [Cnpy][Cl] was attributed to the electron donor anion [Cl]- undergoing hydrogen bonding or electron donor-acceptor interactions with asphaltenes. Boukherissa et al. (2009) studied 1-propyl boronic acid-3-alkylimidazolium bromides and 1-propenyl-3-alkylimidazolium bromides ILs as asphaltene inhibitors. It was inferred that the ionic liquids were soluble in crude oil and their ability to disperse asphaltenes was due to hydrogen bonding or charge-transfer complex formation. It was also concluded that in order for effective steric stabilization of ionic liquids and asphaltene complexes, the required minimum length of the carbon number of the ionic liquids should be eight or more. From the literature, it is evident that the potential for application of IL in preventing asphaltene deposition in oil reservoirs is good. However, not many experiments have been conducted at high pressure and high temperature (HP-HT) conditions which prevail in 1047

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hydrocarbon reservoirs except the information shared by Hu and Guo (2005). Most importantly, no published work could be found which relates to IL application in CO2 injected reservoir conditions, where the severity of asphaltene deposition and related problems would be enhanced. In this study, we investigated the effectiveness of two ionic liquids, namely 1-Butyl-3-methylimidaziolium chloride [BMIM][Cl] and 1Butyl-3-methyl-imidazolium bromide [BMIM][Br]. They were chosen for asphaltene inhibition studies with the following rationale. From the literature, it is evident that [Cnpy], [Cniql] and [BMIM] group of ILs are seen to be promising in inhibiting asphaltenes. [Cnpy] and [Cniql] ILs are studied in detail by Hu and Guo (2005) under HT-HP condition in the presence of CO2. [BMIM] group of ILs are studied for tank bottom sludge dissolution and asphaltenes inhibition at ambient conditions only without the presence of CO2. [BMIM][Cl] and [BMIM][NO3] were seen to have asphaltene inhibitive properties. In this project, we planned to investigate inhibitive properties of [BMIM]+ ILs with Cl−, Br− and I− anions so that the impact of the size of the anion can be ascertained. However [BMIM][I] was not available in the market thus we had to limit our study with [BMIM]+ ILs with Cl− and Br− as anions. The studies were conducted at the ambient condition as well as temperature up to 120 °C and system pressure up to 6000 psi. Depressurization experiments were simulated to miscible CO2 EOR reservoir depletion. Initial screenings were conducted at ambient conditions followed by employing a state-of-the-art HT-HP flow assurance setup equipped with Near Infrared Solid Detection System (NIR-SDS) and High-Pressure Microscope (HPM) to capture real-time microscopic images. The NIR-SDS technique helped to determine the asphaltene onset pressure (AOP) of recombined crude oil with and without additives supported by captured HPM images.

2.2. Experimental setup and procedure 2.2.1. Asphaltene dispersion test (ADT) through the centrifuge method ADT was conducted to evaluate the performance of the inhibitors in dispersing asphaltenes. The dispersion efficiency was measured by comparing the amount of asphaltene sediment deposited in the presence and absence of additives. A stock solution of the commercial inhibitors CI-1 and CI-2 were prepared in n-Heptane and then diluted to 250, 450, 550, 750 and 1000 ppm. For the ionic liquids IL-1 and IL-2, the stock solutions were prepared in 10:90 isopropanol: n-Heptane mixture and diluted to 250, 450, 550, 750 and 1000 ppm. 0.1 mL of crude oil was taken in finely graduated centrifuge tubes along with 10 mL of a corresponding solvent (with and without inhibitors). The tubes were aged in dark for 12 h and subsequently centrifuged at 2500 rpm for 15 min. The crude oil in solvent only (without inhibitor) was marked as the control. The effectiveness of these inhibitors was visually measured by the quantity of asphaltenes settled at the bottom of the tubes. 2.2.2. Asphaltene dispersion test (ADT) through turbidity measurements 10 mL of a sample from the previous test was taken in 25 mm round glass cuvettes, shaken thoroughly and turbidity measurements were conducted under 860 nm infra-red light passing through the solution. The intensity of scattered light by suspended asphaltene particles is measured by a detector, providing a qualitative measurement of asphaltene particles in solution. This data is used in combination with the previous test for screening of asphaltene inhibitors. The turbidity meter (Hanna Instruments HI88713) has a measuring range of 1–2000 Nephelo Turbidity Unit (NTU). It was calibrated with solutions of specific turbidity (1, 10, 100 and 1000 NTU). Relatively high turbidity reading values indicate a relatively ineffective asphaltene inhibition while low turbidity readings indicate effective asphaltene inhibition due to the formation of a lesser number of asphaltene agglomerates (Shadman et al., 2016). Turbidity readings were found to be fairly consistent with the uncertainty limit of ± 1 unit. Since this test used more as qualitative rather than quantitative, such minor uncertainty should not have any impact on the conclusion.

2. Materials and methods 2.1. Materials The Crude oil used in this research was from a high temperature (120 °C) and high pressure (4000 + psi) oil reservoir in the Middle East, known to have asphaltene deposition problem on the production tubings and flow lines. The crude oil properties are presented in Table 1. Two commercial asphaltene inhibitors, CI-1 and CI-2, were received from local service companies which work as a replicate of natural crude oil resin (the rest of the information on their chemical and physical properties retained as commercial secret). These inhibitors have been used in the local oilfields under normal circumstances (not in CO2 injected fields) with reasonable success. In addition, two Ionic liquids 1-Butyl-3-methylimidaziolium chloride [BMIM][Cl] (98.4% purity) and 1-Butyl-3-methyl-imidazolium bromide [BMIM][Br] (98.8% purity), referred here as IL-1 and IL-2 respectively, were used and their chemical structures and properties are provided in Table 2. Technical grade solvents such as n-heptane, toluene, isopropanol with 99% purity were used for the experiments. The carbon dioxide used for the experiments had > 99% purity.

2.2.3. Isothermal depressurization experiments with near infra-red (NIR) solid detection system (SDS) and high-pressure microscopy (HPM) A high-pressure, high-temperature (HP-HT) flow assurance system (named FLASS, manufactured by Vinci Technologies, France) was used for isothermal depressurization of recombined oil, replicating the oil reservoir pressure depletion pattern. The system consisted of a 200 cc PVT cell with a viewing window, equipped with computer-controlled positive displacement pumps, a magnetically driven stirrer for fluid homogenization and fiber optic NIR source and detector fitted across the PVT cell. The NIR-SDS responds to asphaltene onset by measuring the light transmittance in the NIR region with a high level of sensitivity (see Fig. 2). The recombination process of crude oil with gas samples was carried out in a sample cylinder. After charging and pressurizing the oil and gas sample to experimental pressure, the sample cylinder was rocked for 12 h to homogenize the mixture. The presence of a steel rolling ball in the sample cylinder aided the homogenization process. A pressure gauge was connected to the sample cylinder for visual monitoring of the cylinder pressure. Any change in pressure from the initial set pressure indicated that the sample was not well mixed and was not in a single phase. A programmable positive displacement pump was therefore used to compensate the lost pressure and maintain the set experimental pressure. While rocking, the sample cylinder is heated to experiment temperature using a silicon heating tape. The recombined crude oil at the reservoir gas-oil ratio of 370 scf/bbl was injected into the FLASS PVT cell via a high pressure 0.45-μm filter at a pressure 4500 psi and temperature of 90 °C. The PVT cell was heated to a temperature of 90 °C before injecting the recombined oil.

Table 1 Crude oil properties at 25 °C. Property

Crude Oil

Gas Oil Ratio (SCF/Bbl) Density (g/cm3) Viscosity (cP) API gravity Molecular weight (g/mol) Saturates (wt %) Aromatics (wt %) Resins (wt %) n-Heptane Asphaltenes (wt %)

370 0.834 5.34 37.36 212 63.67 29.56 5.78 0.850

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Table 2 Properties of Ionic liquids. Molecular Weight, g/mol

Melting Point, oC

Flashpoint, oC

C8H15ClN2

174.67

70

192

C8H15BrN2

219.12

65–75

Very High

Chemical Name

Molecular Formula

1-Butyl-3-methylimidaziolium chloride [BMIM][Cl]

1-Butyl-3-methylimidazolium bromide [BMIM][Br]

Structure

The sample temperature and pressure in the PVT cell were raised and maintained at 120 °C and 6000 psi with the help of computer-controlled pumps. The recombined oil sample was homogenized by the magnetically driven stirrer and HPM camera was used to monitor the presence of any particles for nearly 24 h before depressurization experiments were initiated. While depressurizing, the NIR-SDS was turned on and the transmitted data were captured by the data acquisition system at a pre-set time interval. With the help of a separate high-pressure pump, a small fraction of the depressurized crude oil was recycled through a sapphire window, attached with the high-pressure microscope (HPM) for visualization

and micro-imaging. The high-magnification and high-resolution microscope monitor real-time changes in the fluid phase and captures images at user-defined intervals. An initial experiment was conducted to determine the AOP without any additive (inhibitor and or CO2) in order to serve as a benchmark for the comparison with subsequent experiments in which the selected inhibitor (IL-2) was added to investigate its potential as an AOP reducing agent. With the knowledge of the volume of oil sample to be fed into the PVT cell (precisely 40 mL), the appropriate volume of the inhibitor needed to obtain the desired concentration was added to the oil before recombination. The oil with inhibitor IL-2 was then recombined

Fig. 2. Schematic diagram showing the loading of recombined crude oil into the FLASS system. 1049

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Fig. 3. Schematic diagram showing the injection of CO2 into FLASS for depressurization experiments.

and injected into the PVT cell for depressurization. The PVT cell is housed inside a temperature controlled oven and its maximum operating pressure and temperature are 15,000 psi and 200 °C respectively. The above basic protocol was adopted with the necessary modifications for the injection of 35 mol % of CO2 gas to investigate the efficiency of the inhibitors in the presence of CO2. The CO2 gas was first injected into the gas injection cylinder of the FLASS system using a gas booster at a pressure of 4800 psi, resulting in a supercritical state. The gas was further pressurized to 5000 psi in the gas injection cylinder using an internal injection pump and the calculated volume of CO2 gas injected into the PVT cell charged with recombined oil. The mixture was then agitated and homogenized for 24 h with constant stirring before initiating the isothermal depressurization experiments. The schematics of the injection of recombined oil into the FLASS system and the injection of CO2 gas are presented in Figs. 2 and 3.

Table 3 ADT results of Crude Oil C with inhibitors CI-1, CI-2, IL-1 and IL-2. Sample

Dosage (ppm)

Measured Sediment Levels (mL)

Crude Oil C Treated with CI-1

0 250 450 550 750 1000 250 450 550 750 1000 250 450 550 750 1000 250 450 550 750 1000

0.2 trace trace trace trace trace trace trace trace trace trace trace trace trace trace trace trace trace Nearly clear Clear clear

Treated with CI-2

Treated with IL-1

3. Results and discussion

Treated with IL-2

3.1. Asphaltene dispersion test The results of the ADT tests are presented in Table 3 below. The effectiveness of the chemical inhibitors was ascertained from the amount of sediments observed at the bottom of the tubes after aging and centrifugation. When the amount of sediments at the bottom of the tube is significantly lower compared to the control, it indicates an effective inhibition. Visual observation and comparison made between the untreated crude oil against crude oil treated with inhibitors show that the untreated crude (in heptane) has a sediment level much higher

than those treated with inhibitors. Due to the very low level of asphaltene sediments in the treated samples, a clear distinction between the samples could not be made and recorded as a trace. The samples treated with IL-2 showed no asphaltene precipitate at or above 750 ppm. 1050

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Fig. 4. Turbidity readings of oil in n-Heptane containing asphaltene inhibitors (CI-1, CI-2, IL-1 and IL-2).

3.2. Turbidity measurements

1A). From NIR-SDS and HPM image, the AOP was found to be near 3778 psi. This indicates that the oil has the potential to precipitate asphaltene at the sand-face or near wellbore reservoir. However, as discussed earlier, there is a time lag between precipitation and deposition. This theory is validated with the well behavior in this reservoir. The asphaltene deposition takes place not at the sand-face (which is evident from the transient well tests) but from the packer depth and downstream as is evident during well logs. In Exp-2A, the same oil sample was mixed with the IL-2 at 1000 ppm concentration and the AOP is observed at 2968 psi (Fig. 6). This is indicative of some dispersive interaction between IL-2 and asphaltene molecules, resulting in reduced AOP by nearly 21% and bringing the AOP to bottomhole node instead of inside the reservoir. Exp-A-3 and A-4 (Figs. 7 and 8 respectively) were performed with recombined oil, homogeneously mixed with 35 mol% CO2. Fig. 7 represents the NIR plot of untreated oil and Fig. 8 represents IL-2 treated oil. From the NIR transmittance data and close observation of HPM images, the AOP without the addition of IL-2 was detected at 4195 psi. However, with the addition of IL-2, the AOP was detected at 3678 psi, indicating about 12.5% reduction of AOP. The increase of AOP as a result of mixing CO2 is reported in numerous experimental studies. The main concern for this particular reservoir is that if CO2 is injected without appropriate inhibition and if the local concentration of CO2 is developed, the asphaltene precipitation would occur far away from the sand face with a high possibility of plugging the near wellbore formation, which is the most sensitive area in terms of skin damage. The addition of IL-2, though effective in depressing the AOP, is not significant enough to bring the AOP towards a safer downstream node. Though the AOP is reduced and appears around the sand-face node, the asphaltene depositions could be expected at the bottomhole and subsurface nodes, affecting the sensitive subsurface equipment such as downhole chokes and valves. The observations from these experiments are encouraging because it proves that even in the presence of high CO2 fraction, IL-2 is partially effective in depressing asphaltene onset pressures.

Turbidity experiments were conducted to supplement the ADT results and to conclude the pre-screening process. It is obvious that a superior asphaltene inhibitor would stabilize and keep asphaltenes in solution, resulting in lower turbidity as compared to an inferior one. The plots of turbidity readings shown in Fig. 4 are the outcome of the average of three stabilized readings. The experimental error was limited to ± 1 NTU. The key elements of the information derived from Fig. 4 include; (a) the untreated crude oil in n-heptane underwent significant precipitation (as indicated by high turbidity readings); (b) addition of CI-1 resulted in a reduced turbidity (in the range of 110–112 NTU) and the optimum dosing is close to 250 ppm, (c) CI-2 as an asphaltene inhibitor is less effective compared to CI-1. Though the optimum dose is similar to CI-1, the turbidity readings are comparatively higher (115-120 NTU); (d) the ionic liquids showed mixed performance at 250 ppm dosing (where the turbidity is minimum, the IL-1 reduced the turbidity from 145 to 101 NTU whereas at 250 ppm dosing of IL-2, the turbidity is reduced to a significantly low value (24 NTU). It was also noticed that unlike IL-1, which manifested higher turbidity at higher dosing, the turbidity readings with IL-2 remained consistent. At a dosing level of 700 + ppm, the turbidity value reduced to about 16 NTU, indicating a significant reduction in asphaltene precipitates. Based on the above results, it was evident that CI-1, CI-2, and IL-1 are quite inept as asphaltene inhibitors compared to IL-2. Based on these results, IL-2 was selected for further studies at high temperature and high-pressure conditions to measure its effect in depressing the asphaltene onset pressure. 3.3. Isothermal depressurization with NIR-SDS and HPM analysis 3.3.1. Effect of IL-2 on asphaltene onset pressure (AOP) The depressurization experiments were conducted @ 2 psi/min while NIR-SDS was recording the light transmittance through the sample and the HPM was capturing sample images intermittently which helped in determining the AOP. The experiments were conducted with two distinctly different crude oil compositions: (Set-A) with recombined crude oil and (Set-B) the recombined crude oil homogeneously mixed with 35% mole fraction of CO2. The normalized NIR light transmittance plotted against pressure drop for the eight experiments are given in Figs. 5–12 along with the HPM images placed in appropriate positions. The results are summarized in Table 4. Fig. 5 represents the benchmark AOP without any additive (Exp-

3.3.2. Effect of IL-2 dissolution in solvent (isopropanol-heptane) A similar set of experiments (Set-B) were conducted with IL-2 dissolved in a solvent (isopropanol-heptane) instead of direct injection into the oil and/or oil-CO2 mixture as in Set-A. IL-2 dissolved in 50 mL of isopropanol-heptane (10:90 V/V) were injected into the oil in the recombination cell to obtain a final concentration of 1000 ppm. An equivalent quantity of solvent without the IL-2 was used for blank experiments. In Exp-B-1 (Fig. 9), the AOP was found at 3855 psi which is 1051

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Fig. 5. NIR transmittance data from isothermal depressurization experiment with untreated recombined oil.

comparable with Exp-A-1. A slight excess in AOP could be attributed to the presence of n-heptane which enhances asphaltene precipitation. Exp-B-2 (Fig. 10) was the repetition of Exp-B-1 with the exception of the presence of IL-2 (1000 ppm), resulting in the depression of AOP to 2641 psi, about 32% reduction at nearly identical CTP condition. From the above observation, it becomes evident that IL-2 is more effective in preventing asphaltene aggregation when dissolved in an appropriate solvent rather than when injected directly into crude oil. This could be explained with the assumption that (a) the introduction of Heptane-IPA solvent reduced the oil viscosity resulting in a more homogeneous distribution of IL and thus better interaction with the asphaltenes. (b) IPA being a well-known mutual solvent for polar and non-polar fluids, its presence enhanced the miscibility of polar IL with the non-polar oil through molecular level diffusion thus enhancing its activity. (c) It is also possible that IPA itself has contributed in suppressing asphaltene

aggregation by neutralizing the asphaltene π-electron cloud through Hbonding. Recently it is established that the petroleum asphaltenes are strong hydrogen bond acceptors and weak hydrogen bond donors. Isopropanol being a polar protic solvent with a high dielectric constant of 18 (compared to that of heptane with dielectric constant 1.9) makes it favorable for H-bonding through SN1 type reactions. (Babamohammadi et al., 2015). The final set of experiments (Exp-B-3 & B-4) were similar to those above. However, in Exp-B-3, the oil was mixed with 35 mol% of CO2 without any inhibitor added and in Exp-B-4, 1000 ppm of IL-2 was injected as a solution in isopropanol-heptane (10:90 V/V). A comparison was made between the AOP of untreated (Fig. 11) and treated (Fig. 12) samples. The results in Fig. 11 show that the AOP jumped to 4390 psi due to the addition of CO2 and the solvent, implying that the asphaltene precipitation could occur deep inside the reservoir and deposition could

Fig. 6. NIR transmittance data from isothermal depressurization experiment with recombined oil treated with IL-2. 1052

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Fig. 7. NIR Transmittance data from depressurization of untreated recombined oil and injected CO2 gas.

be expected at the downstream of the CO2 flood front (Fig. 1). This may cause severe formation plugging, from deep in the reservoir up to near wellbore region. When the same oil–CO2–solvent system was treated with IL-2, the AOP was depressed to 2380 psi (46% reduction in AOP), which falls at the midzone of production tubing. Though the IL-2 treatment could not eliminate the asphaltene precipitation entirely, it, however, can facilitate easier well management.

dispersants are amphiphilic molecules, (Hu and Guo, 2005), whose efficiency is dependent on their polarity and structure. Asphaltene flocculation is hindered by strong interactions between asphaltenes and dispersants as well as by steric hindrance process, which in turn depends on the solvation energy and the energy of stabilization, i.e the interaction energy between asphaltenes and dispersants (Leon et al., 2002). Mutelet et al. (2004) have stated that the petroleum asphaltenes are strong hydrogen bond acceptors and they participate in the chargetransfer process corresponding to partial removal of an electron from a bonding orbital. IR spectroscopy also suggested asphaltenes peptization in crude oils through the electron donor-acceptor interaction (hydrogen bonding, and charge-transfer complexation). Based on literature information, Boukherissa et al. (2009) summarized the varieties of molecular level interactions between ionic liquid dispersants and asphaltenes as; dispersive π-π, acid-base (hydrogen bonding, charge transfer), dipolar, and Coulombic interactions which facilitate the stabilization of asphaltenes and the electron donor-acceptor properties of dispersants.

3.4. Discussion The results presented in sub-sections 3.1 and 3.2 indicate that the commercial asphaltene inhibitors used in the local oil fields for wellbore treatment and the ionic liquid [BMIM][Cl] are far less efficient in asphaltene inhibition in the ambient conditions compared to the [BMIM][Br] IL. Therefore, we decided to continue our investigation only the most efficient ionic liquids (i.e., [BMIM][Br]). Historically it is observed that asphaltene flocculation inhibitors or

Fig. 8. NIR Transmittance data from depressurization of treated recombined oil with ionic liquid inhibitor (IL-2) and CO2 injected. 1053

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Fig. 9. NIR transmittance data from depressurization of untreated recombined oil with isopropanol-heptane.

are partially responsible for the inhibition effect. The π-π interaction between cations of IL and asphaltenes via hydrogen bonding seems to be more plausible. In addition, the charge neutralization effect of acidic groups such as –O-, -COO- and –S- etc. by the cations of ILs while neutralizing the positively charged basic cations such as = N+ and –NH+ present in the periphery of the asphaltene molecules must too have a significant contribution towards preventing asphaltene aggregation. On the question of why [BMIM][Br] is more effective in inhibiting asphaltene aggregation compared to [BMIM][Cl], we must look from two different angles. Though the [BMIM][Cl] and [BMIM][Br] both have the same cation, the size of their anions differs in size. The bromide anion in [BMIM][Br] having a larger ionic radius than the

Bai et al. (2013) studied the interaction energies of [BMIM]Cl and [BMIM]NO3 with asphaltene and found that [BMIM]Cl has the least interaction energy (unfortunately no information could be traced on the interaction energies between [BMIM]Br and the asphaltene). Ogunlaja et al. (2014) repeated the study and through computer simulation concluded that [BMIM] ILs depress asphaltene onset through π-π interaction between cations of IL and asphaltenes via hydrogen bonding, contradicting Hu and Guo (2005) who stated that due to the low density of the cation charge delocalized on the whole of aromatic rings of the ILs the cations cannot bond with asphaltene moieties to form stable complexes. The inhibitive effect was attributed to the electron donor anion undergoing hydrogen bonding or electron donor-acceptor interactions with the asphaltenes. In our view, both anions and cations of ILs

Fig. 10. NIR Transmittance data from depressurization of IL-2 treated recombined oil mixed with isopropanol-heptane solvent. 1054

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Fig. 11. NIR Transmittance data from depressurization of untreated recombined oil with isopropanol-heptane solvent and 35 mol % CO2 injected.

chloride, exhibited better steric stabilization of ionic liquid-asphaltene complexes, exerting better dispersion of asphaltene molecules. On the other hand, larger ion size would lead to a weaker ionic attraction between anions and cations of the IL, leading to more freedom for both the cations and anions to bond with the respective sites of the asphaltene molecules. These are however speculative and no firm conclusion can be drawn at this stage without detailed studies deeper into their molecular level interactions. In miscible injection, the CO2 is injected as a supercritical fluid, thus resulting in changes in oil composition and asphaltene destabilization. This is evidenced in the 1st HT-HP depressurization experiment, showing a significant jump of AOP indicating the possibilities of asphaltene precipitation deeper into the reservoir. On subsequent experiments, the reduction of AOP on the introduction of IL indicated that [BMIM][Br] interacted with asphaltene molecules and prevent them from flocculation even when CO2 has solvated the oil. The reason for

[BMIM][Br] being more effective when CO2 is present can be attributed to the fact that [BMIM] ILs also chemically interact with CO2 (Aki et al., 2004; Gonzalez. et al., 2013; Kodama et al., 2018). Extraordinalry solubility of [BMIM] ILs with CO2 was attributed to the Lewis acid−Lewis base (LA-LB) interactions with the anions of IL by Bhargava and Balasubramanian (2007) and Seki et al. (2009). Besnard et al. (2012) studied this process in detail using 1H, 13C, and 15N NMR supported by Raman and IR spectroscopy and found evidence of the formation of [BMIM]carboxylate in the mixture of CO2 with [BMIM]Ac even under near atmospheric conditions. On further investigations they established that the carboxylation process of the imidazolium ring occurs through the formation of intermediate “CO2-1-butyl-3-methylimidazole 2-ylidene” carbene species (Cabaço et al., 2012). Though no literature support could be found for similar process with [BMIM][Br], Br− ion being a moderately strong Lewis base, similar reaction could be expected. However this inference is speculative and further investigation

Fig. 12. NIR Transmittance from depressurization of treated recombined oil with isopropanol-heptane solvent, ionic liquid (IL-2) and 35 mol % CO2 injected. 1055

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Table 4 Results of isothermal depressurization experiments with and without additives. Exp. No.

Oil

A-1 A-2. A-3. A-4. B-1 B-2. B-3. B-4.

Recombined Recombined Recombined Recombined Recombined Recombined Recombined Recombined

oil oil oil oil oil oil oil oil

(GOR (GOR (GOR (GOR (GOR (GOR (GOR (GOR

370) 370) 370) 370) 370) 370) 370) 370)

Solvent-1

Solvent-2

Inhibitor

AOP (psi)

Ref.

Nil Nil CO2 CO2 Nil Nil CO2 CO2

Nil Nil Nil Nil isopropanol-n-heptane isopropanol-n-heptane isopropanol-n-heptane isopropanol-n-heptane

Nil IL-2 Nil IL-2 Nil IL-2 Nil IL-2

3778 2968 4195 3678 3855 2641 4390 2380

Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.

is necessary. Reduction of oil viscosity of the CO2 mixed oil may also played a role in improved dispersion and diffusion of the additives. Kanakubo et al. (2016) reported a decrease in viscosity and electrical conductivity for the CO2-saturated [BMIM] ILs which is attributed to the formation of the zwitterion [BMIM+-CO2-] and acetic acid. Thus higher activity of [BMIM][Br] may possibly due to the formation of [BMIM+-CO2-] zwitterion and/or replacement of Br_ anion by larger CO32− ions which is present in much higher concentration than the bromides. When experiments B-2 and B-4 are compared (both having IPAHeptane solvent), AOP is seen to be far lower in B-4 than B-2, i.e when CO2 is added. This implies that solvent additives have a ternary interaction with the CO2 and IL-2. Heptane being non-polar with low dielectric constant, the factor responsible for this possible interaction could be attributed to IPA with high dielectric constant. Cadena al. (2004) demonstrated through their studies on imidazolium-based ionic liquids of [BF4] and [Tf2N] anions with CO2, that the solubility of CO2 in ionic liquids takes place because of the formation of a strong network of anions and cations with CO2 filling the interstices in the fluid. The efficiency of asphaltene dispersant is dependent on their polarity and structure. So it may be assumed that the presence of polar protic solvent (IPA) in the media will enhance the polarity of the entire system, thereby enhancing the ability of the [BMIM+-CO2-] zwitterion dispersants to be absorbed on the surface of the asphaltene aggregates or bonding with the polar group of asphaltene molecule, resulting in the formation of a dominant repulsive forces between asphaltene species.

1000 ppm 1000 ppm 1000 ppm 1000 ppm

Remarks 5 6 7 8 9 10 11 12

Considered as benchmark AOP reduced by 21.5% Considered as benchmark AOP reduced by 12.5% Considered as benchmark AOP reduced by 31.5% Considered as benchmark AOP reduced by 46%

AOP AOP AOP AOP

asphaltene inhibitors fail to inhibit asphaltene precipitation in severe cases such as in miscible CO2 EOR, its application may be justified. If the application is sized judiciously, e.g. either as squeeze inhibitor in the near wellbore formation (similar to scale inhibitor squeeze) or injected at the bottomhole through chemical injection line, the consumption can be reduced significantly. 4. Conclusions

• The efficiency of commercial asphaltene inhibitors and prospective

•

•

3.4.1. Way forward It must be reiterated that prior studies on the ternary system (ILasphaltene-CO2) at HT-HP condition are almost non-existent, particularly involving [BMIM][Br] IL and the justifications given above are mostly speculative. Detailed studies on the molecular level interactions is planned in the next phase of our study. Also in the first phase, the study was restricted to the oil phase only, as the introduction of another phase (water) would inevitably increase the number of variants and make results interpretation more difficult. It is well known that CO2 injection is a part of water alternating gas (WAG) injection process and the presence of water will have a significant impact on the dispersion process. As way forward, a two-phase system with varying water content and salinity will be studied on reservoir rock core, simulating twophase flow through reservoir condition coreflood studies as well as HTHP depressurization studies in PVT cell in the next phase of this research.

•

• •

ionic liquid inhibitors to inhibit asphaltene precipitation were evaluated at ambient conditions through asphaltene dispersion tests and it was established that the ionic liquid 1-butyl-3-imidazolium bromide [BMIM][Br] was the most effective in dispersing and keeping asphaltenes in solution. Studies conducted through isothermal depressurization tests with the reservoir oil show that there is a potential threat of asphaltene precipitation at the near wellbore node at primary recovery mode and deep into the reservoir when miscible CO2 flood is conducted. The later could be a bigger threat as it can lead to severe formation damage and loss of production. AOP was reduced to 2380 psi from 4390 psi (46% reduction) when the oil–CO2–solvent system was treated with 1000 ppm of [BMIM] [Br]. This would bring the precipitation zone to the lower part of the production tubing and deposition could be expected at the upper part of the tubing and surface flow lines, which will be far easier to handle. The action of [BMIM][Br], in keeping asphaltene particles dispersed, could be attributed to dispersive interaction between asphaltene aromatic ring and the cations of the ionic liquid. The good performance of [BMIM][Br] over [BMIM][Cl] is assumed to be because of the higher steric hindrance of larger [Br] anion compared to [Cl] anions. [BMIM][Br] was seen to be more effective when injected as a solution in isopropanol-heptane rather than as a neat chemical. IPA seems to have contribution because of its high di-electric constant and ability to act as a mutual solvent. [BMIM][Br] was more efficient as AOP depressant in the presence of CO2 which is attributed to the viscosity reduction, its chemical interaction with CO2 and formation of carbene species or zwitterion or ion exchange as reported by earlier investigators.

Notes

3.4.2. Economic prospect With the current scale of production and the complicated synthesis mechanism, ILs are currently expensive compared to commercially available asphaltene inhibitors. Optimistically the prices are expected to come down with more efficient synthesis process and higher consumption. According to Aghaie et al. (2018), though the price of ILs in large scale can be much lower than the lab scale price, the cost is still much higher and their large scale use in reservoir scale could be cost prohibitive. However, in cases when the commercially available

The authors hereby declare no competing financial interests. Acknowledgments The authors acknowledge the Khalifa University for the financial support provided in undertaking this study. 1056

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Appendix A. Supplementary data

Sci. 13 (2), 280–291. González, G., Middea, A., 1991. Peptization of asphaltene by various oil soluble amphiphiles. Colloids Surface. 52, 207–217. Gonzalez-Miquel, M., Bedia, J., Abrusci, C., Palomar, J., Rodriguez, F., 2013. Anion effects on kinetics and thermodynamics of CO2 absorption in ionic liquids. J. Phys. Chem. B 117 (12), 3398–3406. Hammami, A., Phelps, C.H., Monger-McClure, T., Little, T.M., 2000. Asphaltene precipitation from live Oils: an experimental investigation of onset conditions and reversibility. Energy Fuels 14 (1), 14–18. Hu, Y., Guo, T.M., 2005. Effect of the structures of ionic liquids and Alkylbenzene-derived amphiphiles on the inhibition of asphaltene precipitation from CO2-injected reservoir oils. Langmuir 21 (18), 8168–8174. Kanakubo, M., Makino, T., Umecky, T., 2016. CO2 solubility and physical properties for ionic liquid mixtures of [BMIM]Ac and [BMIM][TFSA]. J. Mol. Liq. 217, 112–119. Kodama, D., Sato, K., Watanabe, M., 2018. Density, viscosity, and CO2 solubility in the ionic liquid mixtures of [BMIM][PF6] and [BMIM][TFSA] at 313.15 K. J. Chem. Eng. Data 63 (4), 1036–1043. Kokal, S., Sayegh, G., 1995. Asphaltenes: the Cholesterol of Petroleum, SPE 29787. Presented at the SPE Middle East Oil Show. Bahrain. March 1995. Leon, O., Contreras, E., Rogel, E., Espidel, Y., 2002. Adsorption of native resins on asphaltene particles: a correlation between adsorption and activity. Langmuir 18 (13), 5106–5112. Marcano, F., Moura, L.G.M., Cardoso, F.M.R., Rosa, P.T.V., 2015. Evaluation of the chemical additive effect on asphaltene aggregation in Dead oils: a comparative study between UV-visible and NIR-laser light scattering techniques. Energy Fuels 29 (5), 2813–2822. Murillo, H., Aburto, J., 2011. Current Knowledge and Potential Applications of Ionic Liquids in the Petroleum Industry. INTECH Open Access. Mutelet, F., Ekulu, G., Solimando, R., Rogalski, M., 2004. Solubility parameters of crude oils and asphaltenes. Energy Fuels 18 (3), 10391–10397. Ogunlaja, A.S., Hosten, E., Tshentu, Z.R., 2014. Dispersion of asphaltenes in petroleum with ionic liquids: evaluation of molecular interactions in the binary mixture. Ind. Eng. Chem. Res. 53 (48), 18390–18401. Permsukarome, P., Chang, C., Fogler, H.S., 1997. Kinetic study of asphaltene dissolution in Amphiphile/alkane solutions. Ind. Eng. Chem. Res. 36 (9), 3960–3967. Seki, T., Grunwaldt, J.D., Baiker, A.J., 2009. In situ attenuated total reflection infrared spectroscopy of imidazolium-based room-temperature ionic liquids under supercritical CO2. J. Phys. Chem. B 113 (1), 114–122. Shadman, M.M., Vafaie-Sefti, M., Ahmadi, S., Assaf, M.A., Veisi, S., 2016. Effect of dispersants on the kinetics of asphaltene settling using turbidity measurement method. Petrol. Sci. Technol. 34 (14), 1233–1239. Srivastava, K., Huang, R.,S., Dong, M., 1999. Asphaltene deposition during CO2 flooding. SPE Prod. Facil. 14 (4), 235–245. Sulemana, N., Ghosh, B., 2017. Application of ionic liquids in the upstream oil industry-A review. Int. J. Petrochem. Res. 1, 50–60. Yonebayashi, H., Miyagawa, Y., Ikarashi, M., Watanabe, T., Maeda, H., Yazawa, N., 2018. Determination of asphaltene-onset pressure using multiple techniques in parallel. SPE Prod. Oper. 33 (3), 486–497. Zanganeh, P., Ayatollahi, S., Alamdari, A., Dashti, H., Kord, S., 2012. Asphaltene deposition during CO2 injection and pressure depletion: a visual study. Energy Fuels 26 (10), 1412–1419.

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.06.017. References Aghaie, M., Rezaei, N., Zendehboudi, S., 2018. A systematic review on CO2 capture with ionic liquids: current status and future prospects. Renew. Sustain. Energy Rev. 96, 502–525. Aki, S.N.V.K., Mellein, B.R., Saurer, E.M., Brennecke, J.F., 2004. High-pressure phase behavior of carbon dioxide with imidazolium-based ionic liquids. J. Phys. Chem. B 108 (52), 20355–20365. Babamohammadi, S., Shamiri, A., Aroua Mohamed, K., 2015. A review of CO2 capture by absorption in ionic liquid-based solvents. Rev. Chem. Eng. 31 (4), 383–412. Bai, L., Nie, Y., Li, Y., Dong, H., Zhang, X., 2013. Protic ionic liquids extract asphaltenes from direct coal liquefaction residue at room temperature. Fuel Process. Technol. 108, 94–100. Behbahani, T.J., Ghotbi, C., Taghikhani, V., Shahrabadi, A., 2012. Investigation on asphaltene deposition mechanisms during CO2 flooding processes in porous media: a novel experimental study and a modified model based on multilayer theory for asphaltene adsorption. Energy Fuels. 26, 5080–5091. Besnard, M., Cabaco, M.I., Vaca-Chavez, F., Pinaud, N., Sebastiao, P.J., Coutinho, J.A.P., Danten, Y., 2012. On the spontaneous carboxylation of 1-butyl-3-methylimidazolium acetate by carbon dioxide. Chem. Commun. 48, 1245–1247. Bhargava, B.L., Balasubramanian, S.J., 2007. Insights into the structure and Dynamics of a room-temperature ionic Liquid: Ab Initio molecular Dynamics simulation studies of 1-n-Butyl-3-methylimidazolium Hexafluorophosphate ([BMIM][PF6]) and the [BMIM][PF6]−CO2 mixture. J. Phys. Chem. B 111 (17), 4477–4487. Boukherissa, M., Mutelet, F., Modarressi, A., Dicko, A., Dafri, D., Rogalski, M., 2009. Ionic liquids as dispersants of petroleum asphaltenes. Energy Fuels 23 (5), 2557–2564. Cabaço, M.I., Besnard, M., Danten, Y., Coutinho, J.A.P., 2012. Carbon dioxide in 1-Butyl3-methylimidazolium acetate. I. Unusual solubility investigated by Raman spectroscopy and DFT Calculations. J. Phys. Chem. A 116 (6), 1605–1620. Cadena, C., Anthony, J.L., Shah, J.K., Morrow, T.I., Brennecke, J.J., Maginn, E.J., 2004. Why is CO2 so soluble in imidazolium-based ionic liquids? J. Am. Chem. Soc. 126 (16), 5300–5308. Chang, C.L., Fogler, H.S., 1993. Asphaltene stabilization in alkyl solvents using oil-soluble amphiphiles. In: SPE International Symposium on Oilfield Chemistry. Society of Petroleum Engineers, New Orleans, Louisiana. Chang, C.L., Fogler, H.S., 1994a. Stabilization of asphaltenes in aliphatic solvents using Alkylbenzene-derived amphiphiles. 1. Effect of the chemical structure of amphiphiles on asphaltene stabilization. Langmuir 10 (6), 1749–1757. Chang, C.L., Fogler, H.S., 1994b. Stabilization of asphaltenes in aliphatic solvents using Alkylbenzene-derived amphiphiles. 2. Study of the asphaltene-Amphiphile interactions and structures using fourier transform infrared spectroscopy and small-Angle Xray scattering techniques. Langmuir 10 (6), 1758–1766. Firoozinia, H., Fouladi, H.A.K., Varamesh, A., 2016. A comprehensive experimental evaluation of asphaltene dispersants for injection under reservoir conditions. Petrol.

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