CO2-Miscible Flooding for Three Saskatchewan Crude Oils

Energy & Fuels 2004, 18, 743-754

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CO2-Miscible Flooding for Three Saskatchewan Crude Oils: Interrelationships between Asphaltene Precipitation Inhibitor Effectiveness, Asphaltenes Characteristics, and Precipitation Behavior Hussam H. Ibrahim and Raphael O. Idem* Process and Petroleum Systems Engineering Laboratory, Faculty of Engineering, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan, Canada S4S 0A2 Received August 21, 2003. Revised Manuscript Received February 3, 2004

Studies were conducted to determine the asphaltene precipitation inhibition effectiveness of three carefully chosen chemicals (dodecylbenzenesulfonic acid (DDBSA), nonyl phenol (NP), and toluene) during CO2 flooding of three Saskatchewan crude oils, as well as to evaluate the interrelationships between the chemicals’ inhibition effectiveness, crude oil/asphaltenes characteristics, and asphaltene precipitation behavior (in terms of kinetic and equilibrium parameters). Results showed that both the asphaltene precipitation rate dependence on asphaltene content and apparent rate constant for asphaltene precipitation were strong functions of the paraffin fraction of the asphaltenes and the propensity of the asphaltene molecules for aggregation. On the other hand, the precipitation rate dependence on the amount of CO2 added was a strong function of the heteroatoms (nitrogen, sulfur, and oxygen) content of the oil and asphaltenes, the aromatic carbon fraction, and the degree of branching of the asphaltene molecules. The equilibrium parameter (onset point) increased with the paraffin fraction of the asphaltene molecule but decreased with the propensity of the asphaltene molecule for aggregation. In terms of kinetic parameters, NP with the -OH functional group in its molecule was most effective with the morearomatic (and more-substituted and more-polycondensed) shorter-alkyl-chain-length oil, whereas toluene (the most-aromatic additive) was most effective with the least-aromatic oil. In terms of the onset point, all three chemical additives showed maximum effectiveness with the least-stable oil that had the lowest metal content, and in the asphaltenes molecules that had the lowest paraffin fraction, highest degree of condensation, highest aromatic carbon fraction, and highest propensity for aggregation.

1. Introduction During tertiary oil production, many light and medium reservoirs are subjected to miscible or nearmiscible CO2 or hydrocarbon flooding for enhanced oil recovery after the initial water flooding. For example, 60 active miscible CO2 projects were in operation in the United States in 1996, whereas in Canada, hydrocarbon miscible floods are more common and there are ∼40 such active projects.1 In Saskatchewan, Canada, most of the light-oil reservoirs have reached their economic limits of production by water flooding2 and have thus become suitable candidates for miscible/near-miscible CO2 flooding,3 which currently is the most popular method of improved oil recovery.4 The attractive features of CO2 that make it effective in displacing oil in subsurface porous rocks include swelling effects, a * Author to whom correspondence should be addressed. E-mail address: [email protected]. (1) Moritis, G. OGJ Special EOR Survey and Analysis. New Technology, Improved Economics Boost EOR Hopes. Oil Gas J. 1996, 94 (16), 39. (2) Saskatchewan Energy and Mines Reservoir Annual Report, Regina, Saskatchewan, Canada, 1993. (3) Huang, S. S.; Dyer, S. B. Miscible Displacement in the Weyburn Reservoir. A Laboratory Study. J. Can. Pet. Technol. 1993, 32, 42.

reduction in oil viscosity, and its high solubility in water. On the other hand, the flooding process causes several changes in the flow and phase behavior of the reservoir fluids and can significantly alter formation properties that favor precipitation of organic solids, mainly asphaltenes.5 Asphaltene precipitation can change the wettability of the reservoir matrix and, consequently, affect the flood performance.6 It can also cause formation damage and wellbore plugging, which requires expensive treatment and clean-up procedures.7-11 (4) Yin, Y. R.; Yen, A. T. Asphaltene Deposition and Chemical Control in CO2 Floods. Presented at the 2000 SPE/DOE Improved Oil Recovery Symposium, Tulsa, OK, April 3-5, 2000, Paper No. SPE 59293. (5) Kokal, S. L.; Sayegh, S. G. Asphaltenes: The Cholesterol of Petroleum. Presented at the Middle East Oil Show, Bahrain, March 11-14, 1995, Paper No. SPE 29787. (6) Buckley, J. S. Asphaltene Precipitation and Crude Oil Wetting. In SPE Advances in Technology Series 53; SPE J, March 1995, 3 (1), 53-59. (7) Barker, K. M.; Germer, J. W.; Lesile, M. P. Removal and Inhibition of Asphaltene Deposition on Formation Minerals. Presented at the SPE International Petroleum Conference and Exhibition of Mexico, Villahermosa, Tabasco, Mexico, March 5-7, 1996, Paper No. SPE 35342. (8) Kamath, V. A.; Yang, J.; Sharma, G. D. Effect of Asphaltene Deposition on Dynamic Displacements of Oil by Water. Presented at the SPE Western Regional Meeting, Anchorage, AK, May 26-28, 1993, Paper No. SPE 26046,

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Generally, asphaltene deposition can be controlled using predictive, corrective, or preventive methods. In the predictive method, a combination of thermodynamic and kinetic models (which are mostly derived from the polymer solution, colloidal stability, and fractal aggregation theories) are used to predict the composition, pressure, and temperature conditions for the onset of asphaltene precipitation after the introduction of experimentally determined oil characteristics (typically fluid composition and physicochemical properties) data. It has been noticed that purely thermodynamic predictive models are not sufficient in real reservoir simulations.12 This could be attributed to the fact that the models are expected to consider the chemical and structural nature of the asphaltenes oil system; however, the considerable effort that has been put into their characterization13-16 has shown that they are not unique.17 In the case of corrective methods, actions usually taken include the removal of precipitated and deposited asphaltenes with mechanical or chemical treatments that involve deposit dissolution using aromatic solvents.18 These methods and their combinations are used when the problem is already present and important production losses have already occurred. According to von Albretch,19 the most effective asphaltene-precipitation preventive action is reservoir pressure maintenance above the asphaltene precipitation onset. In many cases, however, this condition is quite difficult or even impossible to accomplish, because of reservoir depletion and/or inconvenience in gas injection. In these cases, asphaltene dispersants and precipitation inhibitors constitute a good alternative.20-22 These products can keep the small initially formed (9) Novosad, Z.; Costain, T. G. Experimental and Modeling Studies of Asphaltene Equilibria for a Reservoir Under CO2 Injection. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, LA, Sept. 23-26, 1990, Paper No. SPE 20530. (10) Leontaritis, K. J.; Mansoori, G. A. Fast Crude-Oil HeavyComponent Characterization Using Combination of ASTM, HPLC, and CPC Methods. J. Pet. Sci. Eng. 1989, 2, 1-12. (11) Leontaritis, K. J.; Amaefule, J. O.; Charles, R. E. A Systematic Approach for the Prevention and Treatment of Formation Damage Caused by Asphaltene Deposition. Presented at the Symposium on Formation Damage Control, Lafayette, LA, February 26-27, 1992, Paper No. SPE 23810. (12) Wang, J. X.; Brower, K. R.; Buckley, J. S. Presented at the SPE International Symposium on Oilfield Chemistry, Houston, TX, February 16-19, 1999, Paper No. SPE 50745. (13) Escobedo, J.; Mansoori, G. A. Viscometric Determination of the Onset of Asphaltene Flocculation. A Novel Methodology. SPE Prod. Facil. 1995, (May), 115. (14) Mansoori, G. A. Modeling of Asphaltene and Other Heavy Organic Depositions. J. Pet. Sci. Eng. 1997, 17, 101. (15) Pacheco-Sanchez, J. H.; Mansoori, G. A. Prediction of the Phase Behavior of Asphaltene Micelle/Aromatic Hydrocarbon Systems. J. Pet. Sci. Technol. 1998, 16 (3&4), 377. (16) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Structural Characterization of Asphaltenes of Different Origins. Energy Fuels 1995, 9, 225. (17) Sheu, E. Y. Petroleum AsphaltenesProperties, Characterization, and Issues. Energy Fuels 2002, 16, 74. (18) PTAC Oil Production Technical Subcommittee. Potential Solutions for Top Four Problems/Opportunities. In Petroleum Technology Alliance Canada (PTAC) Workshop Notes; PTAC: Calgary, Alberta, Canada, April 21, 1997; pp 10-13. (19) von Albretch, C.; Diaz, B.; Salathiel, W. M.; Nierode, D. E. Stimulation of Asphaltic Deep Wells and Shallow Wells in Lake Maracaibo, Venezuela. Proc. 10th Pet. Congr. 1980, 3, 55. (20) Broaddus, G. Well- and Formation-Damage Removal with Nonacid Fluids. J. Pet. Technol. 1988, 40 (6), 685. (21) Carbognani, L.; Espidel, J.; Izquierdo, A. Characterization of Asphaltenic Deposits from Oil Productions and Transportation Operations. In Asphaltenes and Asphalts 2. Development in Petroleum Science; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier Science B.V.: Amsterdam, 2000; p 335.

Ibrahim and Idem

asphaltene particles suspended into the oil matrix (in the case of dispersants) or prevent their formation (when inhibitors are used). These methods can be more effective and cost reductive, because plugging and production losses are prevented. Extensive research has been done to obtain insights in inhibition mechanisms and the correlations between crude oil composition, the structural characteristics of asphaltenes,17,23 and the effectiveness of chemical additives as asphaltene precipitation inhibitors.24,25 However, no studies that investigate possible interrelationships between these parameters (as they affect CO2 flooding) have been reported in the literature. Also, the inhibition or dispersion effectiveness of a chemical additive is typically measured in terms of a delay in the onset point of asphaltene precipitation (i.e., the equilibrium parameter).13 However, because asphaltene precipitation can be considered to be a rate process,26-30 the effectiveness of a chemical additive could also be evaluated in terms of the kinetic parameters. Therefore, it would be essential as well as informative to directly correlate the effectiveness of chemical additives with the overall asphaltene precipitation behavior involving both the equilibrium and the kinetics. At the present time, very limited kinetic evaluation has been performed for asphaltene precipitation during CO2 flooding. Some of the reported kinetics of precipitation have focused on studying the critical micelle concentration (CMC) as a function of concentration and time, using asphaltenes samples that were redissolved in toluene and/or heptane.26,27 The kinetics of asphaltenes dissolution has also been the subject of more-recent studies.28,29 However, the kinetics of CO2-induced asphaltenes precipitation from actual crude oil samples has not been reported previously in the open literature, to the best of our knowledge, possibly because of the difficulty in using the conventional techniques to evaluate the kinetics. In a recent study,30 we developed a technique based on a molar programmed titration of the oil with CO2 to simulate CO2 flooding to evaluate the kinetics of CO2-induced precipitation of asphaltene from three Saskatchewan crude oilssnamely, a light oil (L-O) and (22) Acosta, A. Efecto de las resinas en la deposicion de asfaltenos, Trabajo especial de Grada. Escuela de Ingenieriay Ciencias Aplicadas. University de Oriente, 1981. (23) Yen, T. F. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1979, 24, 901. (24) Gonzalez, G.; Middea, A. Peptization of Asphaltene by Various Oil Soluble Amphiphiles. Colloids Surf. 1991, 52, 207. (25) Chang, C.-L.; Fogler, H. S. Stabilization of Asphaltenes in Aliphatic Solvents Using Alkylbenzene-Derived Amphiphiles. 1. Effect of the Chemical Structure of Amphiphiles on Asphaltene Stabilization. Langmuir 1994, 10, 1749. (26) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura, M. Structure and Reactivity of Petroleum-Derived Asphaltene. Energy Fuels 1999, 13, 287. (27) Priyanto, S.; Mansoori, G. A.; Suwono, A. Structure and Properties of Micelles and Micelle Coacervates of Asphaltene Macromolecule. Prepared for Presentation at the 2001 AIChE Annual Meeting, Session 90sNanotools. (28) Permsukarome, P.; Chang, C.; Foggler, H. S. Kinetic Study of Asphaltene Dissolution in Amphiphile/Alkane Solutions. Ind. Eng. Chem. Res. 1997, 36, 3960. (29) Kaminski, T. J.; Fogler, H. S.; Wolf, N.; Wattana, P.; Mairal, A. Classification of Asphaltenes via Fractionation and the Effect of Heteroatom Content on Dissolution Kinetics. Energy Fuels 2000, 14, 25. (30) Idem, R. O.; Ibrahim, H. H. Kinetics of CO2-Induced Asphaltene Precipitation from Various Saskatchewan Crude Oils during CO2 Miscible Flooding. J. Pet. Sci. Eng. 2002, 35, 233-246.

CO2-Miscible Flooding for Saskatchewan Crude Oils

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length of the alkyl side chain, for these chemicals, increases in the following order: toluene < NP < DDBSA. Generally, toluene and other aromatic-based solvents concentrate in the micellar shell, which leads to a decrease in the interfacial tension between the asphaltene micellar core and shell, and the micelle becomes more stable. On the other hand, oil-soluble amphiphiles such as DDBSA and NP behave in a manner similar to that of resin species of the crude and co-adsorb onto the micellar core with resins. The adsorption enthalpy of an amphiphile onto the micellar core is much higher than that of the resin, thus accounting for the effectiveness of amphiphiles as inhibitors.31 The results for the inhibition performance of these chemicals during CO2 flooding are presented and discussed in this paper. 2. Experimental Section Figure 1. Chemical structure for toluene, dodecylbenzenesulfonic acid (DDBSA), and nonyl phenol (NP).

two medium oils (M1-O and M2-O)sin which the kinetic data were fitted to an empirical power-law model. This model was of the form

rA )

β dNAp n ) kN AmoN CO 2 Mo dNCO2

(1)

where rA is the rate of CO2-induced precipitation of asphaltenes from crude oil (in units of moles of asphaltene per mole of crude oil, per minute), β is the molar programmed rate of titration or addition of CO2 (β ) dNCO2/dt), dNAp/dNCO2 is the asphaltene precipitation as a function of CO2 added, k is the apparent rate constant (in units of moles of asphaltene per mole of oil, per minute), NAo represents the moles of asphaltene in the oil at any time, NCO2 represents the moles of CO2 in the oil at any time, Mo represents the moles of crude oil charged to the sample cell, m is the rate dependence of asphaltene precipitation on asphaltene content in the oil, and n is the rate dependence of asphaltene precipitation on CO2 content in the oil (introduced during flooding). This work enabled us to determine asphaltene precipitation behavior in terms of the equilibrium (i.e., the onset point of precipitation) and kinetic (i.e., rA, k, m, and n) parameters of the tertiary oil production process that involves CO2 flooding. In this paper, we have used this technique to evaluate three carefully chosen chemicals as asphaltene precipitation inhibition or stabilizing additives, based on the interrelationships between oil and asphaltenes characteristics, CO2-induced asphaltene precipitation behavior, and inhibitor or stabilizer effectiveness during CO2miscible flooding for three Saskatchewan crude oils to better understand the inhibition mechanism involved in CO2-induced asphaltene precipitation. The chemicals are toluene, dodecylbenzenesulfonic acid (DDBSA), and nonyl phenol (NP), and their chemical structures are given in Figure 1, which shows that all three chemicals contain the benzene ring. However, although toluene does not contain any other functional group attached to the benzene ring, DDBSA and NP have a sulfonic acid (SO2-OH) group and a hydroxyl (OH) group, respectively, attached to the benzene ring. Also, the

2.1. Crude Oil Samples and Their Corresponding Asphaltenes. Three crude oil samplessone light oil (L-O) and two medium oils (M1-O and M2-O)sfrom three different reservoirs were used for this study. Their identities have been described in detail in our earlier work.30,32 Asphaltene samples were extracted from each oil sample, and the extracted samples were then thoroughly characterized. Details on the extraction procedure have been given elsewhere.32 Briefly, the procedure was as follows. Ten grams of the oil sample was weighed to the nearest 0.001 g while being rigorously stirred at room temperature. The precipitating agent (n-heptane, 400 mL) then was added in a gradual fashion, to ensure complete mixing between the oil sample and n-heptane (to avoid the formation of lumps). After mixing for 1 h, the precipitate was allowed to form overnight under an argon-gas blanket. Asphaltenes, which separated as brown/ dark-brown granular solids, were isolated by filtration, using a membrane filter (Millipore) with a pore size of 0.45 µm. The flask and the filter cake were washed thoroughly with several small volumes of n-heptane, until the washings were colorless; this washing was done to ensure that all dissolved oil was transferred through the filter. In the case of characterization, all the crude oils and their n-heptane-derived asphaltenes were characterized using a multiple-technique approach including carbon, hydrogen, nitrogen, sulfur, and oxygen (i.e., CHNSO) analysis, measurement of the metal content and average molecular weight, Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (1H NMR), carbon nuclear magnetic resonance (13C NMR), and gated spin-echo (GASPE) analysis, as we described in our previous work.32 2.2. Inhibitors and Stabilizers. Two non-aromatic-based surfactants or inhibitorssnamely, dodecylbenzenesulfonic acid (DDBSA) and nonyl phenol (NP)s in addition to a well-known industry-used aromatic-based solvent (toluene, which was used as a reference inhibitor) were evaluated for their ability to peptize (i.e., stabilize) asphaltenes in the asphaltene-oil system. All the chemicals are commercially available and they were used as-received. The selection of these chemicals was based on four different criteria: (i) environmentally friendly inhibitors (DDBSA and NP) versus nonenvironmentally friendly solvent (toluene), (ii) functional groups (SO2-OH in DDBSA, -OH in NP, and the benzene ring in toluene), (iii) length of the alkyl side chain, and (iv) polarity (DDBSA > NP > (31) Pan, H.; Firoozabadi, A. Thermodynamic Micellization Model for Asphaltene Precipitation Inhibition. Presented at the Third International Symposium on the Thermodynamics of Heavy Oils and Asphaltenes, AIChE Spring Conference, New Orleans, LA, March 1518, 1999. (32) Ibrahim, H. H.; Idem, R. O. Structural and Molecular Characteristics of Asphaltenes from Various Saskatchewan Crude Oils. submitted to Energy Fuels, 2003.

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toluene). According to the literature,33,34 a typical range of concentrations of inhibitors used is 500-3000 ppm. We decided to use 1000 ppm for all of our runs. 2.3. Effectiveness of Additives as Asphaltene Precipitation Inhibitors or Stabilizers during CO2-Miscible Flooding. The effectiveness of the chemicals as inhibitors or stabilizers for asphaltene precipitation was evaluated by comparing the asphaltene precipitation behavior during CO2miscible flooding for the three oils with and without chemical additives or inhibitors. 2.3.1. Equipment. Asphaltene precipitation behavior during CO2-miscible flooding was evaluated under isothermal (304, 313, and 338 K) and isobaric (17.2 MPa) conditions for both noninhibited and inhibited crude oils for the three crude oils. CO2-miscible flooding was simulated using a molar CO2 programmed titration technique30 in a solids detection system (SDS) obtained from DB Robinson & Associates, Ltd. (Edmonton, Canada). This equipment consisted of a mercury-free, variable-volume, fully visual, JEFRI pressure-volume-temperature (PVT) cell retrofit with fiber-optic light-transmission probes (source and detector). The SDS enabled us to conduct experiments using crude oil samples from the field, rather than pre-precipitated and toluene-redissolved asphaltenes samples that are usually used in most reported experiments to circumvent the problem of opaqueness of the oil samples.7,33 2.3.2. Procedure for Nonstabilized Oil (i.e., without Inhibitor). A known quantity of a crude oil sample was fed into the PVT cell from a high-pressure cylinder using a JEFRI displacement pump (JDP). When the required amount of sample (0.069 mol for L-O, 0.064 mol for M1-O, and 0.67 mol for M2O) was in place, the sample cell was then completely isolated. The CO2 pressure was increased to 17.2 MPa, and the remaining oil in the tubing was accounted for as dead volume. The oil sample and the CO2 were then respectively allowed to equilibrate overnight for ∼15 h at the desired pressure and temperature in the PVT cell and the 1000-mL high-pressure solvent transfer cylinder. Before introducing CO2 from the solvent cylinder to the PVT (sample) cell, pressure in the lines connecting the CO2 cylinder to the sample cell were equilibrated, to avoid any backflash. The system was then opened to the back-pressure regulator (BPR), to maintain the desired constant pressure needed for the duration of the experiment. The SDS operates by transmitting a laser beam through the sample in the PVT sample cell; the beam transmittance was recorded as an output power signal, the variation of which was inversely proportional to the solids content of the sample cell. 2.3.3. Procedure for Stabilized Oil (i.e., with Inhibitor). For the case of inhibited oil, a known amount of the inhibitor (1000 ppm) was added to the crude oil under ambient conditions (0.1 MPa, 293 K). The resulting solution was then stirred vigorously, using a magnetic stirrer at 1200 ppm, to ensure that all the inhibitor dissolved in the oil under an argon blanket. A test sample was charged to the high-pressure transfer cylinder, from which the sample was fed into the PVT cell using a JDP. Other than this prior addition of the inhibitor to the crude oil, the procedure to evaluate the asphaltene precipitation behavior for inhibited oil was the same as that already described for nonstabilized oil. 2.3.4. Typical Experimental Run. Each experimental run involved a programmed addition of pressure- and temperatureequilibrated CO2 that was transferred from the solvent cylinder to the crude oil sample contained in the sample cell. Vigorous stirring of the contents of the sample cell was performed at a rate of 2400 rpm, to prevent the settling of any solids formed during CO2 titration or addition in the cell. (33) Aquino-Olivos, M. A.; Buenrostro-Gonzales, E.; Anderson, S. I.; Lira-Galeana, C. Investigation of Inhibition of Asphaltene Precipitation at High-Pressure Using Bottomhole Samples. Energy Fuels 2001, 15, 236. (34) Garcia, M. C.; Carbognani, L. Asphaltene-Paraffin Structural Interactions. Effect on Crude Oil Stability. Energy Fuels 2001, 15, 1021.

Ibrahim and Idem Table 1. Crude Oil Characteristics Value characteristic carbon content (wt %) hydrogen content (wt %) nitrogen content (wt %) sulfur content (wt %) oxygen content (wt %) heteroatoms content (wt %) molecular weight (g/g-mol) viscosity @ 15 °C (cSt) density @ 15 °C (g/cm3) °API metals content (µg/g) iron nickel vanadium total saturates content (wt %) aromatics content (wt %) resins content (wt %) n-heptane-derived asphaltenes (wt %) n-pentane-derived asphaltenes (wt %) a

L-O oil M1-O oil M2-O oil 85.43 12.87 0.11 1.49 0.41 2.01 372.8 10.7 0.8555 33.9

85.06 12.83 0.19 2.22 0.35 2.76 403.5 9.21 0.8619 32.7

84.52 12.30 0.22 2.98 0.39 3.59 398.3 24 0.8853 28.3

2 2 3.1 7.1 45.5 17.7 8.6 1.2 1.8

16 16 24 56 37.3 22.6 17.2 3.15 3.76

0.6 17 31 48.6 N/Pa N/Pa N/Pa 4.77 5.17

Not performed.

The addition of CO2 continued past the point where asphaltene started to precipitate from the sample in the cell and further, until there was no net asphaltene precipitation. This point was indicated when further additions of CO2 did not result in any significant decrease in the recorded power output signal. The inverse proportionality between the power output signal and the solids content of the cell was used in conjunction with knowledge of the asphaltene contents obtained from saturates, aromatics, resins, and asphaltenes (SARA) analysis to quantify the amount of asphaltene precipitated as a function of time or amount of CO2 added. A programmed CO2 flow rate of 0.5 mL/min was used in the presence and absence of an inhibitor in the oil matrix at a temperature of 338 K in various experimental runs for the three crude oils. Also, a constant pressure of 17.2 MPa was maintained throughout all the experimental runs. Details of the experimental procedure, as well as the typical run, are as described in our previous work,30 which also outlines the procedure for evaluating the precipitation behavior parameters (i.e., when and how fast asphaltene is precipitated) for noninhibited and inhibited oil for CO2induced asphaltene precipitation. Also, each experiment was performed at least twice, to ensure reproducibility, and the maximum error observed was (2%.

3. Results and Discussion 3.1. Relationship between Asphaltene Characteristics and Precipitation Behavior for Noninhibited Oils. The characteristics of the crude oils are presented in Table 1 whereas the characteristics of their n-heptane-derived asphaltene are presented in Table 2.32 Characteristics in these tables include the elemental composition, molecular weight, viscosity, density, specific gravity, metal content, SARA analysis, the average number of carbon per alkyl side chain (n), methyleneto-methyl ratio (CH2/CH3), aromatic carbon fraction (fa), and the degree of condensation (Cb/Cnb). On the other hand, the equilibrium and kinetic parameters for CO2induced asphaltene precipitation for the three noninhibited crude oils (and the inhibited oils) are summarized in Table 3.30 The relationships discussed below were derived upon relating the characteristics given in Tables 1 and 2 to the asphaltene precipitation behavior given in Table 3. 3.1.1. Asphaltene Precipitation Rate Dependence on Asphaltenes Content in the Oil (m). Figure 2 shows the



Table 2. Asphaltenes Samples Characteristics Value characteristics carbon content (wt %) hydrogen content (wt %) nitrogen content (wt %) sulfur content (wt %) oxygen content (wt %) total heteroatoms content (wt %) molecular weight (g/g-mol) metals content (µg/g) iron nickel vanadium total nc CdO RCH2/CH3 S1H/4H CH2/CH3 (by FTIR) fa nNMR As (%) r Cb/Cnb nGASPE NR NB C CH CH2 CH3 CH3/CH CH2/CH3 (by NMR) a

L-Oa

M1-Oa

M2-Oa

83.60 6.95 1.06 4.64 2.60 8.30 3345.7

83.09 7.40 1.34 5.90 1.29 8.53 4550.2

82.78 7.20 1.28 6.91 1.40 9.59 3380.4

79 100 140 319 2.69 0.48 2.34 1.40

260 290 440 990 2.58 0.35 2.46 1.20

62 240 410 712 2.97 0.36 2.62 1.20

1.50 0.60 4.48 18.62 0.86 1.83 13.5 0.68 1.78

1.56 0.59 4.06 18.24 0.80 1.72 13.1 0.66 1.74

1.64 0.58 4.46 17.69 0.85 1.70 13.2 0.65 1.73

physical meaning

average number of carbons per alkyl side chain, from FTIR empirical index of carbonyl abundances, from FTIR molar ratio of CH2 and CH3 groups, from FTIR ratio of intensities of aromatic C-H out-of-plane deformation with 1 adjacent proton to 4 adjacent protons methylene-to-methyl group ratio aromatic carbon fraction average number of carbons per alkyl side chain, from NMR average percent of substitution of aromatic carbon number of substituent rings degree of condensation average number of carbons per alkyl side chain, from GASPE 13C NMR average number of rings per molecule average number of branches per molecule quaternary carbon methine group methylene group methyl group methyl-to-methine group ratio methylene-to-methyl group ratio

0.132 0.688 0.180 1.360 3.834

0.133 0.683 0.184 1.386 3.713

0.131 0.685 0.184 1.404 3.729

n-Heptane-extracted asphaltenes.

Table 3. Estimates of the Values of Kinetic and Equilibrium Parameters of CO2-Induced Asphaltene Precipitation for Noninhibited and Inhibited Crude Oils during CO2 Flooding at 338 K Estimates of Parameters parameter

inhibitora

ln k m n onset point (mL CO2)

ln k m n onset point (mL CO2) a

DDBSA NP toluene DDBSA NP toluene DDBSA NP toluene DDBSA NP toluene

L-O oil

M1-O oil

M2-O oil

Data for Noninhibited Crude Oil -21.4 ( 5.8 -5.18 ( 0.7 2.36 ( 0.53 0.07 ( 0.05 62.8 ( 17.6 41.7 ( 2.6 11.90 6.85

-11.0 ( 1.0 0.27 ( 0.13 28.6 ( 9.8 8.45

Data for Inhibited Crude Oils -32.96 ( 1.45 -109.37 ( 81.70 -37.05 ( 1.45 -76 ( 14.17 -324.26 ( 79.23 -136.12 ( 5.37 1.47 ( 0.04 2.6 ( 1.65 0.97 ( 0.05 1.10 ( 0.26 4.97 ( 1.32 1.85 ( 0.092 69.16 ( 3.13 220.7 ( 167.69 55.71 ( 2.86 144.26 ( 29.26 521.93 ( 132.12 270.45 ( 11.34 11.85 11.75 12.30 12.00 13.50 11.55

-100.7 ( 37.1 -58.77 ( 8.96 -5.44 × 10-12 ( 1.04 × 10-12 3.26 ( 0.35 0.26 ( 0.053 -6.4 × 10-15 ( 5.7 × 10-15 192.54 ( 62.23 80.65 ( 13.69 -2.4 × 10-12 ( 1.71 × 10-12 12.65 13.35 12.60

DDBSA, dodecylbenzenesulfonic acid; NP, nonyl phenol.

relationships between m and the asphaltene molecular weight, as well as that between m and the total metals content in the asphaltene. The figure shows that the asphaltene precipitation rate dependence on asphaltene content in the oil decreases as both the asphaltene molecular weight and the total metals content increase. However, it is observed that the trend for molecular weight does not strictly follow a monotonic decrease with molecular weight. This indicates that there is another characteristic that seems to be interacting with the result. This characteristic is the metals content, in particular, the iron content, which did not follow the logical sequence exhibited by other characteristics, as

shown in Table 2. This made the molecular weight of the M2-O crude oil smaller than expected. On the other hand, we did not find any meaningful relationship between the heteroatoms (S and N) and m and n, or between n and the metals content. Hence, these were not included in the results. Another set of characteristics that were evaluated was the carbonyl (CdO) abundances index, as defined in our previous work,32 as well as the propensity for aggregation of asphaltenes through hydrogen bonding (measured in terms of the relative abundance of FTIR peaks at 3435 cm-1 (representing OH and NH groups) and 3050 cm-1 (representing aromatic CH stretching),

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Figure 2. Variation of asphaltene precipitation rate dependence on asphaltene content (m), relative to (O) asphaltene molecular weight and (4) total metals content.

Figure 3. Variation of asphaltene precipitation rate dependence on asphaltene content (m), relative to (O) the propensity of asphaltenes molecules for aggregation (I3435/I3050) and (4) the carbonyl abundances index (CdO).

I3435/I3050). Their relationships with m are given in Figure 3, which shows that m decreases as I3435/I3050 increases but increases with the CdO content. The results for m versus I3435/I3050 and the CdO content show that a higher propensity for aggregation is actually beneficial, in terms of the kinetics of CO2-induced asphaltene precipitation, whereas a higher CdO content is not. On the other hand, m versus the average number of carbons per alkyl side chain (measured as nNMR and nGASPE32) in Figure 4 shows that more carbons in the alkyl side chain increases m. This is detrimental to the kinetics of CO2-induced asphaltene precipitation. A larger number of carbons in the alkyl side chain implies higher paraffinicity; therefore, the result shows that the higher the paraffinic content of the asphaltene, the higher the contribution of asphaltene content in the oil to the rate of CO2-induced precipitation. 3.1.2. Asphaltene Precipitation Rate Dependence on CO2 Content in the Oil (n). Figure 5 relates n to the

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Figure 4. Variation of asphaltene precipitation rate dependence on asphaltene content (m), relative to the average number of carbons per alkyl side chain of the asphaltenes molecule ((×) nNMR and (O) nGASPE).

Figure 5. Variation of asphaltene precipitation rate dependence on CO2 added (n), relative to (O) the heteroatoms content and (4) the percent substitution of peripheral carbons of asphaltenes (As).

content of heteroatoms and the average percent substitution of peripheral aromatic carbon (As). This figure shows that n increases with As, indicating the detrimental effect on CO2-induced asphaltene precipitation kinetics of an increase in As. On the other hand, the figure shows that n decreases with an increase in the total content of heteroatoms. This latter result shows that an increase in the content of heteroatoms is actually beneficial to the kinetics of CO2-induced asphaltene precipitation. Crude oil characteristics were also considered for possible direct relationships with precipitation behavior parameters. One such characteristic was the asphaltenes content of the oil. According to Table 1, this content increased in the order L-O < M1-O < M2-O, whereas Table 3 shows that n decreased in the order L-O > M1-O > M2-O. A plot of asphaltene precipitation rate dependence on CO2 content in the oil (n) with the asphaltenes content is given in Figure 6. This figure shows that ns and, therefore, the ratesdecreases as the asphaltenes


Figure 6. Variation of asphaltene precipitation rate dependence on CO2 added (n), relative to (×) the asphaltene content and the (O) CH2/CH3 and (4) RCH2/CH3 ratios of the asphaltene molecules.


Figure 8. Variation of the apparent rate constant for asphaltene precipitation (ln k), relative to (O) the asphaltenes molecular weight and (4) the total metals content.

Figure 9. Variation of the apparent rate constant for asphaltene precipitation (ln k), relative to (O) the propensity of asphaltenes molecules for aggregation (I3435/I3050) and (4) the carbonyl abundances index (CdO). Figure 7. Variation of asphaltene precipitation rate dependence on CO2 added (n), relative to various asphaltene characteristics: (O) aromatic carbon fraction (fa), (×) average number of rings per molecule (NR), (4) degree of condensation (Cb/Cnb), and (0) degree of branching of the alkyl side chains (NB).

content of the oil increases. This is an interesting result for CO2-induced asphaltene precipitation and represents the first attempt at confirming what influence the asphaltenes content of the oil has on the kinetics of asphaltene precipitation during CO2 flooding. Similar arguments are applicable to other oil characteristics, such as the total content of heteroatoms and the density of the crude oil. Figure 6 also presents the variation of n, relative to the length of the alkyl side chain (measured in terms of CH2/CH3 and RCH2/CH3 ratios). Because a high CH2/ CH3 ratio is an indication of a low degree of branching, this figure shows that the longer and straighter the length of the alkyl side chain is, the smaller the effect n has on the kinetics. This assertion was verified by making a direct plot of the variation of n with the degree of branching (NB), as illustrated in Figure 7, which also contains relationships of n with other asphaltene characteristics (aromatic carbon fraction (fa), average number of aromatic rings per molecule (NR), and degree of condensation (Cb/Cbn)). This figure shows that n increases with NB, fa, NR, and Cb/Cbn. In the case of n vs

NB, the result confirms our earlier assertion that n decreases as the amount of normal alkyl side chains increases (i.e., the degree of straightness of the alkyl side chain; see Figure 7), which, conversely, implies that n increases with the degree of branching of the alkyl side chain. Because the other parameters (fa, NR, and Cb/Cbn) provide, in a general manner, a measure of the aromaticity of the asphaltenes, the results of their relationship with n indicate that n increases with the aromaticity of the asphaltenes. 3.1.3. Apparent Rate Constant of Asphaltene Precipitation. The relationships between the apparent rate constant (evaluated as ln k) and various asphaltenes characteristics are presented in Figures 8-11. Figure 8 depicts the relationship of ln k to the characteristics of molecular weight and total metals content; Figure 9 shows the propensity for aggregation (I3435/I3050) and the carbonyl abundances index (CdO), relative to ln k; Figure 10 shows the relation of ln k to the number of carbons per alkyl side chain (in terms of nNMR and nGASPE); and Figure 11 shows the relation of ln k to the number of substituent rings (r). Figure 8 shows that ln k increases with both the molecular weight and the total metals content of asphaltenes, in contrast to the case for m (see Figure 2). Also, Figure 9 shows that ln k increases with the propensity for aggregation (I3435/I3050) but decreases sharply with an increase in the carbonyl abundances index (CdO). These trends are completely

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Figure 10. Variation of the apparent rate constant for asphaltene precipitation (ln k), relative to the average number of carbons per alkyl side chain of the asphaltenes molecule ((4) nNMR and (O) nGASPE).

Figure 12. Variation of the onset point for asphaltene precipitation (w), relative to (O) the asphaltenes molecular weight and (4) the total metals content.

Figure 11. Variation of the apparent rate constant for asphaltene precipitation (ln k), relative to the number of substituent rings in the asphaltene molecule (r).

opposite to the situation for m (Figure 3). In the case of Figure 10, the results show that ln k decreases very sharply as the number of carbons per alkyl side chain increases. In contrast to the case for m (Figure 4), this is favorable, in terms of the kinetics of CO2-induced asphaltene precipitation. Because a larger number of carbons in the alkyl side chain implies higher paraffinicity, the result shows that the higher the paraffinic content of the asphaltenes, the smaller the contribution of the apparent rate constant to the rate of CO2-induced precipitation. Finally, Figure 11 shows that the apparent rate constant (ln k) decreases as the number of substituent rings in the asphaltene molecules increases, whereas there was no discernible variation of m with this parameter (r). Thus far, we have only examined how various asphaltene and oil characteristics influence the kinetic parameters in CO2-induced asphaltene precipitation. This effort has made it possible to identify the oil/ asphaltenes characteristics that affect the kinetic parameters (m, n, ln k) positively in which an increase in the value of the characteristic contributes to a reduction of the rate of CO2-induced asphaltene precipitation according to eq 1. We have also identified those factors that influence the kinetic parameters negatively. However, in some cases, the variations of two or more of the kinetic parameters (m, n, and ln k) with a single oil or asphaltene characteristic have resulted in conflicting effects on the overall rate of precipitation. For example,

m versus I3435/I3050 (Figure 3) shows a beneficial effect on the rate whereas ln k versus I3435/I3050 (Figure 9) shows an adverse effect on the rate. In such a case, the overall effect on the rate would be the composite effect that these kinetic parameters would have on the rate, and the overall direction of the effect would be dependent on which parameter has the overriding influence. We did not find any meaningful relationship between the heteroatoms (S and N) and m and n, nor between n and the metals content. Hence, these parameters were not included in the results. 3.1.4. Onset Point of Asphaltene Precipitation (w). The relationships between the onset point of CO2-induced asphaltene precipitation and various asphaltenes characteristics are presented in Figures 12-15. Figure 12 shows the relationship between the onset point and molecular weight and total metals content; Figure 13 shows the effect of the onset point on the propensity for aggregation (I3435/I3050) and the carbonyl abundances index (CdO); Figure 14 shows the relationship between the onset point and the number of carbons per alkyl side chain (in terms of nNMR and nGASPE); and Figure 15 indicates the relationship between the onset point and the number of substituent rings (r). Figure 12 shows that the onset point decreases as both the molecular weight and the total metals content of the asphaltenes molecule increase; these are detrimental effects. Also, as expected, Figure 13 shows that the higher the propensity for the asphaltenes molecules to aggregate through hydrogen bonding (I3435/I3050), the smaller the onset point (i.e., amount of CO2 required to induce asphaltene precipitation); this is a detrimental but wellknown effect.35 However, this is the first time this relationship has been proven experimentally for CO2induced asphaltene precipitation. On the other hand, variation of the onset point with the carbonyl abun(35) Moschopedis, S. E.; Speight, J. G. Investigation of Hydrogen Bonding by Oxygen Functions in Athabasca Bitumen. Fuel 1976, 55, 187.



Figure 15. Variation of the onset point for asphaltene precipitation (w), relative to the number of substituent rings in the asphaltene molecule (r). Figure 13. Variation of the onset point for asphaltene precipitation (w), relative to (O) the propensity of asphaltenes molecules for aggregation (I3435/I3050) and (4) the carbonyl abundances index (CdO).

Table 4. Inhibition Effectiveness of Dodecylbenzenesulfonic Acid (DDBSA) during CO2 Flooding Value parameter

L-O oil

M1-O oil

M2-O oil

∆m/m ∆n/n ∆k/k ∆w/w

0.388 -0.101 1 0.004

-36.143 -4.293 1 -0.374

-11.074 -5.732 1 -0.246

unlike in the case of the kinetic parameter n, we did not observe any definite trend in the relationship between the equilibrium parameter w and the asphaltenes content of the oil. This suggests that there may not necessarily exist a direct influence of asphaltenes content on the asphaltene precipitation onset point (i.e., the point at which asphaltenes start to precipitate) during CO2 flooding. 3.2. Inhibitor Effectiveness. The effectiveness of each chemical additive to inhibit CO2-induced asphaltene precipitation was evaluated from Table 3, in terms of the fractional difference ∆i/i, which is defined as Figure 14. Variation of the onset point for asphaltene precipitation (w), relative to the average number of carbons per alkyl side chain of the asphaltenes molecule ((4) nNMR and (O) nGASPE).

dances index (CdO), which is also given in Figure 13, shows that the onset point improves with CdO, implying a positive influence. This means that the preponderance of CdO groups should not automatically be interpreted to imply a high propensity for aggregation of asphaltenes molecules. Instead, the parameter that is responsible for the adverse effect is the abundance of OH and NH groups. Figure 14 shows that the onset point improves as the number of carbons per alkyl side chain increases. A larger number of carbons in the alkyl side chain implies higher paraffinicity; therefore, the result shows that the higher the paraffinic content of the asphaltenes, the more stable the oil becomes, in terms of CO2-induced asphaltene precipitation. In the case of Figure 15, the result shows that the onset point increases (i.e., improves) as the number of substituent rings in the asphaltene molecules increases. On the other hand,

∆i iNoninhibited - iInhibited ) i iNoninhibited

(2)

where i represents m, n, k, and w. In this definition, a positive value for the kinetic parameters k, m, and n indicate that the chemical additive is an effective inhibitor and the magnitude represents the degree of effectiveness. The converse is also true. On the other hand, a negative value for the equilibrium parameter (i.e., the onset point of precipitation, w) shows that the inhibitor is effective and the magnitude represents the degree of effectiveness; here, the converse is also true. 3.2.1. Dodecylbenzenesulfonic Acid (DDBSA). Table 4 presents the effectiveness of DDBSA on the asphaltene precipitation behavior of the three crude oil samples during CO2 flooding. The results show that the addition of DDBSA reduces the asphaltene content dependence of the precipitation rate m for L-O, whereas this parameter is enhanced in M1-O and M2-O. This result indicates that, in terms of suppressing the effect of m, DDBSA is effective when used with the more-aromatic (and more-substituted and polycondensed) shorter-alkylchain-length oil L-O.32 On the other hand, the table also

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Table 5. Inhibition Effectiveness of Nonyl Phenol (NP) during CO2 Flooding

Table 6. Inhibition Effectiveness of Toluene during CO2 Flooding

Value

Value

parameter

L-O oil

M1-O oil

M2-O oil

parameter

L-O oil

M1-O oil

M2-O oil


0.589 0.113 1 -0.034

-14.714 -2.459 1 -0.404

0.037 -1.820 1 -0.315


-1.106 -7.311 1 -0.134

-25.429 -5.486 1 -0.351

1.000 1.000 -258.82284 -0.241

shows that the CO2 content dependence of the precipitation rate n is enhanced by 10%, 429%, and 573% for L-O, M1-O, and M2-O, respectively. This is a negative result in that, with DDBSA, the amount of CO2 that can be added during an actual CO2 flooding is restricted. The effect worsens with the asphaltene content of the oil. In contrast, other results in Table 4 indicate that the apparent rate constant of asphaltene precipitation (k) in the presence of DDBSA is reduced by 100% (i.e., to zero) for L-O, M1-O, and M2-O. This is a positive effect for DDBSA. Also, the onset point with DDBSA for the L-O light oil remained more or less the same but produced beneficial effects for the M1-O and M2-O medium oils. The onset points (i.e., the CO2 amount required for precipitation) increased by 72% and 50%, respectively. One can conclude from Table 4 that DDBSA was effective for all the oils in terms of reducing the apparent rate constants, whereas it was effective only with L-O, in terms of reducing the rate dependence of precipitation on the asphaltene content. Also, DDBSA was effective with M1-O and M2-O, in terms of stabilizing the oil (i.e., increasing the onset points). 3.2.2. Nonyl Phenol (NP). Table 5 shows the effectiveness of NP on asphaltene precipitation from the three crude oil samples. The results show that the addition of NP reduces the asphaltene content dependence of the precipitation rate m for L-O and M2-O and enhanced this dependence for M1-O. This result indicates that, in terms of suppressing the effect of m, NP was effective when used with the L-O and M2-O oil samples. Thus, similar to the case of DDBSA, NP is effective in terms of reducing m with the more-aromatic (more-substituted and more-polycondensed) shorter-alkyl-chain-length L-O oil.32 Table 5 shows that, unlike the case for DDBSA, the CO2 dependence of the precipitation rate n was reduced in the case of NP by 11.3% for L-O but enhanced by 246% and 182% for M1-O and M2-O, respectively. Other results in Table 5 indicate that the apparent rate constant of asphaltene precipitation in the presence of NP was reduced by 100% (i.e., virtually to zero) for L-O, M1-O, and M2-O. Also, in terms of onset points, NP was effective for all three oils. Onset points were increased by 3%, 75%, and 58% for L-O, M1-O, and M2-O, respectively. It can be concluded from Table 5 that NP was effective with L-O (the most-aromatic crude oil) for all the kinetic behavior parameters. In addition, its effects were positive for all three oils, in terms of the equilibrium parameter (onset point). 3.2.3. Toluene. Table 6 shows the effectiveness of toluene on asphaltene precipitation from the three crude oil samples. The results show that the addition of toluene enhanced the asphaltene content dependence of the precipitation rate m for L-O and M1-O but decreased this effect (m) by 100% when used with M2O. In the case of the CO2 content dependence of the precipitation rate n, similar results were obtained where

it decreased the effect by 100% for M2-O but increased the value for L-O and M1-O. Other results in Table 6 indicate that the apparent rate constant of asphaltene precipitation was decreased by 100% (almost to zero) for L-O and M1-O but increased by 25 900% for M2-O. In contrast, the onset points of all the toluene-inhibited oils were improved. This parameter increased by 13%, 69%, and 49% for L-O, M1-O, and M2-O, respectively, similar to the trend for NP. On the basis of the results, toluene was only effective with M2-O (the least-aromatic oil), in terms of the kinetic parameters m and n, in contrast to NP, which was effective with the mostaromatic oil (L-O). 3.2.4. Summary of Inhibitor Effectiveness. On the basis of the results, non-aromatic-based surfactants that are used as chemical additives during CO2 flooding seem to be the most likely to have positive effects on aromaticbased oils, in terms of asphaltene precipitation inhibition kinetic parameters. NP with the -OH functional group in its molecule was the more-effective chemical additive. It worked best with the more-aromatic (and the more-substituted and more-polycondensed) shorteralkyl-chain-length oil (i.e., L-O), in terms of the kinetic parameters. In contrast, toluene (the most-aromatic additive) was very effective, in terms of the kinetic parameters m and n, with the least-aromatic oil (and the less-substituted and less-polycondensed) longeralkyl-chain-length oil (i.e., M2-O). On the other hand, in terms of the equilibrium parameter (i.e., the onset point), all the three chemical additives (DDBSA, NP, and toluene) were more or less equally effective. The results show that the additives were more effective with the less-stable and less-aromatic oils (M1-O and M2-O) and less effective with the more-stable and morearomatic oil (L-O). These results represent the first attempt at evaluating the effectiveness of chemical additives during CO2induced asphaltenes precipitation from crude oils, in terms of both the equilibrium and kinetic parameters of precipitation. It shows that if the amount of CO2 required for flooding is calculated to be less than the new onset point when an inhibitor is added, then controlling w is more important. On the other hand, if it cannot be controlled by w (i.e., the CO2 required for flooding is larger than the inhibited w value, then the rate should be minimized either by n, k, or both. The parameter m is not a significant factor, judging by its values as shown in Table 3. 3.3. Interrelationship between Additive Effectiveness, Precipitation Behavior, and Oil-Asphaltene Characteristics. It should be mentioned that not all of the inhibitor effectiveness parameters exhibited discernible relationships with the oil-asphaltene characteristics. Thus, because the additives were effective in terms of ∆w/w for all the oils, we decided to use this parameter to evaluate the interrelationships between



Figure 16. Interrelationships between DDBSA effectiveness (measured in terms of onset point, ∆w/w), onset point (w), and various asphaltenes characteristics (aromatic carbon fraction (fa), average number of carbons per alkyl side chain (nGASPE), degree of condensation (Cb/Cnb), and propensity for aggregation (I3435/I3050). Legend is as follows: (4) ∆w/w, (]) fa, (+) nGASPE, (×) Cb/Cnb, (0) total metals content (× 103 ppm), and (*) I3435/ I3050.

Figure 17. Interrelationships between NP effectiveness (measured in terms of onset point, ∆w/w), onset point (w), and various asphaltenes characteristics (aromatic carbon fraction (fa), average number of carbons per alkyl side chain (nGASPE), degree of condensation (Cb/Cnb), and propensity for aggregation (I3435/I3050). Legend is as follows: (4) ∆w/w, (]) fa, (+) nGASPE, (×) Cb/Cnb, (0) total metals content (× 103 ppm), and (*) I3435/ I3050.

additive effectiveness, precipitation behavior, and oilasphaltenes characteristics. A typical set of interrelationships is given below, with respect to the onset point for asphaltene precipitation (i.e., the amount of CO2 needed to start asphaltene precipitation). Note that the ∆w/w values, as calculated by eq 2, are negative values, as shown in Tables 4-6. However, absolute ∆w/w values are used in this section, based on our definition of effectiveness for the onset point. 3.3.1. Dodecylbenzenesulfonic Acid (DDBSA). Figure 16 shows the interrelationships between DDBSA effectiveness (measured as an improvement in the onset point (∆w/w), precipitation behavior (in terms of the onset point), and some asphaltene characteristics (namely, the aromatic carbon fraction (fa), the metals content, the average number of carbon per alkyl side chain (nGASPE), the degree of condensation (Cb/Cnb), and the propensity for aggregation of the asphaltenes molecules (I3435/I3050)). Figure 16 suggests that DDBSA has the highest inhibition efficiency (∼72%), in terms of the onset point on the oil that has the lowest average number of carbon per alkyl side chain (nGASPE), lowest onset point (i.e., the least-stable oil), lowest metals content, highest degree of condensation, highest aromatic carbon fraction, and highest propensity for aggregation. Figure 16 postulates that the efficiency of DDBSA, in terms of the onset point (∆w/w), tends to zero as the oil becomes richer in metals content, becomes higher in the average number of carbons per alkyl side chain, or becomes more stable. 3.3.2. Nonyl Phenol (NP). Figure 17 illustrates the interrelationships between NP effectiveness (measured as an improvement in the onset point, ∆w/w), precipitation behavior (in terms of the onset point), and asphaltene characteristics similar to those used in the case of DDBSA (namely, fa, the metals content, nGASPE, Cb/Cnb, and I3435/I3050). The results shown in Figure 17 have trends that are similar to those shown in Figure 16. However, NP showed better efficiency (an increase to

Figure 18. Interrelationships between toluene effectiveness (measured in terms of onset point, ∆w/w), onset point (w), and various asphaltenes characteristics (aromatic carbon fraction (fa), average number of carbons per alkyl side chain (nGASPE), degree of condensation (Cb/Cnb), and propensity for aggregation (I3435/I3050). Legend is as follows: (4) ∆w/w, (]) fa, (+) nGASPE, (×) Cb/Cnb, (0) total metals content (× 103 ppm), and (*) I3435/ I3050.

∼75%) when used for the same type of oil. Also, Figure 17 shows the zero ∆w/w effect as the oil becomes more stable, has greater metals content, or has a higher degree of condensation. 3.3.3. Toluene. Figure 18 shows the interrelationship between toluene effectiveness (measured as an improvement in the onset point, ∆w/w), precipitation behavior (in terms of the onset point), and asphaltene characteristics similar to those used in the case of DDBSA and NP (namely, fa, the metals content, nGASPE, Cb/Cnb, and I3435/I3050). The results shown in Figure 18 have trends that are similar to those given in Figures 16 and 17. However, toluene efficiency decreased to ∼69% when used with the same type of oil. Also, Figure 18 shows a

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minimum efficiency of ∼13%, which is much higher than that of DDBSA and NP for the same oil. 3.3.4. Industrial Applications. The results from this work should enable the correct choice of chemical inhibitor for the prevention by inhibition of CO2-induced asphaltene precipitation during CO2 flooding. The information needed to make this choice includes the nature of the oil (whether paraffinic or aromatic) and the amount of CO2 required for flooding. If the oil is aromatic, the inhibitor to use should be an amphiphilebased chemical, such as NP, which minimizes the rate of asphaltene precipitation, as well as allows more CO2 to be used in the flooding process. In the case of paraffinic oil, an aromatic-based inhibitor such as toluene is preferable. For paraffinic oils, however, toluene, in a manner similar to that of the amphiphilebased chemicals, works best in terms of minimizing the rate of CO2-induced asphaltene precipitation during CO2 flooding. It does not necessarily work toward improving the onset point. Inhibitor effectiveness studies were performed at one pressure (17.2 MPa) and temperature (338 K). However, it is worthwhile to perform studies at other temperatures and pressures. Previous studies have shown that m, n, k, and w are affected by temperature,30 whereas w is affected by pressure.36 Therefore, it is expected that inhibition effectiveness of the chemicals, in terms of m, n, k, and w, will also be affected by both temperature and pressure. 4. Conclusions (1) Asphaltene precipitation rate dependence on asphaltene content (m) increased with the paraffinicity of the asphaltenes but, interestingly, decreased as the propensity of the asphaltene molecules for aggregation (36) Ramachandran, S.; Breen, P.; Ray, R. Chemical Programs Assure Flow and Prevent Corrosion in Deepwater Facilities and Flowlines; Baker Hughes, Inc.: Sugar Land, TX, 2000. (Available via the Internet at http://www.bakerhughes.com/bakerhughes/inDepth/ 72k/Petrolite.pdf.)

Ibrahim and Idem

increased. The trends for the apparent rate constant for asphaltene precipitation (k) were the exact opposite of those for m. On the other hand, the precipitation rate dependence on the amount of CO2 added (n) decreased with an increase in the heteroatoms content of both the crude oil and asphaltenes but increased with the aromatic carbon fraction and degree of branching of the asphaltene molecules. (2) The equilibrium parameter (onset point) increased with the paraffin fraction of the asphaltene molecules but decreased as the propensity of the asphaltene molecules for aggregation increased. (3) Nonyl phenol (NP) with the -OH functional group in its molecule was the most-effective chemical additive with the more-aromatic (and more-substituted and more-polycondensed) shorter-alkyl-chain-length oil, in terms of the kinetic parameters. In contrast, toluene (the most-aromatic additive) was very effective, in terms of the kinetic parameters m and n, with the leastaromatic oil. In terms of the onset point, all three chemical additives were more effective with the lessstable and less-aromatic oils but less effective with the more-stable and more-aromatic oil. (4) Dodecylbenzenesulfonic acid (DDBSA) exhibited its highest inhibition efficiency (∼72%), in terms of the onset point, on the oil that had the lowest average number of carbons per alkyl side chain (nGASPE), lowest onset point (i.e., the least-stable oil), lowest metals content, highest degree of condensation, highest aromatic carbon fraction, and highest propensity for aggregation. NP and toluene exhibited similar relationships. However, the highest efficiencies for NP and toluene were 75% and 69%, respectively. Acknowledgment. The authors thank the Petroleum Technology Research Center (PTRC), Regina, for financial support, and Dr. Sam Huang and Mr. Bart Schnell of the Saskatchewan Research Council (SRC), Petroleum Branch, Regina, for their technical support. EF0340458

CO2-Miscible Flooding for Three Saskatchewan Crude Oils

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