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Wettability modification and adsorption characteristics of imidazole-based ionic liquid on carbonate rock: Implications for enhanced oil recovery Prathibha Pillai, and Ajay Mandal Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03376 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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C8mimBF4 C10mimBF4

30

C12mimBF4 Pure carbonate Crude oil aged carbonate

Zeta potential

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0 ppm 1000ppm 2000ppm 5000ppm

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Oil Wet

Water Wet

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Amount adsorbed (mM/g)

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0 0

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3.0 2.5 2.0 1.5

C mimBF 8 4

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C mimBF 10 4 C mimBF 12 4

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Wettability modification by IL adsorption

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Wettability modification and adsorption characteristics of imidazole-based ionic liquid on carbonate rock: Implications for enhanced oil recovery Prathibha Pillai, Ajay Mandal * Department of Petroleum Engineering Indian Institute of Technology (Indian School of Mines), Dhanbad -826004, India Corresponding Author, Email: [email protected] Abstract Carbonate reservoirs contain a significant amount of oil and gas reserves but the ultimate recovery of these reserves are very low because of higher wettability of carbonate rock relative to crude oil. Surfactant flooding is the most common method employed to increase the recovery with the main mechanism of wettability alteration which to an extent depends on the mineralogical attributes of the reservoir. Ionic liquid (IL) which acts as a surface active chemical was employed to investigate its potential impact on oil recovery in carbonate reservoirs. XRD of carbonate reservoir rock was done to determine the quantitative mineralogy of rock samples. FTIR of carbonate reservoir rock samples and crude oil sample indicated that the polar components of crude oil are adsorbed onto the surfaces making it oil wet. SARA analysis of the crude oil determined the percentage of heavier fractions (asphaltene and resins) which have a direct impact on altering the wettability towards oilwet. Experimental investigation revealed that, imidazolium-based ionic liquids (ILs) were able to alter the wettability from oil-wet to water-wet condition. In the presence of salt and alkali, IL worked efficiently in reducing the contact angle and altering the wettability towards more water1 ACS Paragon Plus Environment

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wet. The loss of IL by adsorption on carbonate rock was investigated and Langmuir isotherm was found to be the best fit. Effect of salt had a detrimental effect on adsorption density of IL with a double adsorption density. On the other hand, addition of alkali with an optimum concentration of 1.5%-2% was found to reduce the adsorption of IL on the rock surface as alkali itself preferentially adsorb on the rock. Keywords: Enhanced Oil Recovery, Carbonate Reservoir, Ionic liquid, Wettability, Adsorption 1. Introduction Carbonate reservoirs are estimated to have more than 60% of the world's oil and 40% of the world's gas reserves but the ultimate recovery of these reserves are lower than 30% 1. The complex structures, formation heterogeneities and oil-wet/mixed-wet condition of the carbonate reservoirs lead to lower recoveries in comparison to sandstone reservoir 1,2. Primary and secondary recovery methods fail to yield above 20%–40% of original oil in place from these reserves, thus a considerable amount of residual oil remains in the reservoir matrix, emerging the need of enhanced oil recovery (EOR) techniques for incremental oil recovery. In the last few decades, extensive research in the field of chemical EOR has been carried out globally. Chemical EOR research in carbonate reservoirs has been focusing on using surfactants to change the wettability from oil-wet to water-wet, as oil-wet condition is one of the main reason for high remaining oil saturation in the carbonate reservoirs 1,3,4. Carbonate reservoirs are mostly believed to be neutral to oil-wet in nature and its recovery factor is much dependent on the wettability of the 5,6. Carbonate surface is usually positively charged and adsorb negatively charged acidic groups of crude oil readily. Wetting conditions of carbonate reservoirs can be modified by adding surface active chemicals, which form ion-pairs with adsorbed organic carboxylates of crude oil and stabilize them into the oil thereby changing the rock surface to water-wet 7. 2 ACS Paragon Plus Environment

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Application of surfactant to alter the wettability and recover oil from carbonate reservoir has been investigated by many researchers 3,8–10. For every reservoir conditions, the compatible surfactants are selected after a series of investigations, based on the reservoir temperature and salinity, surfactant structure, formation type, pH, the permeability of rock, cost of the surfactant, adsorption of the surfactant on the matrix rock, and finally, the oil recovery

11.

One of the most relevant

problems of the surfactant flooding system is the loss of surfactant due to adsorption in the reservoir. It affects the economic feasibility of the surfactant thus it is important to analyze the adsorption properties of the surfactant. Thus adsorption properties have been extensively studied for various surfactants and reservoir rocks 12–14. The increasing demand for energy has directed the oil industry to explore other surface active chemicals which can alter wettability as well as reduce the adsorption compared to conventional surfactants. IL is emerging as an alternative to the conventional surfactants and various studies highlighting its surface active nature has been investigated 15–17. ILs are organic salts which are liquid in the room temperature with some encouraging properties such as high thermal stability, non-volatility, salt tolerance, non-flammability, and low critical micellar concentration (CMC) etc. 18,19.

Various studies to investigate the effect of IL on IFT and its favorable impact on the recovery

of oil has been conducted 20. IL are acknowledged for their surface activity and micelle formation behavior which plays a pivotal role in flooding processes

21.

IL tunability allows them to be

utilized in different reservoirs, with different fluid properties. Very few research focusing on IL as a surface active chemical for carbonate reservoir has been investigated. In the present research, the IL synthesized in our earlier work 20,21 was studied to test its effectiveness in wettability alteration and its resistance to loss of IL due to adsorption. Effect of salinity and alkalinity was also tested during wettability alteration and adsorption phenomenon. 3 ACS Paragon Plus Environment

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The characterization of carbonate rock sample and crude oil was done to understand the effect of different parameters on wettability. 2. Materials Required Three ILs of imidazolium family were synthesized and characterized as reported in our previous papers 20,21 were used for performing the experiments. Sodium chloride and sodium carbonate were purchased from Sigma Aldrich Co. LLC. The aqueous solutions were prepared in double distilled water. The crude oil sample used was from ONGC (Ahmedabad, India) with 25° API at 30 °C. 3. Experimental Section 3.1. Characterization The mineralogy of dry carbonate rock was studied by XRD (Rigaku 9kW, SmartLab, with Cu Kα radiation (λ = 1.5406 Å)) was utilized. The functional groups present in the dry carbonate rock, oil-aged carbonate rock, and crude oil was investigated by using a FTIR spectrophotometer PerkinElmer Spectrum-2 between 400 and 4000 cm−1. For the morphological study of the dry carbonate rock, oil-aged carbonate rock and IL-treated carbonate rock SEM analysis was conducted using SUPRA 55 ZEISS (Germany), where microscopic images were analyzed to identify the wettability change. 3.2. Analysis of crude oil ASTM procedure (ASTM D2007-932) is the standard testing method followed for SARA analysis of crude oil using n-hexane to separate the asphaltenes. ASTM procedure was conducted by diluting the sample with solvent and then charging it to a glass-percolating column containing clay in the upper section for adsorbing resins and lower section with silica gel plus clay which separated aromatics from the saturate fraction. A toluene and acetone mixture was used in the clay section 4 ACS Paragon Plus Environment

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for desorption of resin from clay packed column. The aromatics from the lower section of the column was recovered by Soxhlet extraction of the silica gel in hot toluene. During the process the volatile components lost can be calculated by weight difference. ASTM D-664 procedure was used to determine the total acid number (TAN) for crude oil by potentiometric titration. Perkin Elmer Clarus 500 Gas Chromatograph, with chromatographic column of 30 m x 0.25 mm ID HP-5ms capillary column was used to determine the constituents of the crude oil sample used. 3.3. Wettability The ability of IL on alteration of wettability in presence of salts and alkali was studied by sessile drop method was applied. Sessile drop method was availed by Kruss DSA25 Drop Shape Analyzer to measure the contact angle of the sample. To perform the test, small slices of a clean carbonate rock sample were cut, polished (30 × 30 × 5 mm) and aged with crude oil for 45 days before performing the experiments. The samples were later washed with n-heptane and finally were dried overnight in an oven. The contact angle of the IL solution was evaluated by dropping a small drop of IL solution through the needle tip of 0.5mm diameter on the rock surface, and dynamic study of contact angle was measured at 30 ℃. Each experiment was performed on unaltered rock samples. Experiments were repeated multiple times before reporting. FTIR spectra of samples i.e. dry carbonate rock, oil - aged carbonate and oil - aged carbonate treated with IL were investigated using FTIR spectrophotometer PerkinElmer Spectrum-2 in a range of 400 and 4000 cm−1. The presence or absence of functional groups would give an indication of wettability alteration behavior. The zeta potential of aqueous rock suspension was analyzed using a using Horiba Nano Particle Analyzer SZ-100 at 30 ℃. Zeta potential of carbonate rock powder was evaluated by preparing a

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suspension by mixing 2.5 gm of powdered carbonate rock sample in 1000 ml of distilled water. The solution was sonicated by Fisherbrand FB15051 sonicator for an hour to obtain carbonate suspension. These carbonate suspension was left untouched to stabilize and tested with varying concentration of IL. In a similar way, crude oil aged carbonate rock powder sample suspension was also prepared with different IL concentration to investigate the interaction of the IL with oil aged carbonate particles. The experiments were repeated multiple times for accurate results. 3.4.

Adsorption analysis

Adsorption of synthesized ILs on carbonate rock was determined by batch tests. In this test, finely crushed carbonate sample (4 g) was mixed with the aqueous ILs solution (40 ml each) of different concentrations. The solution was stirred continuously in a horizontal shaker for a period of 24 hours. The solution was centrifuged to allow the carbonate sample to separate from the supernatant solution. The IL solution before and after adsorption with carbonate sample was further tested to evaluate the adsorbed amount of IL on the rock surface. SHIMADZU UV-1800 ultraviolet spectrophotometer was used to determine the residual concentration of IL. Adsorption on the carbonate rock surface was determined by estimating the difference between the IL concentration in the aqueous phase before and after adsorption. Adsorption (q) 22can be calculated using, Eq. (1) 𝑣

𝑞 = (𝐶𝑜 ― 𝐶𝑒)𝑚

(1)

where q is the adsorption density (mg/g), Co is the initial IL concentration in solution (ppm), and Ce equilibrium IL concentrations in solution (ppm), m is the mass of the powdered carbonate sample (g) and v is the volume of solution (mL). Static adsorption was obtained by comparing the initial IL concentration with the concentration of IL after reaching equilibrium. Adsorption Isotherms 6 ACS Paragon Plus Environment

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An adsorption isotherm is a plot used to analyze the equilibria between the amount of adsorbate that accumulates on the adsorbent and the concentration of the dissolved adsorbate 23. In this study, the adsorption models were used for describing the adsorption equilibrium behavior elaborately. Langmuir Isotherm Langmuir equation is the common model used to illustrate the adsorption process for a monolayer system. The maximum adsorption capacity can be evaluated 24 as shown

𝑞𝑒 =

𝑞𝑜𝐾𝑎𝑑𝐶𝑒

(2)

1 + 𝐾𝑎𝑑𝐶𝑒

where qe is the amount of adsorbate per unit mass of adsorbent (mg/g), qo is the maximum amount adsorbed (mg/g), Ce is the equilibrium concentration of adsorbate (ppm) and Kad is the Langmuir constant (L/mM). The qo and Kad values can be determined from the slope and intercept of the straight line respectively. The isotherm includes some assumptions which are monolayer coverage, homogeneous adsorption sites and identical sorption sites with equivalent energy. Another non-dimensional equilibrium parameter of Langmuir isotherm RL, which describes the feasibility of the adsorption process can be calculated as 25, 𝑅𝐿 = 1 +

1 𝐾𝑎𝑑𝐶𝑒

(3)

The value of RL indicates the process of adsorption to be favorable in the case of (RL < 1), unfavorable for (RL > 1), linear type for (RL = 1) and irreversible for (RL = 0). Freundlich Isotherm


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The Freundlich isotherm equation is considered to be more convenient for a heterogeneous adsorption pattern where there is almost negligible saturation of adsorbent to adsorbate. Hence, infinite surface coverage is predicted which indicates multilayer adsorption on the surface 26. The amount of solute adsorbed, qe, can be determined from Freundlich equation as 𝑞𝑒 = 𝐾𝑓𝐶1/𝑛 𝑒

(4)

where 1/n is the adsorption intensity and KF is the adsorption capacity. Adsorption feasibility can be determined from the adsorption intensity index (1/n) 25. Adsorption phenomenon is considered to be favourable when 1/n lies in between 0.1 to 0.5, easy to adsorb when 1/n lies in between 0.5 to 1 and difficult to adsorb when 1/n is above 1. Temkin Isotherm Temkin isotherm model considers the influence of adsorbate interactions on the adsorption processes. The molecular interaction between the adsorbate leads to decline in the heat of adsorption of molecule in a linear pattern between the adsorbed layer. The Temkin equation 22 is calculated using the following equation, 𝑞𝑒 = 𝐵 𝑙𝑛𝐾𝑡 +𝐵 𝑙𝑛𝐶𝑒

(5)

where B is the Temkin constant and Kt is the equilibrium binding constant respectively. 4. Results and Discussions 4.1.

Characterization of carbonate rock

The mineralogical attribute of a reservoir rock is one of the fundamental factors which determines the type of interaction that controls the adsorption of polar components on the rock which effects oil recovery. Initially, most of the reservoir rocks are considered to be water-rich i.e. water-wet. 8 ACS Paragon Plus Environment

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During migration when polar components enter the system, their ability to alter rock wettability depends, in part, on the rock type and surface charge. The surface charge of the minerals will affect the rock-crude oil interactions, mainly in the way crude oil adheres to the surface of the porous media 4,27. Sandstone surfaces are anionic in nature while limestone surfaces are typically cationic and reservoir solution chemistry affects both

28.

The surface chemistry of carbonates is

significantly more complex than that of sandstones. This is due to the presence of dolomite, Mgrich calcites, clays, pyrite, and anhydrite 29. Carbonate surfaces are found to be positively charged at pH values below 8 to 9.7, hence they show higher sensitivity to acidic components 3,30,31. Thus adsorption of the negatively charged carboxylates, RCOO-, which are found in crude oil is more likely to occur 32. XRD characterization of carbonate rock sample was performed to evaluate its nature of the charge and mineral composition of the rock. Figure 1. shows the XRD diffractogram of the powdered carbonate rock sample. Single major peak at 29.68° corresponding to calcite was observed depicting that the rock is mostly composed of calcite mineral which indicates the mineral surface sites to be positively charged. FTIR was used to analyze the functional groups present in the crude oil, dry carbonate rock and oil-wet carbonate rock to determine the main components affecting rock characteristics. Figure 2. shows the FTIR spectra of all three samples analyzed. In dry carbonate rock, absorptions peaks at 711 cm−1, 876 cm−1, and 1796 cm−1 were observed which corresponds to the stretching vibration of Ca−C bonds indicating the presence of calcite which is in coordination with XRD results reported. In crude oil sample, adsorptions peak at 2927 cm−1 and 2851 cm−1 corresponding to the C-H and CH2 stretching vibrations were observed. Peaks at 1461 cm−1 indicating C-H stretch was noted. Peak due to O-H bond stretching vibrations corresponding to alcoholic content was 9 ACS Paragon Plus Environment

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observed at 3428 cm–1. Further peaks at 810 cm–1 and 1603 cm–1 due to CH bending and C=C stretching was seen. Additional peaks at 1306 cm-1 and 1032 cm-1 can be assigned to the presence of C-O bond. In oil-wet carbonate rock, absorption band at about 1306 cm-1 and 1032 cm-1 was seen corresponding to the bending vibration of C-O bonds. The peak corresponding to the carboxylic acid O−H group was observed at 3428 cm−1 which confirms that the carbonate rock is oil-wet. 4.2.

Characterization of crude oil components affecting wettability

The composition of crude oil was determined by SARA (saturate, aromatic, resin, and asphaltene) ASTM Column Separation method as shown in Table i. As depicted in Table i, crude oil has major fraction of saturate (56.3%) and aromatic fraction (25.8%). However, significant fraction of heavier components i.e. asphaltene (6.3%) and resin (11.5%) were also obtained, which plays a major role in wettability alteration. An oil composed of saturates and aromatics would be nonwetting relative to water on mineral surfaces. Thus focus on asphaltene and resin components has been directed. Crude oil components containing the carboxyl group -COOH, are mostly found in the heavy end fraction of crude oils, mainly asphaltene and resins. This carboxylic group is held responsible for altering the reservoir rock wettability to oil-wet from initial water-wet state. The bond between the negatively charged carboxylic group, -COO-, and the positively charged mineral surface is very strong, and thus the carbonate surface gets adhered with large crude oil molecules, thus altering the wettability towards more oil-wet 33.The carboxylic material present in the crude oil can be estimated from acid number, AN (mgKOH/g). Results indicate that the crude oil sample used in the present study is having an acid number of 0.044mgKOH/g. Acid and base number of oil play an important role in determining the wetting state and wetting mechanism. As the AN of


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the crude oil increases, the water-wetness decreases. It was observed by some researchers that the imbibition rate and oil recovery decreased dramatically as the AN of the oil increased 3,33. Along with SARA analysis, FTIR spectra of the crude oil sample was also analyzed as shown in Figure 3. Adsorption peaks corresponding to C-H and CH2 stretching vibrations were evident at 2927 cm−1 and 2851 cm−1. Peaks were also observed at 1461 cm−1, 1378 cm–1 and 719 cm–1 corresponding to CH, CH3 bending vibrations and CH2 bending. The presence of these peaks confirm the existence of the paraffinic fraction in crude oil sample 34. Further peaks at 810 cm–1 and 1603 cm–1 due to CH bending and C=C stretching ensured the presence of aromatic fraction of the crude oil. Peak due to O-H bond stretching vibrations corresponding to alcoholic content was observed at 3428 cm–1 and a small peak at 1721 cm–1 corresponded to ketone as carbonyl compound was also noted. Additional peaks at 1306 cm-1 and 1032 cm-1 can be assigned to the presence of esters and ester linkages which are found in asphaltene molecule 35. Crude oil sample was further characterized by HTGC (high-temperature gas chromatograph) as shown in Figure 4. The resulting chromatogram shows that the crude oil also contains high molecular weight hydrocarbons fraction. It may be seen from Figure 4. that shown C7+ - C35+ are uniformly present in the crude oil with C36+ more than 56%. These heavier components are mainly characterized as resins and asphaltenes. Asphaltenes are the constituents of crude oil having the highest molecular weight and consists of molecules mainly of sulfur, carbon, nitrogen, hydrogen, and oxygen 36. Resins are the components of crude oil consisting of polar molecules often containing organic compounds that contain nitrogen, sulfur, and oxygen. 37. Further within these two components, asphaltenes are considered to be the major component that adsorb on the carbonate surface, primarily by interactions between their aromatic and peripheral heteroatom groups and the sites present in the carbonate surface. 11 ACS Paragon Plus Environment

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Asphaltene molecules are susceptible to intermolecular forces due to the presence of polar peripheral groups which are responsible for their ability to self-associate, form aggregates and to adsorb on mineral surfaces of rocks. The resins form a chief stabilizing component for asphaltene due to their intermediate polarity and reduced number of aromatic ring. Even in the absence of water, asphaltene adsorption on the mineral surfaces is driven by polar interaction, surface precipitation and H-bond formation. The adsorption process produces marked changes in the wettability of the solid and reduces oil production from the reservoir. 4.3.

Characterization of Oil-Carbonate Rocks Interaction

Only one-third of the crude oil stored in underground reservoirs are successfully recovered by conventional recovery methods. An in-depth investigation of the interaction between crude oil and the rock surface of the reservoir is essential to develop innovative methods for optimum oil recovery. Oil recovery in carbonate reservoirs is generally less than 30% due to low water wetness, natural fractures, low permeability, and heterogeneous rock properties

33.

Carbonate rocks are

chemically unstable rocks; at higher temperatures many mineralization reactions take place in a carbonate reservoir which affects wettability. Carbonate surface is initially positively charged, when it comes in contact with crude oil, negatively charged carboxylic group (-COO) from crude oil forms an electrostatic bond with positively charged carbonate surface. As discussed earlier this carboxylic group (-COO) is an important parameter responsible for wettability alteration in a carbonate reservoir. If only formation water exists in the reservoir, all of the surface areas is in contact with water, which means the reservoir is water-wet for everywhere. However, the aging process will happen when crude oil comes in contact with the rock surface for a long time in the reservoir surface 38. In a reservoir, when oil enters it interacts with water and the interface becomes charged thus 12 ACS Paragon Plus Environment

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exposing the electrical attributes of the polar compounds. Based on the mineral surface charge the interface can take part in different electrostatic interactions 39. Crude oil adsorption onto the rock surface takes place due to the intermolecular or inter-ionic forces interaction between opposite charges of the rock surfaces and oil-water interface comes into contact 4,40. The result of the aging process is the strong bond between the negatively charged carboxylic group and the positively charged sites on carbonate. The surface is oil wet due to the occupation of the oil component on the mineral surface sites. The zeta potential of the crude-oil/brine interface and of the mineral/ brine interface is also considered as an important factor in the determination of the wettability. For understanding zeta potential, some important factors needs to be considered are pH of the environment and formation salinity

41.

The zeta potentials of the mineral/ brine interfaces as a function of pH in a saline

environment is shown in Figure 5. Zeta-potential values tend to decrease and then increase with pH. The isoelectric point (IEP) was observed to be at 7.25 pH. Calcite particles acquire a positive charge below the IEP and a negative charge above the IEP. 4.4.

Characterization of ILs

The ionic liquid was characterized by 1H NMR, FTIR, and TGA analysis. Successful synthesis of IL was confirmed by FTIR peak at 1570 cm-1 and 1H NMR peak at 4.183 ppm as discussed in our earlier work21. From the TGA analysis of the IL it was observed that ILs were stable up to 400 ºC and thus exhibited good thermal stability at reservoir temperature conditions. The aggregation and surface activity of C8mimBF4, C10mimBF4 and C12mimBF4 in aqueous media was studied as shown in Figure 6. The CMC of synthesized ionic liquid was found to be 12000 ppm in the case of C8mimBF4, 5000 ppm in the case of C10mimBF4 and 2000 ppm in the case of C12mimBF4. Decrease in CMC value with increasing number of aliphatic methyl chains is indicative of 13 ACS Paragon Plus Environment

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enhanced hydrophobicity of ionic liquids with increasing tail chain lengths. The surface activity of ionic liquids was found to critically depend on temperature as well as the length of alkyl chain attached to the imidazole group. Interfacial activity as a function of the concentration of the ionic liquid, salt and alkali was investigated and it was observed that the IL was efficient in reducing IFT as shown in Figure 7. Alkali produces in-situ surfactants once it reacts with the crude oil inducing synergistic effect which further reduces IFT. From obtained results, it was found that as the alkyl chain length increased, the CMC and IFT values decreased and the minimum IFT of 0.041 mN/m was obtained. 4.5.

Wettability Alteration of oil-wet carbonate

Wettability is a fundamental property of the interactions between the pore filling fluids and the reservoir rock components. Wettability of a rock surface affects most of the parameters necessary for reservoir management. Wettability is a controlling factor on the location of the reservoir fluids within its porous structure and subsequent distribution of fluids in the reservoir

42.

Original

wettability of the reservoir is dependent on various factors which include the adsorption of polar compounds, mineral surface charge etc. Crude oil consists of polar compounds which have a polar and a hydrocarbon end. The polar end of the crude oil adsorbs on the rock surface and the hydrocarbon end on the opposite side makes the surface oil-wet. These compounds are water soluble allowing them to penetrate the original water film and render the surface oil-wet 43. Thus the adsorption of these polar compounds from the crude oil on the originally water-wet rock can alter the wettability to oil-wet. Figure 8. shows a schematic representation of wettability alteration of carbonate rocks. The degree of wettability alteration is determined by the interaction of the oil constituents, the mineral surface, IL, and the brine chemistry. Along with the polar compounds, mineral surfaces are also important in determining wettability as discussed above. The carbonates 14 ACS Paragon Plus Environment

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have positively charged surface thus adsorb compounds of opposite polarity (polar compounds of crude oil) by acid/base reaction

44,

thus altering the wettability towards oil-wet. With the

introduction of these surface active chemical like ILs, the reservoir wettability can be changed back to water-wet condition, as they form ion-pairs with the carboxylic groups of the crude oil thus desorbing from the rock surface. 4.5.1.

Surface morphology

Surface morphology of the rocks prominently affects the wettability. SEM images of dry carbonate rock, oil-wet carbonate and oil-wet carbonate treated with IL at 3000 X magnification zoom are shown in Figure 9. Dry carbonate rock shows a rough texture owing to surface morphology of carbonate. The oil-aged sample showed a smooth texture in comparison to dry carbonate due to adsorption of crude oil. As the crude oil and calcite interfaces are predominately oppositely charged hence they are mutually attractive, thus altering the sample wettability towards oil-wet. Further on treatment of the oil-wet carbonate sample with IL, the sample texture was observed to be rough again. It was observed that the adsorption of IL on the surface of oil-wet carbonate takes place which forms ion-pair with the carboxylic groups of the crude oil and it results in the desorption of crude oil layer from the carbonate surface, thus altering the wettability towards water wet. 4.5.2. Contact Angle measurements 4.5.2.1.

Effect of concentration

Most of the carbonate reservoir are having salinity on higher side, hence the efficiency of salinity on wettability alteration by ILs is of utmost important. In this work, the wettability of the carbonate rock sample surface was evaluated by using the contact angle method. Initially contact angle


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measurements in distilled water was performed which indicated the oil-wet state (about θ = 148°), of the aged carbonate rock. Then the contact angle of oil aged rock surface was measured for different concentration of three synthesized ILs as shown in Figure 10. (a), (b) and (c). The equilibrium contact angle reduces with increase in the concentration of ionic liquid solution but the decrease is only marginal after CMC because of saturation of the IL at the rock-fluid interface. The proposed mechanism for wettability alteration to water wet by IL is due to the ion-pair formation between the positive head groups of the IL molecules and the negatively charged adsorbed material, mostly carboxylic groups from crude oil on the surface of the carbonate

19.

Electrostatic interactions between the head groups drive the ion-pair formation. Formed ion-pairs could strip away the crude oil components adsorbed on the initially water-wet rock. Thus, ILs were able to alter the wettability of the carbonate rock surface towards more water-wet state effectively. Among the synthesized IL, C12mimBF4 was observed to alter the wettability effectively to waterwet. It can be observed that the contact angle recedes along with increasing time and attains a minimum angle within a short time span. Based on experimental results the average contact angle of distilled water and rock surface was observed to be 146° confirming the sample to be oil-wet. The average contact angle after treating with IL was 26° which indicates a water-wet system as a result of IL solution. 4.5.2.2.

Effect of salt and alkali on wettability

In recent studies, it was found that salinity of injected water and its composition also plays a major role in wettability alteration of carbonate reservoir

33.

The effect of salinity on contact angle at

CMC of IL was studied as shown in Figure 11. To investigate the effect of salt concentration on contact angle, the concentration of NaCl was varied between 0% to 5%. It was observed that contact angle started decreasing with increase in NaCl concentration. The presence of NaCl in the 16 ACS Paragon Plus Environment

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aqueous phase has the ability to increase IL molecules available at the interface thus reducing the contact angle

45.

When salt concentration is increased beyond a certain limit, IL molecules are

prevented from dissolving in the aqueous phase, thus increasing contact angle. It has been observed that as the alkyl chains of synthesized IL increases contact angle shows a better reduction. Alkali also have additional benefits of in situ- surfactant formation and emulsification which improves oil recovery. High-pH basic components such as sodium carbonate can act as wettability modifier and alter wettability of rock from oil-wet to water-wet

46.

In this study effect of alkali (Na2CO3) on the

contact angle with time for all three IL was investigated, while analysing the effect of alkali on contact angle it became difficult to measure the contact angle with time due to the occurrence of complex mechanisms and adsorption phenomenon within a small time span (unsteady state condition). Thus for better explanation of the phenomenon, effect of pH on contact angle has been explained with the equilibrium data (⁓ 200 seconds) for all three ILs (revised Figure 12). It was observed that the contact angle increased with increasing pH from 9 to 11(0.5% and 1% Na2CO3), reaching a maximum at around 11(1.5% Na2CO3) and decreasing further with increasing pH (2% Na2CO3). As explained by Peng et.al.47 in their work, the values of contact angle reflect the degree of hydrophobicity of mineral surface. Larger contact angle represents stronger hydrophobicity of the surface. In fact, the hydrophobicity of the surface is attributed to the adsorption quantity and aggregation structures of IL on the surface. Initially at low pH the IL molecules are adsorbed on the surface such that the lower layer is attached by means of electrostatic force and the upper layer is attached by hydrophobic association with the alkyl chains. Thus in the upper layer the polar head is oriented towards the solution, causing the hydrophilicity of the surface. Increase in pH causes an increase in the concentration of neutral IL molecules. These neutral IL molecules are mostly 17 ACS Paragon Plus Environment

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present in the upper layer of the adsorbed IL bilayer by hydrophobic association. As the adsorbed layer has neutral charge, the surface was found to be hydrophobic in nature. Thus, with increase in pH, increase in hydrophobicity was observed. However, with further increase in concentration of sodium carbonate, it has been reported that increase in ionic strength in solution is greater in comparison to increase in pH 48. Thus at 2% sodium carbonate the greater ionic strength plays a dominating role and a greater amount of polar IL molecules are expelled from the aqueous solution to the interface causing better water-wetting of the surface. Thus it was observed that contact angle of surface increased with increase in sodium carbonate concentration as a result of increase in pH and further decrease in contact angle was observed as a result of increase in ionic strength. 4.5.3. Zeta Potential measurements The charges at the interfaces of oil/brine and brine/rock surface control the stability of the water film between the oil and rock, thus the rock wetting state of the reservoir. If similar charges are exhibited in both surfaces a strong repulsive force is created and strong attractive force if surface charges are opposite. A strong electrostatic repulsion between the interfaces will create a stable and thick water film which would result in water-wet rock and vice versa 49. Due to the generation of electrostatic attractive or repulsive forces at the interface, rock wettability depends on the sign and magnitude of the electrical charge at the interface. The zeta potential of carbonate rock was performed to determine the charge distribution of the rock in the reservoir system as shown in Figure 13. It was observed that carbonate surface showed positive values of the zeta potential of +6.1 mV as the pH was below zero potential of carbonate. The zeta potential of carbonate powder after aging decreased to -21mV, which is due to the negative charge of crude oil. In the presence of water, the dissociation of the acidic component of crude oil changes the carbonate charge to a negative value. Oil molecules, mainly resins and 18 ACS Paragon Plus Environment

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asphaltenes are adsorbed onto the surface by electrostatic forces due to their high polarity. The aged carbonate rock was treated with IL solution of different concentration. The IL-treated carbonate solutions exhibited positive zeta potential value with increasing concentration due to the desorption of the acidic components from the carbonate surface which is in good agreement with the FTIR and FESEM results. From the three synthesized ILs, C12mimBF4 increased the zeta potential of carbonate from -22mV to +36mV and exhibited the best result in altering the wettability of carbonate rock. It was observed that with increase in chain length of IL, the ability to desorb oil was improved. This improvement was due to increased hydrophobicity (i.e. better ability to undergo hydrophobic interactions). As C12mimBF4 has a high number of carbon atoms of alkyl chain length thus can adsorb on crude oil particles more efficiently than other ILs. 4.6.

Adsorption

The IL adsorption by carbonate reservoir rock is measured to find the amount of excess IL required in the injection stream for an effective chemical flooding. IL loss due to adsorption on rock surface cannot be avoided but can be reduced. Rock surface charge plays an important role in adsorption and charge developed at the solid-liquid interface depends to an extent on pH and ionic strength. The mechanism responsible for the IL adsorption is mainly the electrostatic attraction between the charged surface of the solid and the charged head group of the IL molecule

50.

During the

adsorption process, IL molecules are transported from the bulk solution to the interface (rock surface). This surface charge causes oppositely charged IL molecule to adsorb and like charge to repel 51. The surface charge of carbonate rock sample is positive, thus positively charged cationic head of IL intensify the repulsive force between rock surface and IL molecules to minimize adsorption. The main mechanism governing adsorption pattern of IL is believed to be van der Waals’ 19 ACS Paragon Plus Environment

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interactions and electrostatic interaction 15. Despite this, adsorption is believed to be a complex phenomenon and the driving force of IL adsorption can be a combination of electrostatic interactions, hydrogen bonding, chemical interactions, covalent bonding, non-polar interactions, and de-solvation of the adsorbate moieties. From the Figure 15., it can be seen that the adsorption trend can be divided into three stages. Initially, the adsorption was due to the electrostatic interaction between individual charged monomeric species and charged solid surface. Then adsorption linearly increases resulting from the interaction of the hydrophobic chains of ongoing IL with previously adsorbed IL. In next zone, adsorption is due to electrostatic attraction between the surface sites and the charged IL species and hydrophobic interactions between the hydrocarbon chains. Later the surface becomes electrically neutralized and further adsorption takes place due to chain–chain hydrophobic interactions alone

13.

Thus above the CMC of the synthesized IL

adsorption also remains constant. As observed from Figure 14, after attaining an adsorption density of 4.2 mg/g at 12000 ppm for C8mimBF4, 2.4 mg/g at 5000 ppm for C10mimBF4 and 0.9 mg/g at 2000ppm for C12mimBF4, adsorption further increases marginally with increasing availability of IL. The electrostatic interaction between IL molecules and solid surface as well as the lateral chain-chain interactions between the adsorbed IL molecules contribute to the driving force of adsorption 52. 4.6.1. Adsorption Isotherms An adsorption isotherm is used to characterize the equilibria between the amount of adsorbate that accumulates on the adsorbent and the concentration of the dissolved adsorbate 23. To optimize the design of the adsorption system, it is essential to evaluate the most appropriate correlations for equilibrium curves. In present work, adsorption isotherms namely Langmuir, Freundlich and Temkin were incorporated to explain the adsorption process. The adsorption isotherm of ILs are 20 ACS Paragon Plus Environment

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presented in Figure 15. Parameters for all equilibrium adsorption isotherms along with regression coefficient (R2) are presented in Table ii. From Figure 15. and Table ii it can be observed that the Langmuir isotherm plot provided maximum regression coefficient in all three synthesized IL. The parameters, equilibrium constant Kad and maximum monolayer coverage capacity qo evaluated from Langmuir isotherm are presented in Table ii. The non-dimensional separation factor RL was also evaluated as shown with a variation between 0.12–0.28 which depicted the favorability of the adsorption process. Freundlich and Temkin isotherm plot fitness was not as significant as the Langmuir model. From the above discussion, it can be concluded from the adsorption isotherms that the Langmuir isotherm model was the best fit model for the experimental values of batch experiments. The results revealed that the surface of the adsorbent was homogeneous with monolayer adsorption and no interaction between the adsorbed molecules. 4.6.2.

Effect of NaCl

The effect of formation water on the adsorption behavior of carbonate reservoir was investigated using NaCl. Adsorption of C12mimBF4 (as it depicted lowest adsorption values in comparison to other two ILS) at CMC, and concentration above and below CMC on carbonate rock sample was investigated with varying NaCl concentration as shown in Figure 16. As observed from the figure, adsorption of IL increases with increasing NaCl concentration. The presence of salt affects the physical and chemical adsorption of IL by interfering with the electrostatic interactions during the adsorption process. When the salinity of the aqueous phase is increased, cations and anions are dissociated from IL and electrolytes which compete for water solvation. Smaller size of Na+ ion in comparison to the bulky cationic part of IL causes higher surface charge density of Na+ ion, hence reducing the number of reachable water molecules for the hydration of IL 53. This results in 21 ACS Paragon Plus Environment

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reduction in the solvation power of the solution for the IL and results in the movement of IL toward the carbonate surface. Thus adsorption of more IL molecules on the surface takes place, which leads to increased adsorption density. 4.6.3. Effect of Alkali in reducing adsorption: The effectiveness of injected IL slug is reduced due to adsorption of the IL on the rock surface. The attraction of IL on to the rock-water interface, results in the loss of IL to reservoir rock surface by adsorption. Alkali increases the pH and decreases the number of positive sites available for the adsorption on the carbonate surface, thus reducing the adsorption 54. The effect of alkali, Na2CO3 on IL adsorption on carbonate rock sample was analyzed as shown in Figure 17. In case of synthesized IL C12mimBF4 it was observed that adsorption increased with increasing alkali concentration and reaches a maximum at around 1.5% and further adsorption decreases with increasing concentration. This adsorption behavior in the presence of alkali was similar to that observed in the contact angle results with alkali. Initially at lower alkali concentration movement of some IL molecules at the interface from the initial adsorption position at the interface to the top of the adsorption monolayer takes place, increasing the hydrophobicity of the surface to a certain extent, thus increasing the adsorption. Further increase in alkali concentration (at 1.5%), IL molecules are considered to be arranged in a compact monolayer formation with polar head groups adsorbed towards the rock surface, thus establishing maximum hydrophobicity 47. Increasing hydrophobicity of the surface is attributed to increase in adsorption values as well, thus at 1.5% alkali concentration adsorption was maximum. With further increase in alkali concentration, the hydrophobicity effect starts decreasing and strong electrostatic repulsion between the mineral sites and IL molecules starts dominating the stability of the carbonate surface, thus reducing adsorption values 46. 22 ACS Paragon Plus Environment

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Conclusion This study aimed to further investigate the efficiency of imidazolium-based ILs as a surface active agents in altering wettability and reducing IL loss in carbonate reservoirs. Carbonate reservoirs due to its complex nature and heterogeneities, depends largely on its mineralogical attributes for recovery. XRD of the carbonate reservoir rock was used to determine the mineralogical effect and calcite was found to be the main mineral present in the rock sample. Further, SARA analysis of crude oil sample was performed to determine the heavier fractions present in the crude, which is considered as one of the important parameter which affects the wettability of carbonate surface. FTIR of dry carbonate rock, oil-wet carbonate rock and crude oil samples were performed which indicated that the polar components of crude oil are adsorbed onto the surface making it oil wet. The synthesized ILs make rock surfaces more water wet by reducing the contact angle and has a great potential as high-quality EOR agent. ILs were also effective in altering the wettability efficiently in the presence of salt and alkali. With increase in the IL concentration, the adsorption on the carbonate surface increased until the saturation point. To predict the saturation condition, suitable adsorption models were fitted. Based on the correlation coefficient R2, Langmuir model was considered to be appropriate for predicting the IL adsorption in comparison to Freundlich and Temkin model. Addition of alkali exhibited further reduction in adsorption as it tends to adsorb onto rock surface instead of IL. Thus from the above discussion it can be concluded that that ionic liquids can be used as an alternative for the surfactants in chemical EOR processes especially for the carbonate reservoir. Reference (1)

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B 12000

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Figure 4. HTGC of crude oil sample

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Figure 8. Schematic representation of wettability alteration of carbonate reservoir rock


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Figure 9. SEM microscopic images of (a) Dry carbonate rock, (b) Oil-aged carbonate rock and (c) IL-treated aged carbonate rock

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600

Time (Sec)

Figure 10. Effect of varying IL concentration on contact angle of (a) C8mimBF4, (b) C10mimBF4 and (c) C12mimBF4

35

C8mimBF4 C10mimBF4 C12mimBF4

30

25

Contact Angle ()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 41

20

15

10

5

0 0

1

2

3

4

5

6

Salt Concentration (%)

Figure 11. Effect of varying NaCl concentration on the contact angle of IL at CMC and time frame of 400sec


Page 37 of 41

C8mimBF4 C10mimBF4

25

C12mimBF4

Contact Angle (C)

20

15

10

5

0 8

9

10

11

pH

Figure 12. Effect of pH on contact angle of (a) C8mimBF4, (b) C10mimBF4 and (c) C12mimBF4 at CMC and 200s

C8mimBF4 C10mimBF4

30

C12mimBF4 Pure carbonate Crude oil aged carbonate

20

Zeta potential

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

10

0 2000 -10

4000

6000

8000

10000

12000

Concentration (ppm)

-20

Figure 13. Zeta potential of (a) pure carbonate rock sample, (b) crude oil and (c) carbonate sample with varying IL concentration 36 ACS Paragon Plus Environment

Energy & Fuels

5.0 4.5

Amount adsorbed (mM/g)

4.0 3.5 3.0 2.5 2.0 1.5

C mimBF 8 4

1.0

C C

0.5 0

2000

4000

6000

8000

10 12

mimBF mimBF

4 4

10000 12000 14000 16000

Concentration (ppm)

Figure 14. Effect of varying IL concentration on adsorption density 5.0

(a)

3.0

(b)

4.5 4.0

2.5

3.5

Amount adsorbed (mg/g)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 41

3.0 2.5 2.0

Experimental Data

1.5

Langmuir Isotherm 1.0

2.0

1.5

Experimental Data Langmuir Isotherm Freundlich Isotherm Temkin Isotherm

1.0

Freundlich Isotherm Temkin Isotherm

0.5 0

2000

4000

6000

8000

10000 12000 14000 16000

0.5 0

2000

IL Concentration (ppm)

4000

6000

8000

10000 12000 14000 16000



Page 39 of 41


1.0

(c)

0.9

0.8

0.7

Experimental Data Langmuir Isotherm Freundlich Isotherm Temkin Isotherm

0.6 0

1000

2000

3000

4000

5000

6000

7000


Figure 15. Adsorption Isotherms of experimental data’s of (a) C8mimBF4, (b) C10mimBF4 and (c) C12mimBF4 0% NaCl 1% NaCl 2% NaCl 3% NaCl

6

5

Adsorption (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

4

3

2

1

0 0

1000

2000

3000

4000


Figure 16. Effect of NaCl concentration on adsorption density with varying IL concentration of C12mimBF4


Energy & Fuels

1.0

1% Na2CO3 1.5% Na2CO3 2% Na2CO3

0.8

Adsorption (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 41

0.6

0.4

0.2

0.0 0

1000

2000

3000

4000

Concentration (ppm)

Figure 17. Effect of alkali concentration on adsorption density with varying IL concentration of C12mimBF4

Table i. SARA analysis of the crude oil sample Sl. No.

Parameters

(%)

1

Saturates

56.3

2

Aromatics

25.8

3

Resins

11.5

4

Asphaltenes

6.3

Acid Number (AN)

0.044 mg KOH/g

Table ii Calculated parameters for the three adsorption isotherms; Langmuir, Freundlich and Temkin


Page 41 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Models

Parameters qo (mg/g)

Langmuir

Freundlich

Temkin

C8mimBF4

C10mimBF4

C12mimBF4

5.39139

3.25798

0.98194

2.86466E-4

5.57644E-4

0.00379

R2

0.99393

0.86348

0.97103

RL

0.234026

0.286305

0.12134

Kf

0.06772

0.15733

0.29455

n

2.27233

3.20143

7.32939

R2

0.97603

0.69764

0.80261

B

1.13662

0.70892

0.11623

Kt

0.00323

0.00534

0.66813

R2

0.98602

0.80036

0.84255

Kad (L/mg)


Wettability Modification and Adsorption ... - ACS Publications

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