Article pubs.acs.org/EF
Experimental Investigation on the Effect of Asphaltene Types on the Interfacial Tension of CO2−Hydrocarbon Systems Ehsan Mahdavi,† Fatemeh Sadat Zebarjad,‡ Shahab Ayatollahi,*,† and Vahid Taghikhani†,§ †
Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran Harold Vance Department of Petroleum Engineering, Texas A&M University, College Station, Texas 77843, United States § Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, United States ‡
ABSTRACT: Interfacial tension (IFT) is known as the critical parameter affecting the efficiency of CO2 flooding during the enhanced oil recovery (EOR) process. Besides, the asphaltene precipitation phenomenon is reported as the most significant problem during CO2 injection into asphaltenic oil reservoirs. Accordingly, it is important to examine the effect of asphaltene precipitation on the IFT behavior of the oil−CO2 system at reservoir conditions. The main objective of this research work is to study of the effect of asphaltene and its type on the IFT behavior of the oil−CO2 system. The IFT between pure CO2 and a model oil both with and without asphaltene was measured using an axisymmetric drop shape analysis (ADSA) technique over a wide range of pressures and a constant temperature of 323 K. The asphaltene particles used for the work were precipitated and separated from three different crude oil samples, each having different physical properties. The model oil, consisted of 50 vol % nheptane and 50 vol % toluene (“heptol50”) and was doped with asphaltene particles at a concentration of 4 wt % to ensure that the particles remained suspended in the model oil over the range of pressures studied. The results showed that the IFT between CO2 and the model oil is inversely proportional to the pressure and that the constant proportionality is affected by the presence of asphaltene particles. In fact, the asphaltene aggregates formed in the model oil lead to a reduction in the magnitude of this constant. This, in turn, could result in lower CO2 solubility. Also, the effect of the asphaltene molecular structure on IFT of CO2−model oil was investigated in this work. It was shown that the hydrogen deficiency and aromaticity of asphaltene molecules were important parameters that could significantly affect the IFT for CO2−model oil.
1. INTRODUCTION The use of CO2 for tertiary or enhanced oil recovery (EOR) has increased and become the most common for conventional reservoirs when compared to other EOR techniques. Mitigation of the greenhouse gas emission and relatively lower cost and higher displacement efficiency are the main incentives to use CO2 for tertiary oil recovery.1−4 Miscible/immiscible CO2 flooding have been shown to be a viable EOR method that can be implemented to conventional oil reservoirs. Miscible CO2 injection, by and large, enhances oil recovery as a result of the oil swelling and a reduction in both oil viscosity and interfacial tension (IFT).5,6 However, CO2 injection may exacerbate the precipitation of asphaltene particles during oil production. Asphaltene precipitation has been considered one of the most significant problems that typically occurs in formation and surface facilities.7,8 It is well-known that asphaltene is a polar, polyaromatic molecule and defined as the heaviest fraction of the crude oil that is miscible in light aromatic hydrocarbons, such as toluene and xylene, but not soluble in paraffinic hydrocarbons, such as n-heptane and npentane.9,10 Asphaltenes consist mostly of condensed polynuclear aromatics, small amounts of heteroatoms, such as S, N, and O, and traces of metal elements, such as nickel and vanadium.11 When deposited within porous media, asphaltenes can reduce porosity and permeability, alter rock wettability by adsorbing onto the rock surfaces, and reduce well productivity and injectivity by blocking the pore throats.12,13 Moreover, asphaltene deposition can cause plugging of wellbores, flow lines, and processing equipment.14 © 2015 American Chemical Society
During all stages of oil production, the interfacial properties of reservoir fluids play a crucial role in oil recovery efficiency, affecting the displacement mechanisms, sweep efficiencies, capillary pressure, residual oil saturation, and distribution of fluids within porous media.15 The IFT behavior of multiphase systems is an important property that strongly affects the efficiency of EOR processes. In the CO2 EOR process, the IFT phenomenon under high pressure governs the fluid distribution, such as crude oil and CO2, as well as their flow behavior in porous media.15 Hence, it is of fundamental and practical importance to study the IFT behavior of an oil−CO2 system at high pressures and elevated temperatures. The IFT between CO2 and asphaltenic oil systems at various pressures has been reported in the literature.13,16,17 These studies revealed that the IFT varies with pressure for crude oil− CO2 systems at a constant temperature. It was also reported that the IFT for these systems can change with pressure linearly with multiple slopes at different pressure ranges.17 The change in the slope was explained by the fact that the concentration of the light components in the crude oil reduce, because they can be extracted by CO2, and this results in a change in the oil composition. While for another crude oil−CO2 system, a triple slope behavior has been reported, without any elucidation about the reason.13 Also, it was shown that the IFT between pure hydrocarbon, such as pure C7 or pure C16, and CO2 was Received: September 29, 2015 Revised: November 21, 2015 Published: November 24, 2015 7941
DOI: 10.1021/acs.energyfuels.5b02246 Energy Fuels 2015, 29, 7941−7947
Article
Energy & Fuels linearly reduced with pressure at a certain temperature, while the nonlinear changes for the IFT with pressure were observed in a crude oil−CO2 system.16,18,19 It should be mentioned that multi-slope linear variations for the IFT with pressure for some crude oil−CO2 and crude oil−N2 systems were reported.20−22 Researchers have investigated the IFT of water−synthetic oil and water−crude oil systems in the presence of asphaltenes,23−27 but no attempt has been made to elucidate the effect of different asphaltene types on the IFT behavior of an oil−CO2 system. It is worthwhile to note that there are few studies that investigate the effects of asphaltene structures on the mechanisms and efficiency of EOR methods.12 The main purpose of this study is to explore the effects of asphaltene and asphaltene types on the IFT behavior of CO2−synthetic oil systems. In this paper, a brief background on the asphaltene structure is introduced. Then, an axisymmetric drop shale analysis (ADSA) technique for the pendant drop case is used to measure the equilibrium IFT of the CO2−asphaltenic synthetic oil system at different pressures and the temperature of 323 K. The measured IFT data will provide a better understanding of the effect of asphaltene structures on the interfacial interactions among CO2 and oil under reservoir conditions.
Figure 1. Experimental device for density measurement: (1) mPDS 2000 V3, (2) DMA HPM, (3) interface module, (4) refrigerated heating/cooling bath, and (5) fluid piston.16
2. EXPERIMENTAL SECTION 2.1. Materials. Asphaltenes used in this study were extracted from three asphaltenic crude oil samples labeled types A, B, and C, which were collected from oil fields in the southwest of Iran. n-Heptane and toluene were purchased from Merck with a mole fraction purity of 0.99. The mole fraction purity of carbon dioxide used in this study was up to 0.99, and it was supplied by the Pars Balloon Company. The material and their purities are listed in Table 1.
2000 V3 to calculate fluid density. The setup must be accurately designed to minimize dead volume; also, no gas should be trapped in the apparatus. The setup works on the basis of the dependence of the period of oscillation of a constant-volume unilaterally fixed U-tube on its mass. The mass of the U-tube consists of both its body and the fluid that filled the U-tube. The apparatus can be used to measure the fluid density at oil reservoir conditions (temperatures up to 403 K and pressures up to 137.89 MPa). The apparatus should be initially calibrated by a series of standard gas and liquid samples in the specified pressure and temperature conditions. 2.5. Equilibrium IFT Measurement. In this study, the pendant drop technique was applied to determine the equilibrium IFT between the synthetic oil samples and CO2 at different pressures in the range of 1.72−6.89 MPa and the temperature of 323 K. A schematic diagram of the experimental setup is illustrated in Figure 2. This apparatus consists of a cell with side glass, which has a 20 cm3 inner volume to conduct IFT measurement. The maximum operating pressure and temperature of the cell are 70 MPa and 473 K, respectively. The cell is equipped with a high-quality image acquisition system and accurate image analysis module. Two piston−cylinder systems, which are connected to a pressure generator (HP), are used to introduce oil and gas to the cell. The pressure of the cell is monitored using a pressure transducer (Keller, model PA-33X, Winterthur, Switzerland) with an operating range up to 100 MPa. The temperatures of the cell and piston−cylinder systems are monitored through a temperature sensor (PT100) with an accuracy of 0.1 K. In this apparatus, a stainless-steel needle was installed at the top of the cell with the specification of 1.5 ± 0.001 mm outer diameter to form a pendant drop. The image acquisition system consists of a charge-coupled device (CCD) color camera (1.4 megapixel, macro zoom lens, and panel light) and a drop shape analysis module. The CCD color camera is used to capture the sequential digital images of the dynamic pendant oil drop every second, and meanwhile, the IFT is calculated using the drop shape analysis module. 2.6. Experimental Procedure. As mentioned earlier, heptol50 and three different asphaltene types were used for the preparation of three asphaltenic model oil samples. Prior to performing the IFT tests, the high-pressure IFT cell was cleaned using toluene, followed by deionized water. The cell was dried by purging high-grade nitrogen and was pressurized using CO2, which was injected into the cell using a syringe pump. A few drops of the model oil sample were injected into the pressurized cell, and sufficient time was given for the system to
Table 1. Specification of Chemical Samples
a
chemical name
source
mass fraction purity
analysis methoda
n-heptane toluene carbon dioxide
Merck Merck Pars Balloon Co.
>0.99 >0.99 ≥0.99
GC GC
GC = gas chromatography.
2.2. Asphaltene Extraction. Asphaltenes were extracted from three different crude oil samples using the IP 143/9028 method. In the first step, n-heptane was added to the oil sample at a 40:1 volume ratio of n-heptane/oil sample and the mixture was stirred gently for 6 h. Afterward, the resulting mixture was allowed to settle overnight, and then the supernatant fluid was removed. The precipitant was filtered through a 2 μm Whatman filter paper. Thereafter, the remaining precipitant was washed with n-heptane using Soxhlet until the liquid (n-heptane) flowing through was colorless. 2.3. Sample Preparation. In this study, the model oil consisted of a 1:1 (v/v) mixture of n-heptane and toluene (named “heptol50”) and was used as the synthetic oil sample. Heptol as a mixture of solvent and anti-solvent of asphaltene is usually used to mimic the crude oil samples and to study the asphaltene precipitation phenomenon, because the asphaltene behavior depends upon the interaction between asphaltene, solvents, and anti-solvents, which exist in the oil sample.29,30 In the next step, 4 wt % of the three different asphaltene types were mixed with heptol50 by slow rotation mixing for 18 h to make homogeneous asphaltenic synthetic oil samples. 2.4. Density Measurement. The density of CO2 and synthetic oil samples at different pressures and the temperature of 323 K was measured using a DMA HPM apparatus (Anton-Paar, Austria). Figure 1 shows a simple schematic diagram of this apparatus. The interface module generates and measures the period of oscillation and cell temperature, then the data are transferred to the evaluation unit mPDS 7942
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possibility of attractive interplate interaction when the plates are in close proximity and parallel to each other. A larger plate size leads to greater attraction between asphaltene molecules.12 Thus, it is reasonable that the mechanisms of asphaltene precipitation can be better understood in terms of the molecular structure; albeit, the molecular approach suffers from the amorphous aggregation of asphaltene particles. The available procedures to characterize the asphaltene structure seem not to be very accurate; however, they give an approximation of the chemical structure, which could be helpful in describing the asphaltene precipitation, aggregation, and flocculation.12 2.8. Asphaltene Structure Analysis. The method to determine the asphaltene molecular structure is explained below, and additional details of the procedure can be found elsewhere.32−34 Elemental analysis was performed using the CHNSO analyzer (Thermo Flash EA 1112 series), which determines the mass ratios of C, H, N, S, and O atoms present in the asphaltene samples as well as the mass ratios of vanadium (V) and nickel (Ni). The results of elemental analysis of the three asphaltene types are shown in Table 2. In addition, the Fourier transform infrared (FTIR) spectra (Shimadzu model 8300) were used to determine presence of specific moieties. The presence of aromatic and aliphatic hydrocarbons can be determined from the peak positions at 3070, 2920, and 2850 cm−1, and the peak positions at 1037 and 1325 cm−1 are an indication of S−O and C−N vibrational stretching groups, respectively. Carbon nuclear magnetic resonance (13C NMR) and proton nuclear magnetic resonance (1H NMR) spectra were prepared in deuterated chloroform (CD3Cl). Also, the ratios of hydrogen and carbon atoms each contributing to aliphatic and aromatic parts of the asphaltene molecule were determined using 13C NMR spectra. The results of such a calculation can be used to estimate the number of aromatic rings and number of peripheral aliphatic chains to predict the approximate chemical formula for each asphaltene sample. Figure 3 shows the characterized structure of the three asphaltene samples studied in this work.
Figure 2. Schematic of the experimental apparatus used to measure IFT: (1) pressure generator, (2) bulk tank, (3) drop tank, (4) view cell, and (5) panel light.16 reach equilibrium conditions. Thereafter, the oil sample was injected into the cell from the oil cylinder to form a drop at the needle tip. The drop images were recorded by the image acquisition system, and then the IFT between the two fluids (gas and liquid) was determined using image analysis software at every second. It should be stated that the densities of the two phases were needed to be measured as the input of the IFT analyzing software at the test condition. As mentioned, the densities of CO2 and oil samples were already measured using the DMA HPM apparatus at required pressures and a temperature of 323 K. Each test was repeated at least 3 different times to ensure repeatability of the tests, and the results reported for IFT are the average of the three replicates. It is also worthwhile to note that the dynamic behavior of the IFT for the CO2−oil system reaches the equilibrium value after about 600 s.31 In the recent published works by our group, the accuracy and validation of the data obtained by the setup were examined successfully compared to the literature data.16,18,21,31 2.7. Asphaltene Structure. Crude oils are hydrocarbon mixtures with a complex structure, consisting of polyaromatic fused rings and short side chains, known as asphaltenes. The asphaltene molecular structures used in this study consisted of six-membered aromatic rings fused together to form a rather large graphite-like plate that several alkyl chains are attached to the molecules.11 Hence, carbon and hydrogen are the major elements in the asphaltene molecule. Each asphaltene molecule contains two or three plates, which are connected by alkyl chains. In addition, other elements were detected, such as nitrogen, oxygen, and sulfur, which appear as heteroatoms on the aromatic rings or the alkyl chains. Traces of metals, such as nickel and vanadium, were detected in the extracted asphaltene molecules precipitated from crude oil samples used in the work.12 The fused ring system provides the possibility of overlapping double-bond electrons. The electrons of the double-bond system can resonate, in a concerted way, over aromatic carbon atoms, which is responsible for the high stability of the plate. On the other hand, it provides the
3. RESULTS AND DISCUSSION In this study, heptol50 (50 vol % n-heptane + 50 vol % toluene) and 4 wt % of three different asphaltene types were mixed to prepare synthetic oil samples. Afterward, densities of each synthetic oil sample and CO2 were measured using the AntonPaar apparatus over the pressure range from 1 to 12 MPa and the temperature of 323 K. The equilibrium IFTs between different synthetic oil samples and CO2 were determined at different pressures over the range from 1.72 to 6.89 MPa and at a constant temperature of 323 K using the ADSA technique. Figure 4 compares the density of the model oil samples as a function of the pressure both with and without asphaltene at a constant temperature of 323 K. Note that the density for each sample is slightly increasing with pressure. Also, it observes that the density of model oil is higher when asphaltene type A was added and is also higher than the model oil containing asphaltene types B and C over the entire range of pressures studied. It should be noted that the values for the densities are needed to measure the IFT using the ADSA technique. Figure 5 presents the IFT measured between pure CO2 and the four model oils (heptol50 alone and heptol50 containing asphaltene types A, B, and C) over a range of pressures at a constant temperature. The results show a linear relation between IFT and pressure in all of the pressure ranges for
Table 2. Results of Elemental Analysis for the Three Asphaltene Types12 asphaltene sample
C
H
N
S
O
Ni (ppm)
V (ppm)
H/C
A B C
80.36 74.56 79.84
7.19 6.74 7.71
0.98 0.85 1.50
8.90 6.84 6.09
2.57 10.99 4.84
0.277 0.626 0.166
0.189 0.219 1.052
1.074 1.085 1.159
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Figure 3. Proposed asphaltene structures for three asphaltene types.12
carbons.16,18 As pressure increases, the solubility of CO2 in heptol50 increases and results in a linear decrease in the IFT.16,18,31 This trend can be observed for the model oils containing asphaltene particles up to a certain pressure, whereupon the IFT continues to decrease linearly with an increasing pressure but with a different slope. This was noted for all three model oils containing asphaltene, although the change in slope occurs at a different pressure for each. It is known that asphaltene molecules are randomly suspended in the oil and form a homogeneous solution at low pressures. At higher pressures, the concentration of gas molecules increases; consequently, asphaltene molecules flocculate as a result of the reciprocal effect of pressure and oil composition. The interfacial activity of colloidal solutions is usually increased as a result of the tendency of the particles to accumulate at the interfaces. This accumulation may occur at oil−gas, oil−water, and oil− rock interfaces, and it leads to interfacial behavior alteration, mainly including IFT and wettability alteration. It has been demonstrated that the IFT of fluid systems containing colloidal particles increases as the concentration of the particles at the interface of the two fluids increases.35,36 Also, it was shown that the increasing trend of the IFT is critically amplified when the surface coverage of the particles becomes higher than 60%. To elucidate the mechanism more, asphaltene forms two-dimensional thin islands or nets at the interface at low concentrations (low surface coverage) and the structure transmutes to threedimensional arrangements at higher concentrations.37 Accordingly, it is concluded that the tendency of the colloidal asphaltene particles to accumulate at the oil−gas interface critically affects the double decline rate behavior of the IFT− pressure curves. In consequence of asphaltene agglomeration, the semi-solid three-dimensional asphaltene islands are formed at the oil−CO2 interface and modify the IFT−pressure trend by compensation of the effect of CO2 solubility. In addition, the reason for this behavior could be plausibly justified by the fact that the formation of the asphaltene islands at the interface of oil−CO2 can affect the solubility of CO2 in the model oil. In other word, at high pressure, the formation of a threedimensional asphaltene film at the interface reduces the CO2 dissolution; however, the result shows that the effect of CO2
Figure 4. Density of oil samples at different pressures: (red ●) oil A, (yellow ■) oil B, (blue ◆) oil C, and (green ▲) heptol50.
Figure 5. IFT between CO2 and model oil samples: (●) oil A, (■) oil B, (◆) oil C, and (▲) heptol50.
the heptol50−CO2 system. The same linear behavior has already been reported in the literature for pure hydro7944
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was recently investigated.40,41 A visual setup was used to study the potential of asphaltene precipitation during CO2 injection. The results showed that the potential of asphaltene precipitation for asphaltene type A was about 3 times higher than that for asphaltene type C.40 Figure 6 shows the
solubility on IFT is still more pronounced than surface coverage of asphaltene. Therefore, as the pressure increases in the second pressure region, the IFT reduces with a smaller slope (lower rate) compared to the first stage. It is noteworthy to point out that the slope of the IFT−pressure curves is definitely affected by component extraction and composition change; however, other mechanisms, such as asphaltene precipitation, may be prevailing in the case of asphaltenic oils. On the basis of these explanations, the slope change is the result of asphaltene instability or precipitation and accumulation of the asphaltene particles at the oil−gas interface. At this condition, the slope is reduced, which depends upon the tendency of such molecules to accumulate at the gas−oil interface, their structure at the interface, and the intensity of precipitations. Simple linear regression was used to model the IFT measurements as a function of the pressure by considering the two regions separately, before and after the change in slope. Two regions were analyzed for the data for each model oil divided for the three model oils containing asphaltene for the region before (region I) and after (region II) the change in slope. The results of the simple linear regression for the model oil samples are reported in Table 3, where it is noted that the
Figure 6. Deposited asphaltenes in the model oil samples A and C during CO2 injection at a pressure of 6 MPa and temperature of 363 K.40
deposition of asphaltene aggregates for the model oils of types A and C during CO2 injection at the pressure of 6 MPa and T = 363 K.40 It was also reported that the mean diameter of deposited asphaltene particles of asphaltene type A was larger than that of asphaltene type C.40 The same comparison was made between asphaltene types A and B. This observation confirmed the flocculation tendency for asphaltene type A.41 Therefore, the reason for the IFT changes with pressure with least steepness for the model oil and asphaltene type A can be somehow related to the potential of asphaltene precipitation of this asphaltene type.
Table 3. IFT Measurement Correlations for CO2−Model Oil Systems at Different Pressures sample
region
pressure (MPa)
equation
R2
heptol50
I
1.723−6.893
IFT = − 2.056P + 17.490
0.998
A
I
1.723−4.136
IFT = − 2.279P + 18.552
0.996
A
II
4.480−5.859
IFT = − 0.908P + 12.741
0.969
B
I
1.723−5.170
IFT = − 2.064P + 17.795
0.998
B
II
5.514−6.548
IFT = − 0.926P + 11.270
0.999
C
I
1.723−5.514
IFT = − 2.024P + 17.516
0.999
C
II
5.859−6.893
IFT = − 0.920P + 10.712
0.998
4. CONCLUSION The IFT between pure CO2 and different model oils, such as heptol50, with and without asphaltene has been measured using the ADSA technique at various pressures and a constant temperature. The concentration of asphaltene in the model oil was considered to be 4%. The experimental results showed that the IFT behavior of the heptol50−CO2 system decreases linearly with the pressure in all pressure ranges, while IFT values measured here showed double slope behavior for the asphaltenic oil samples. It was observed that, over the first pressure range, IFT values and the slope of the IFT−pressure curve for non-asphaltenic and three asphaltenic model oil samples are very close to each other and asphaltene and heavy oil components do not play a significant role in this region, where the IFT of the system is mostly controlled by light components. Moreover, it was concluded that the trend of the IFT−pressure curve is affected by asphaltene accumulation at the oil−CO2 interface. As the pressure increases, asphaltene particles aggregate and become larger as a result of mutual effects of pressure and composition. This, in turn, can affect the solubility of the gas phase in the oil. On the other side, the observations showed that, over the second pressure range, the IFT behavior of asphaltenic oil−CO2 systems is strongly affected by asphaltene type. It was shown that the hydrogen deficiency and aromaticity of asphaltene molecules and, as a result, the potential of precipitation are important parameters, which can significantly affect the IFT for CO2−model oils.
slope and intercept for the four model oils, heptol50 alone and heptol50 with three types of asphaltene, are very similar in region I, where the pressures are lower. As collaborated in our previous works,16,18 this behavior is consistent with the fact that accumulation of the paraffinic group (n-heptane in this case) at the interface of CO2−crude oil systems controls the IFT behavior at the first pressure range. On the other side, the results revealed that, in the first pressure range, asphaltene and heavy components do not play a significant role, and in this region, the IFT of the system is mostly controlled by the light components of the samples (n-heptane and toluene). Hence, the slope of the first region does not changed significantly by adding asphaltene to the sample. As inferred from the results, for the model oil with asphaltene type A, slope change of IFT occurs at the lower pressure compared to the other types. This observation can be explained by the difference between the structural parameters, such as the hydrogen deficiency (H/C) and the aromaticity for different asphaltene types. The asphaltene structures and elemental analysis show that molecules of asphaltene type A have lower hydrogen deficiency in comparison to asphaltene types B and C. Asphaltenes with lower hydrogen deficiency have higher aromaticity.11,38 Also, the higher aromaticity of the asphaltene structure leads to lower stability of asphaltene.11,39 The potential of asphaltene precipitation for the three asphaltene types in the model oils 7945
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AUTHOR INFORMATION
Corresponding Author
*Telephone/Fax: +98-21-66166411. E-mail: shahab@sharif. edu. Notes
The authors declare no competing financial interest.
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NOMENCLATURE EOR = enhanced oil recovery IFT = interfacial tension ADSA = axisymmetric drop shape analysis
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REFERENCES
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DOI: 10.1021/acs.energyfuels.5b02246 Energy Fuels 2015, 29, 7941−7947
