Correlations of Characteristics of Saskatchewan ... - ACS Publications

1354

Energy & Fuels 2004, 18, 1354-1369

Correlations of Characteristics of Saskatchewan Crude Oils/Asphaltenes with Their Asphaltenes Precipitation Behavior and Inhibition Mechanisms: Differences between CO2- and n-Heptane-Induced Asphaltene Precipitation Hussam H. Ibrahim and Raphael O. Idem* Process & 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 April 12, 2004

The structural and molecular characteristics of the asphaltenes of four oilsslight oil, L-O; medium oils, M1-O and M2-O; and heavy oil, H-Osfrom the Weyburn and adjacent areas in Saskatchewan, Canada were determined and correlated with the oils’ asphaltene precipitation behavior and mechanism, as well as their chemical inhibitor effectiveness, for the purpose of determining differences between CO2 and hydrocarbon flooding for enhanced oil recovery (EOR). A multitechnique approach involving Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (1H NMR) spectroscopy, 13C NMR spectroscopy, gated spin-echo (GASPE) spectroscopy, inductively coupled plasma (ICP), elemental analysis, saturatesaromatics-resins-asphaltenes (SARA) analysis, molecular weight, and density studies was used for characterization of the crude oils and their n-heptane-derived asphaltenes. Results showed that the asphaltene precipitation behavior and mechanism each were strong functions of the oil and asphaltene characteristics. Interestingly, there were striking contrasts in these relationships, depending on whether CO2 or n-heptane was used as the flooding (i.e., precipitating) agent. In addition, there were differences in the inhibition effectiveness and mechanism, depending on the type of flooding agent used. Furthermore, these differences also were dependent on the type of chemical inhibitor used.

1. Introduction Asphaltenes are the heaviest and most complex fraction of crude oil, consisting of condensed polynuclear aromatic rings and minute amounts of heteroatoms (such as sulfur, nitrogen, and oxygen) and traces of metals such as iron, nickel, and vanadium.1 Asphaltene is usually defined as the brown/black powdery material produced by the treatment of petroleum, petroleum residua, or bituminous materials with a low-boiling paraffin hydrocarbon such as pentane or heptane. It is thus insoluble in these solvents but soluble in benzene (and other aromatics solvents), carbon disulfide, and chloroform (or other chlorinated hydrocarbon solvents). The insolubility of asphaltenes in light paraffin liquids and other incompatible fluids, such as carbon dioxide (CO2), could be a source of problems for certain crude oil production operations.2,3 * Author to whom correspondence should be addressed. Fax: (306)585-4855. E-mail address: [email protected]. (1) Qin, X.; Wang, P.; Sepehrnoori, K.; Pope, G. A. Modeling Asphaltene Precipitation in Reservoir Simulation. Ind. Eng. Chem. Res. 2000, 39, 2644. (2) Garcia, M. C.; Carbognani, L. Asphaltene-Paraffin Structural Interactions. Effect on Crude Oil Stability. Energy Fuels 2001, 15 (5), 1021. (3) 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.

In tertiary oil production, many light and medium reservoirs are subjected to miscible or near-miscible CO2 or hydrocarbon flooding for enhanced oil recovery (EOR) after the initial water flooding. In the United States, 60 active miscible CO2 projects were in operation in 1996, whereas in Canada, hydrocarbon miscible floods are more common and there are ∼40 such active projects.4 In Saskatchewan, Canada, most of the light oil reservoirs have reached their economic limits of production by water flooding5 and have become suitable candidates for miscible/near-miscible CO2 flooding.6,7 However, flooding processes cause many changes in the flow and phase behavior of the reservoir fluids and can significantly alter formation properties, which favors the precipitation of organic solids, mainly asphaltenes.8 Asphaltene precipitation can change the wettability of the reservoir matrix and consequently affect the flood (4) Moritis, G. New Technology, Improved Economics, Boost EOR Hopes. Oil Gas J. 1996, 94 (39). (5) Saskatchewan Energy & Mines Reservoir Annual Report, Regina, Saskatchewan, Canada, 1993. (6) Huang, S. S.; Dyer, S. B. Miscible Displacement in the Weyburn Reservoir: A Laboratory Study. J. Can. Pet. Technol. 1993, 32 (42). (7) 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. 59293. (8) 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.

10.1021/ef034044f CCC: $27.50 © 2004 American Chemical Society Published on Web 07/03/2004

Precipitation and Inhibition of Saskatchewan Crude

performance.9 It can also cause formation damage and wellbore plugging, requiring expensive treatment and cleanup procedures.10-14 There is currently a huge CO2 storage/sequestration and monitoring operation in Weyburn, Saskatchewan, Canada, in which CO2 is used as a flooding agent for EOR. In this CO2 sequestration and monitoring project, the propensity for asphaltene precipitation, as well as the possible inhibition mechanisms, are not well-known. Presently, no studies have been reported on possible relationships between asphaltene precipitation behavior (involving both the kinetics (i.e., rate) and the equilibrium (i.e., the onset point of asphaltene precipitation)) parameters and asphaltene molecular characteristics. We are attempting to obtain structural and molecular characteristics of asphaltenes of oils from the Weyburn and adjacent areas in Saskatchewan with a view to correlating them with the oils’ asphaltene precipitation behavior and their inhibition effectiveness.15-18 Asphaltene precipitation behavior and mechanisms, as well as the inhibition effectiveness, were evaluated in terms of kinetic and equilibrium parameters. These were respectively obtained from a rate equation ((β/Mo)(dNAp/dNH) ) kN Amo N nH) and onset point of asphaltene precipitation derived in our earlier work,15-18 where β is the molar programmed rate of addition of precipitating agent, Mo the number of moles of crude oil charged to the sample cell, k the apparent rate constant, NH the number of moles of n-heptane or CO2 in the oil at any time, NAo the moles of asphaltene in the oil at any time, NAp the amount of asphaltene precipitated in the oil, m the rate dependence of asphaltene precipitation on the asphaltene content in the oil, and n the rate dependence of asphaltene precipitation on the n-heptane or CO2 content in the oil. In this regard, we are evaluating (9) Buckley, J. S. Asphaltene Precipitation and Crude Oil Wetting. SPE Adv. Technol. Ser. 1995, 53. (10) 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. (11) Kamath, V. A.; Yang J.; Sharma, G. D. Effect of Asphaltene Deposition on Dynamic Displacements of Oil by Water. Presented at the Western Regional Meeting, Anchorage, AS, May 26-28, 1993, Paper No. SPE 26046. (12) 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, 1990, 23, Paper No. SPE 20530. (13) 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. (14) 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. (15) 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. (16) Ibrahim, H. H.; Idem, R. O. A Method for Evaluating the Kinetics of n-Heptane-Induced Asphaltene Precipitation from Various Saskatchewan Crude Oils during Light Hydrocarbon Flooding. Submitted to Fuel, 2004. (17) Ibrahim, H. H.; Idem, R. O. CO2 Miscible Flooding for Three Saskatchewan Crude Oils: Interrelationships between Asphaltene Precipitation Inhibitor Effectiveness, Asphaltene Characteristics, and Precipitation Behavior. Energy Fuels 2004, 18 (3), 743-754. (18) Ibrahim, H. H.; Idem, R. O. Interrelationships between Asphaltene Precipitation Inhibitor Effectiveness, Asphaltene Characteristics, and Precipitation Behavior during n-Heptane (Light Paraffin Hydrocarbon)-Induced Asphaltene Precipitation. Energy Fuels 2004, in press.

Energy & Fuels, Vol. 18, No. 5, 2004 1355

asphaltene characteristics from four Saskatchewan oils as a function of the type of oil (i.e., light oil, medium oils, and heavy oil). As a result of the complexity of the asphaltene molecule, a multitechnique approach involving Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (1H NMR) spectroscopy, carbon nuclear magnetic resonance (13C NMR) spectroscopy, gated spin-echo (GASPE) spectroscopy, inductively coupled plasma (ICP), elemental analysis, saturates-aromatics-resins-asphaltenes (SARA) analysis, molecular weight, and density studies was used for characterization of both the crude oils and their nheptane-derived asphaltenes. We are also evaluating the asphaltene precipitation behavior and mechanisms of these oils, as well as their inhibition effectiveness for CO2- and n-heptane-induced asphaltene precipitation, which are respectively used to simulate EOR processes using CO2 and light hydrocarbons. A comparison of the correlations of asphaltene/oil characteristics with precipitation behavior and the inhibition mechanism between CO2- and n-heptane-induced asphaltene precipitation during CO2 and light hydrocarbon EOR processes are presented and discussed in this paper. 2. Experimental Section 2.1. Crude Oil Samples and Their Corresponding Asphaltenes. Four crude oil samples from four different reservoirs were used in this study. Their identities are given as follows. The first oil, designated as L-O oil, was obtained from the light oil pool located southeast of Regina, Saskatchewan, Canada. The field contains light oil in the Frobisher and Midale beds located at a depth of ∼1400 m. Two other oils, designated as M1-O and M2-O, were medium crude oils obtained from the Weyburn pool, which is also located southeast of Regina. A fourth oil, designated as H-O, was a heavy oil obtained from the Senlac pool in Saskatchewan. Asphaltene samples were extracted from each oil sample by weighing 10 g of the oil sample and then adding n-heptane in a gradual fashion (to ensure complete mixing between the oil sample and the n-heptane, to avoid the formation of lumps) while vigorously stirring the mixture at room temperature. After mixing for 1 h, the precipitate was allowed to form overnight under a blanket of argon gas. 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, to ensure that all dissolved oil was transferred through the filter. The extracted asphaltenes samples were then thoroughly characterized. 2.2. Characterization of Crude Oils and Asphaltenes. All the crude oils and their n-heptane-derived asphaltenes were characterized as follows. 2.2.1. CHNSO Analysis. An elemental analyzer (Carlo Erba, model 1108) was used to determine the carbon, hydrogen, nitrogen, sulfur, and oxygen contents (concentration) of the crude oils and the corresponding n-heptane-derived asphaltenes. 2.2.2. Metal Content. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was used to determine the metal content of both the oil and the corresponding asphaltenes. For each analysis, the sample was thermally ashed at 773 K and the ash was dissolved in 3 mL of concentrated hydrochloric acid (HCl) with heating. The solution was diluted to a final volume of 25 mL with distilled water. The analysis was performed using ICP-AES (Thermal Jarrell Ash, model ICAP Trace 61E, with an axial torch).

1356 Energy & Fuels, Vol. 18, No. 5, 2004 2.2.3. Average Molecular Weight. The Corona/Wescan molecular weight apparatus, which is designed for nonvolatile materials within a molecular weight range of 100-100 000 Daltons was used to determine the average molecular weight of both the oils and their n-heptane-derived asphaltene. This instrument worked through application of the differential vapor pressure principle. Calibration was conducted using benzyl, benzoic acid, and naphthalene and used to calculate the calibration constant. 2.2.4. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR was used to obtain information on the type of functional groups present in both the oils and their corresponding n-heptane-derived asphaltene. Infrared spectra were recorded using an FTIR spectrometer (Perkin-Elmer, Spectrum One) in the absorbance mode with a spectral resolution of 4 cm-1 in the 4000-450 cm-1 spectral domain. Solid samples were prepared by mixing the materials with spectroscopic-grade potassium bromide (KBr) to obtain a 0.5% (w/w) asphaltene/ KBr, whereas liquid samples were run as neat samples using NaCl windows. Solid samples were acquired, relative to a pure KBr reference, and the analysis was focused on three regions of the spectrum: 2800-3200 cm-1, 1500-1800 cm-1, and 450950 cm-1. 2.2.5. Proton Nuclear Magnetic Resonance (1H NMR) Spectroscopy. This was used to obtain information about the distribution of H atoms in the aliphatic and aromatic regions of the asphaltenes. Liquid-state proton or 1H NMR spectra were obtained at 300 MHz on a Bruker AM-200 spectrometer at the University of Alberta in Edmonton, Canada. Chemical shifts (δ) are reported (in parts per million, ppm), relative to the appropriate reference signals. 1H NMR spectra were recorded in deuteriochloroform (CDCl3) with a pulse width of 3 µs, a recycle delay of 2 s, a spectral width of 14.4 ppm, a tube diameter of 5 mm, a spectral width of 10 ppm, and at least 128 scans. 2.2.6. 13C NMR. 13C NMR was used to obtain information on the carbon distribution in the oil and asphaltenes samples. 13 C NMR spectra were collected on the same Bruker model AM-200 apparatus as a CDCl3 solution with a pulse width of 17 µs, a tube diameter of 12 mm, a spectral width of 216 ppm, a recycle delay of 2 s, and ∼22 400 scans. Chromium acetylacetonate (Cr(acac)3) (0.01 M in the final solution) was added to ensure complete nuclear magnetic moment relaxation between pulses. An inversed-gated 1H decoupling technique was used to suppress the nuclear Overhauser enhancement (NOE) effect. Solution-state 13C NMR was used in the determination of asphaltene aromaticity, as well as in structural investigations. 2.2.7. GASPE 13C NMR. The gated spin-echo (GASPE) 13C NMR technique was used for asphaltene samples to generate sub-spectra, to alleviate the problem of signals overlapping in the aliphatic region of the 13C NMR spectrum. The main idea behind the (GASPE) 13C NMR subspectral analysis technique was to yield individual 13C NMR subspectra for each CHn group type, namely methyl (CH3), methine (CH), methylene (CH2), and quaternary carbon (C). The spectra for this work were taken with evolution times of 0.007 s, to distinguish methyl and methine (CH + CH3) from methylene and quaternary carbon (CH2 + C), 0.005 s to distinguish between methyl (CH3) and methine (CH) carbon, and 0.004 s to obtain the quaternary resonance (C) only. 2.3. Mechanism of Inhibition. 2.3.1. Inhibitors and Stabilizers. Two non-aromatic-based surfactants or inhibitorss namely, dodecylbenzenesulfonic acid (DDBSA) and nonyl phenol (NP)sin 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

Ibrahim and Idem (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) alkyl side-chain length, and (iv) polarity (DDBSA > NP > toluene). We used 1000 ppm of each chemical inhibitor for the experiments to test inhibition effectiveness. 2.3.2. Effectiveness of Additives as Asphaltene Precipitation Inhibitors or Stabilizers during CO2-Miscible or Light Hydrocarbon Flooding. 2.3.2.1. Equipment. Asphaltene precipitation behavior during CO2-miscible or light hydrocarbon flooding was evaluated under isothermal (304, 313, and 338 K) and isobaric (17.2 MPa) conditions for both noninhibited and inhibited crude oils for three of the crude oils. The heavy oil was too opaque to allow for accurate evaluation; therefore, evaluation of the precipitation behavior was not performed. CO2- or hydrocarbon-miscible flooding was simulated using a molar CO2 or n-heptane (i.e., titrant or precipitating agent) programmed titration technique15-18 in a solids detection system (SDS) obtained from DB Robinson & Manufacturing Limited (Edmonton, Canada). This equipment consisted of a mercury-free, variable-volume, fully visual, JEFRI PVT cell retrofit with fiber-optic light transmission probes (source and detector). The SDS enabled us to conduct experiments using field crude oil samples 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.10,19 2.3.2.2. Procedure for Nonstabilized Oil (i.e., without Inhibitor). A known quantity of a crude oil sample was fed to 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 M2-O) was in place, the sample cell was then completely isolated. The titrant pressure was increased to 17.2 MPa and the remaining oil in the tubing was considered to be dead volume. The oil sample and the titrant 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 the titrant from the solvent cylinder to the PVT (sample) cell, the pressure in the lines connecting the titrant cylinder to the sample cell was 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 works 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. 2.3.2.3. Procedure for Stabilized Oil (i.e., with Inhibitor). In the case of inhibited oil, a known amount of the inhibitor (1000 ppm) was added to the crude oil at 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 JEFRI displacement pump (JDP). Apart from 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. 2.3.2.4. Typical Experimental Run. Each experimental run involved a programmed addition of pressure- and temperatureequilibrated titrant transferred from the solvent cylinder to the crude oil sample contained in the sample cell. There was (19) Aquino-Olivos, M. A.; Buenrostro-Gonzales, E.; Anderson, S. I.; Lira-Galeana, C. Investigation of Inhibition of Asphaltene Precipitation at High-Pressure Bottomhole Samples. Energy Fuels 2001, 15, 1021.



Table 1: Crude Oila Characteristics

Table 2: General Characteristics of Asphaltenesa

Value characteristic composition (wt %) carbon hydrogen nitrogen sulfur oxygen heteroatoms molecular weight (g/g-mol) viscosity @ 15°C (cSt) density @ 15°C (g/cm3) °API metals content (µg/g) iron nickel vanadium total metals SARA analysis (wt %) saturates aromatics resins asphaltenes n-heptane-derived n-pentane-derived

L-O

M1-O

M2-O

Value H-O

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

84.71 11.34 0.36 3.22 0.55 4.13 450.4 N/Ab 0.9719 14.1

2 2 3.1 7.1

16 16 24 56

0.6 17 31 48.6

7.4 39 95 141.4

45.5 17.7 8.6

37.3 22.6 17.2

N/Pc N/Pc N/Pc

23.3 36.9 17.3

1.2 1.8

3.15 3.76

4.77 5.17

9.47 11.8

a The crude oil pool includes L-O, M1-O, M2-O, and H-O. b Not applicable. c Not performed.

vigorous stirring of the contents of the sample cell, at a rate of 2400 rpm, to prevent the settling of any solids formed during titration in the cell. The addition of titrant 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 was indicated when further additions of titrant 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 SARA analysis, to quantify the amount of asphaltene precipitated as a function of time or the amount of titrant added. A programmed titrant 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 a typical run, are as described in our previous work,15-18 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 CO2- or n-heptane-induced 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. Crude Oil Characteristics. The characteristics of L-O, M1-O, M2-O, and H-O oils (i.e., whole crudes) are presented in Table 1. The table shows that the asphaltene content in the oil increased in the following order: L-O < M1-O < M2-O < H-O. Table 1 also shows that the crude oil aromaticity, measured roughly in terms of the total aromatic and polyaromatic components (i.e., aromatic + resins + asphaltenes), as well as the total metals (Fe + Ni + V) content, increased in the same order. Table 1 shows that heteroatoms (i.e., N, S, and O) also increased in the order L-O < M1-O < M2-O < H-O. On the other hand, both the crude oil paraffinicity (measured as the saturates content) and the API gravity increased in the reverse order. The table

characteristic composition (wt %) carbon hydrogen nitrogen sulfur oxygen total heteroatoms molecular weight (g/g-mol) metals content (µg/g) iron nickel vanadium total metals a

L-O

M1-O

M2-O

H-O

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

82.75 8.16 1.16 5.90 1.81 8.87 5091.7

79 100 140 319

260 290 440 990

62 240 410 712

23 220 550 793

n-Heptane-extracted asphaltenes.

shows that the molecular weights of the crude oils were as follows: L-O, 373 g/g-mol; M1-O, 404 g/g-mol; M2-O, 398 g/g-mol; and H-O, 450 g/g-mol (i.e., L-O < M2-O < M1-O < H-O). 3.2. Asphaltene Characteristics. 3.2.1. Elemental Composition and Molecular Weight. Some characteristics (i.e., elemental composition and molecular weight) of n-heptane-derived asphaltene for L-O, M1-O, M2-O, and H-O oils are shown in Table 2. The table shows that the total metals content of the asphaltenes increased in the following order: L-O < M2-O < H-O < M1-O. The substantially high iron and nickel content in M1-O asphaltene resulted in its overall high metals content, such that it was higher than that for the heavy (H-O) asphaltenes. The other medium M2-O asphaltene showed consistency, in that its metal content was higher than that of the light L-O asphaltene but lower than the heavy H-O asphaltene. Also, calculations based on the asphaltene contents show that about half of the metals content in the crude oil end up in the asphaltene fraction. This is consistent with the literature.20 Also, the total heteroatoms (N + S + O) content increased in the following order: L-O < M1-O < H-O < M2-O. In addition, calculations showed that carbon, sulfur, nitrogen, and oxygen were concentrated in the asphaltenes, whereas hydrogen was deficient in the asphaltenes, as compared to their corresponding crude oils. Table 2 also shows that molecular weights of the asphaltenes were as follows: L-O, 3346 g/g-mol; M1-O, 4550 g/g-mol; M2-O, 3380 g/g-mol; and H-O, 5092 g/g-mol (i.e., L-O < M2-O < M1-O < H-O). This trend is as expected and is similar to the trend for the whole oil in which each asphaltene existed. 3.2.2. Fourier Transform Infrared Analysis. FTIR spectra showing different functional groups present in the n-heptane-derived asphaltenes from L-O, M1-O, M2-O, and H-O oils are presented in Figure 1, which shows that all the asphaltene samples are composed of more-or-less similar functional groups. However, the intensities of absorbances for the functional groups were different for different asphaltenes. The ratio of bands at 3435 cm-1 to those at 3050 cm-1 provided information on the OH and NH groups, relative to the aromatic fraction. The results of this ratio, as well as other ratios, for the four asphaltenes are given in Table 3. For the case of the I3435/I3050 ratio, it shows that the O-H and (20) Teh Fu, Y. Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; Chapter 1.

1358 Energy & Fuels, Vol. 18, No. 5, 2004 Ibrahim and Idem

Figure 1. Fourier transform infrared (FTIR) spectra for n-heptane-derived asphaltenes (asphaltene samples extracted from L-O, M1-O, M2-O, and H-O oils, starting at the top and moving downward at the 1600 cm-1 peak ).



Table 3: Average Molecular Parameters Derived from Fourier Transform Infrared (FTIR) Analysisa Value parameter

physical meaning

L-O

M1-O

M2-O

H-O

nIR CdO RCH2/CH3 (I2923/I2852) S1H/4H

average number of carbon per alkyl side chain empirical index of carbonyl abundances molar ratio of CH2 to CH3 groups ratio of intensities of aromatic C-H out-of-plane deformation with one adjacent proton to that of four adjacent protons (degrees of substitution and condensation) methylene-to-methyl group ratio (alkyl side chain length) aromatics/aliphatics ratio ratio of bands at 3435 cm-1 to those at 3050 cm-1 (propensity for H-bond formation)

2.69 0.48 2.34 1.40

2.58 0.35 2.46 1.20

2.97 0.36 2.62 1.20

2.73 0.43 2.54 1.00

1.50 0.263 2.10

1.56 0.202 4.33

1.64 0.160 3.60

1.65 0.153 10.0

CH2/CH3 (I1455/I1376) I1600/I2923 I3435/I3050 a

Data given for n-heptane-extracted asphaltenes.

NH contents decrease in the order H-O > L-O > M1-O > M2-O, whereas the aromatic CH stretching abundance decrease in the order L-O > M1-O > M2-O > H-O. As a result, the propensity for the formation of hydrogen bonds (i.e., the I3435/I3050 ratio)21 decreased in the following order: H-O > M1-O > M2-O > L-O. This shows that, based on hydrogen bonding, the light oil L-O asphaltene has weaker aggregation forces, compared to asphaltenes that have been derived from medium M1-O/M2-O and heavy H-O oils. This result implies that the propensity for aggregation is not necessarily dependent on the amounts of OH and NH groups alone. Instead, it is based entirely on the amount of OH and NH groups, relative to the amount of aromatic fraction. Ratios between aromatic bands at 1600 cm-1 to aliphatic bands at 2923 cm-1 (Table 3) show that the aromaticity decreased in the following order: L-O > M1-O > M2-O > H-O. Another noteworthy result from Figure 1 is the ratio of bands at 1455 cm-1 to bands at 1376 cm-1, which corresponds to the CH2/CH3 ratio (see Table 3). This ratio (eq 1) gives an indication of the alkyl sidechain length. The higher the ratio, the longer the side chain.

CH2/CH3 )

I1455 I1376

(1)

Table 3 shows that the CH2/CH3 ratio decreased in the order H-O > M2-O > M1-O > L-O, representing a corresponding decrease in the aromaticity of the asphaltenes. The trend of the alkyl side-chain length, derived from I1455/I1376, is also consistent with the aromaticity or paraffinicity of the asphaltenes. Table 3 also shows additional molecular parameters that were calculated from the FTIR spectra according to Calemma,22 to further elucidate the variation in the structural characteristics of the asphaltenes from the various oils. The average number of carbons per alkyl side chain (nIR) is defined in eq 2:

nIR ) RCH2/CH3nCH3 + nCH3

(2)

where RCH2/CH3 is the molar ratio between the C-H stretch of the CH2 and CH3 groups (i.e., the higher the (21) Moschopedis, S. E.; Speight, J. G. Investigation of Hydrogen Bonding by Oxygen Functions in Athabasca Bitumen. Fuel 1976, 55, 187. (22) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L.; Structural Characterization of Asphaltenes of Different Origins. Energy Fuels 1995, 9, 225.

ratio, the longer the side chain),

RCH2/CH3 )

nCH2 nCH3

)

(

)

I2927 cm-1 k I2957 cm-1

(3)

where Ix cm-1 is the band intensity at x cm-1, k ) 1.243, and nCH3 is the average molecular methyl number obtained from GASPE 13C NMR (see Table 5). Table 3 shows that the nIR values did not exhibit any particular trend, as it increased in the following order: M1-O < L-O < H-O < M2-O. However, it does show that L-O has a relatively shorter alkyl side chain, compared to the medium M2-O and heavy H-O asphaltenes. The carbonyl abundances index (CdO), which is defined in eq 4, is an empirical index of carbonyl abundances in the asphaltene molecules, with respect to aromatic moieties (see Table 3).

CdO )

I1700 + I1650 I1700 + I1650 + I1600

(4)

Three peaks, centered at 1700 cm-1 (for ketones, aldehydes, and carboxylic acids), 1650 cm-1 (for highly conjugated carbonyls, such as quinone-type structure and amides), and 1600 cm-1 (for aromatic CdC stretching) were used in evaluating CdO. The 1735 cm-1 peak of esters was too weak to be determined quantitatively and was not considered. As shown in Table 3, the carbonyl content as defined by eq 4 did not reveal any definite trend but showed that it was slightly higher for L-O asphaltene than the medium oil (M1-O and M2-O) and heavy oil (H-O) asphaltenes. The ratio of intensities of aromatic C-H out-of-plane deformation with one adjacent proton to four adjacent protons (S1H/4H) is given in eq 5, and it represents the degree of substitution on the peripherial carbon of aromatic ring.

S1H/4H )

I915-852 I760-730

(5)

The S1H/4H ratio is also directly related to the degree of condensation. S1H/4H values for the different asphaltenes are reported in Table 3, which shows that L-O asphaltenes have the highest S1H/4H ratio, whereas H-O asphaltenes have the lowest S1H/4H ratio. The S1H/4H ratios of M1-O and M2-O are between those of L-O and H-O, which is a reflection of their intermediate aromaticity. It seems, from the results so far obtained, that the L-O asphaltenes have shortersbut a larger number

1360 Energy & Fuels, Vol. 18, No. 5, 2004

Ibrahim and Idem

Table 4: Average Molecular Parameters Derived from 1H and

13C

NMR Analysisa Value

a

parameter

physical meaning

L-O

M1-O

M2-O

H-O

fa nNMR As (%) r Cb/Cnb

aromatic carbon fraction average number of carbons per alkyl side chain average percent of substitution of aromatic carbons number of substituent rings degree of condensation

0.60 4.48 18.62 0.86 1.83

0.59 4.06 18.24 0.80 1.72

0.58 4.46 17.69 0.85 1.70

0.51 4.90 14.68 0.98 1.25

Data given for n-heptane-extracted asphaltenes.

ofssubstituted alkyl side chains, compared to the H-O asphaltenes, hence its higher CH3/CH2 ratio, shorter alkyl chain length, and higher degree of substitution. 3.2.3. Nuclear Magnetic Resonance Studies. 3.2.3.1. 1H NMR and 13C NMR. A combination of 1H NMR and 13C NMR data from the designated 1H and 13C regions allowed a more accurate determination of several average structural parameters, such as the aromatic carbon fraction (fa), the average number of carbons per alkyl side chain (nNMR), the percent of substitution of peripheral aromatic carbon (As), the number of substituent rings (r), and the degree of condensation (Cb/Cnb), as outlined by Calemma et al.22 These parameters provided additional tools to further evaluate the variation in the structural characteristics of the four asphaltene from four different types of oils. The aromatic carbon fraction, fa, represents the fraction of aromatic carbons relative to the total number of carbons, as given in eq 6:

fa )

Car Car + Cal

(6)

The fa results are given in Table 4, and they confirm that L-O asphaltene has the highest fa value, whereas H-O has the lowest fa value. The fa values for M1-O and M2-O are between those of L-O and H-O. These results are consistent with the FTIR aromaticity results that were based on I1600/I2923 ratios. The results also indicate that the higher the molecular weight (Table 2), the lower the fa value. The average number of carbons per alkyl side chains, nNMR, is given in eq 7:

nNMR )

HR + Hβ + Hγ HR

(7)

The nNMR values are presented in Table 4, and they show that H-O asphaltene has the highest average number of carbons per alkyl side chain, whereas the medium and light oil asphaltenes exhibited a more-orless similar but fewer number of C atoms per alkyl side chain. These results present a clearer picture as to the trend, compared to those from FTIR analysis based on nIR. The percent of substitution of peripheral aromatic carbon, As, can be obtained directly from the ratio of aromatic protons to aromatic carbons, as given in eq 8:

AS )

100 × C1S C1S ) 100 × C1 C1S + C1U

(8)

where As is the average percentage of substitution of aromatic carbon, and C1s is the percentage of sub-

stituted aromatic carbon:

C1S )

C%Cal n

C1U ) 12HarH%

(9) (10)

where C% is the percentage of carbon (w/w%), H% the percentage of hydrogen (w/w%), Cal the 13C NMR intensity of aliphatic carbon, Har the 1H NMR intensity of aromatic hydrogen, and C1 the percentage of nonbridging aromatic carbon.

C1 ) C1S + C1U

(11)

Table 4 shows that L-O asphaltene has the highest As value, whereas H-O has the lowest. These results are in total agreement with those obtained from FTIR results based on the S1H/4H ratio, which is related to both the degree of condensation and the degree of substitution. In the case of NMR analysis, the degree of condensation (Cb/Cnb) represents the ratio between the number of bridging aromatic carbons (Cb) and the number of peripheral nonbridging carbons (Cnb). This ratio gives an indication of the arrangement of aromatic rings in the asphaltene molecule and whether alkyl side chains (bridges) contribute to the overall structure. This parameter can be estimated using the method described by Leo´n,23 as given in eq 12:

1 - Hal Cb 12 ) Cnb HarH% + HR C%

(12)

where Cb/Cnb is the degree of condensation and HR is the 1H NMR intensity of H in R CH3, CH2, and CH. The Cb/Cnb values, as calculated based on eq 12 for asphaltene from the four oils, are given in Table 4. The table shows that Cb/Cnb decreased in the order L-O > M1-O > M2-O > H-O. This result confirms the aromatic carbon fraction (fa) result that was obtained earlier. The higher degree of condensation of L-O asphaltenes than H-O asphaltenes confirms the predominance of polyaromatic structure in the former and aliphatic chains in the latter. It is interesting to observe that all the different formats for measuring the degree of substitution or condensation (As or Cb/Cnb) reported in Table 4 follow the same trend as those obtained in Table 3 using S1H/4H values from FTIR studies. All these techniques show that L-O asphaltene has the highest degree of condensation and is consistent with its high aromaticity. (23) Leo´n, V. Average Molecular Weight of Oil Fractions by Nuclear Magnetic Resonance. Fuel 1987, 66 (10), 1445-1446.



Table 5: Parameters and CHn Abundances from GASPE

13C

NMR Analysisa Value

parameter

physical meaning

L-O

M1-O

M2-O

H-O

nGASPE NR NB C CH CH2 CH3 CH3/CH CH2/CH3

average number of carbon 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

13.5 0.68 1.78 b 0.132 0.688 0.180 1.360 3.834

13.1 0.66 1.74 b 0.133 0.683 0.184 1.386 3.713

13.2 0.65 1.73 b 0.131 0.685 0.184 1.404 3.729

15.6 0.77 2.01 b 0.128 0.715 0.157 1.225 4.550

a

Data given for n-heptane-extracted asphaltenes. b Negligible.

The number of substituent rings, r, represents the relationship between aromatic rings and their ortho, meta, or para position, with respect to each other, and can be obtained based on the relationship given by Cookson and Smith24 in eq 13:

r)

(

)( )

Hγ n-1 + 0.12 Hβ 2

(13)

Table 4 shows values obtained using eq 13 for the four asphaltene samples. This table shows that H-O asphaltene has the highest r value, whereas L-O asphaltenes, as well as M1-O and M2-O asphaltenes, have low values. The higher number of substituent rings for H-O asphaltenes accounts for its larger size and, hence, higher molecular weight, compared to asphaltenes from the other oils. 3.2.3.2. GASPE 13C NMR. This spectral editing technique was used to determine the percentage abundances of different CHn multiplets for asphaltene samples. The following regions were identified in the SEFT 13C NMR spectra: 10-25 ppm, CH + CH3; and 25-50 ppm, C + CH2. Table 5 shows the relative molar abundances of different (CHn) groups, average n-alkane chain length (nGASPE), and the average number of rings per molecule (NR) obtained using the Cookson and Smith24 method, as explained below. The C, CH, CH2, and CH3 groups were obtained in sub-spectra with evolution times of 0.007 s to distinguish methyl and methine (CH + CH3) from methylene and quaternary carbon (CH2 + C), 0.005 s to distinguish between methyl (CH3) and methine (CH) carbon, and 0.004 s to obtain the quaternary resonance (C) only. The ratio CH3/CH is an indicator of the relative abundance of the CH3 group. The higher the CH3/CH ratio is, the greater the contribution of the methyl group bonds, compared to that of aromatic carbons. Low CH3/CH values are attributed to the presence of a cyclic aliphatic structure for which the presence of CH groups does not require two CH3 end groups. The ratio CH2/CH3 is used to give an indication of the methylene contribution, relative to the methyl contribution. Table 5 shows that H-O asphaltene has the highest CH2/CH3 ratio. This is consistent with FTIR results. H-O asphaltenes also have the lowest CH3/CH ratio. This corroborates other results from fa (see Table 4) and S1H/4H (see Table 3), which (24) Cookson, D. J.; Smith, B. E. Determination of Structural Characteristics of Saturates from Diesel and Kerosene Fuels by Carbon-13 Nuclear Magnetic Resonance Spectrometry. Anal. Chem. 1985, 57, 864.

support the assertion that H-O is composed of groups that are cyclic aliphatic (i.e., naphthenic) in nature, whereas L-O has an aromatic, dominant type of structure. The average n-alkane chain length, nGASPE, is given in eq 14.

nGASPE )

3CH2 + 2CH3 CH3

(14)

The nGASPE values from Table 5 follow the same trend as nNMR obtained from NMR analysis. The results show that the lower the average number of carbons per alkyl side chain, the higher the aromaticity (see Table 4). The average number of rings per molecule, NR, based on GASPE can be evaluated using eq 15.

NR ) 0.5nGASPE(2C + CH - CH3) + 1

(15)

The NR results given in Table 5 show that H-O asphaltenes have the highest NR value. The NR results are in total agreement with the r results obtained from the combined 1H and 13C NMR analysis presented previously in Table 4. H-O asphaltenes exhibited the highest number of aromatic rings per molecule, which is consistent with its high molecular weight, as shown in Table 2. However, most of these rings are bridged by an alkyl chain, as shown by its lower degree of condensation (Cb/Cbn) and, consequently, its lower aromaticity. The average number of branches per molecule (NB) gives an average density of branched alkyl side chains present in a given molecule, as defined in eq 16.

NB ) nGASPE(2C + CH)

(16)

The NB values for the asphaltenes samples are presented in Table 5, and the table shows that H-O has the highest NB value, whereas M2-O, L-O, and M1-O have low values. The results support the assertion that H-O asphaltenes that have the highest molecular weight are also the most highly branched. It also confirms that H-O asphaltenes are mostly naphthenic in nature. 3.3. Comparison of Asphaltene/Oil Characteristics and Precipitation Behavior Correlation for CO2-Induced Precipitation with Those for n-Heptane-Induced Precipitation. An attempt was made to determine possible similarities and differences of correlations involving characteristics of both oil and asphaltenes with asphaltene precipitation behavior between CO2-induced precipitation and n-heptaneinduced precipitation. General crude oil characteristics were used, and these included elemental composition,


Ibrahim and Idem

Table 6: Estimates of the Values of Kinetic and Equilibrium Parameters (a) for CO2-Induced Asphaltene Precipitation for Noninhibited Crude Oils during CO2 Flooding at 338 K and (b) for n-Heptane-Induced Asphaltene Precipitation for Noninhibited Crude Oils at 304-338 K Estimates of Parameters parameter ln k m n onset point (mL CO2) ln k0 k ) k0 exp[-E/(RT)] activation energy, Ea (kJ/mol) m n onset point (mL n-heptane)

L-O

M1-O

M2-O

(a) CO2-Induced Asphaltene Precipitation -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

(b) n-Heptane-Induced Asphaltene Precipitation 7.08 ( 5.26 60.55 ( 23.38 6.7 × 10-1 4.03 × 10-24 171.27 ( 27.15 61.01 ( 17.48 2.4 ( 0.3 1.5 ( 0.2 36.1 ( 6.1 46.9 ( 12.6 28.45 25.51

23.41 ( 6.94 4.99 × 10-17 106.83 ( 20.46 1.1 ( 0.1 39.1 ( 6.4 30.30

Table 7: Estimates of the Values of Kinetic and Equilibrium Parameters for (a) CO2-Induced Asphaltene Precipitation for Inhibited Crude Oils during CO2 Flooding at 338 K and (b) for n-Heptane-Induced Asphaltene Precipitation for Inhibited Crude Oils at 338 K Estimates of Parameters parameter and respective inhibitor

L-O

M1-O

M2-O

(a) CO2-Induced Asphaltene Precipitation ln k dodecylbenzenesulfonic acid, DDBSA nonyl phenol, NP toluene m dodecylbenzenesulfonic acid, DDBSA nonyl phenol, NP toluene n dodecylbenzenesulfonic acid, DDBSA nonyl phenol, NP toluene onset point (mL CO2) dodecylbenzenesulfonic acid, DDBSA nonyl phenol, NP toluene

-32.96 ( 1.45 -37.05 ( 1.45 -324.26 ( 79.23

-109.37 ( 81.70 -76 ( 14.17 -136.12 ( 5.37

-100.7 ( 37.1 -58.77 ( 8.96 -5.44 ( 1.04 × 10-12

1.47 ( 0.04 0.97 ( 0.05 4.97 ( 1.32

2.6 ( 1.65 1.10 ( 0.26 1.85 ( 0.092

3.26 ( 0.35 0.26 ( 0.053 -6.4 × 10-15 ( 5.7 × 10-15

69.16 ( 3.13 55.71 ( 2.86 521.93 ( 132.12

220.7 ( 167.69 144.26 ( 29.26 270.45 ( 11.34

192.54 ( 62.23 80.65 ( 13.69 -2.4 × 10-12 ( 1.71 × 10-12

11.85 12.30 13.50

11.75 12.00 11.55

12.65 13.35 12.60

(b) n-Heptane-Induced Asphaltene Precipitation ln k dodecylbenzenesulfonic acid, DDBSA nonyl phenol, NP toluene m dodecylbenzenesulfonic acid, DDBSA nonyl phenol, NP toluene n dodecylbenzenesulfonic acid, DDBSA nonyl phenol, NP toluene onset point (mL n-heptane) dodecylbenzenesulfonic acid, DDBSA nonyl phenol, NP toluene

-147.81( 19.63 -83.75 ( 3.54 -98.98 ( 14.92

-163.07 ( 2.37 -134.27( 17.79 -103.11 ( 38.79

-14.68 ( 0.005 -177.99 ( 30.81 -146.07 ( 93.16

4.38 ( 0.52 2.78 ( 0.09 2.76 ( 0.33

2.18 ( 0.023 1.91 ( 0.18 1.83 ( 0.39

4.53 × 10-5 ( 6.42 × 10-5 1.86 ( 0.23 1.84 ( 0.66

75.59 ( 9.64 43 ( 1.74 53.91 ( 7.79

88.31 ( 1.27 73.91 ( 9.61 55.30 ( 20.82

-0.998 ( 0.0026 117.37 ( 20.03 90.46 ( 57.84

30.37 29.90 30.50

25.65 26.20 25.85

30.35 33.45 31.10

molecular weight, viscosity, density, specific gravity, metal content, SARA analysis, and heteroatom content, whereas the asphaltene characteristics used were the average number of carbon per alkyl side chain (n), the methylene-to-methyl ratio (CH2/CH3), aromatic carbon fraction (fa), the degree of condensation (Cb/Cnb). On the other hand, precipitation behavior, evaluated in terms of equilibrium and kinetic parameters for the three noninhibited crude oils, are summarized in Table 615,16 and for the inhibited oils in Table 7.17,18 3.3.1. Dependence of Asphaltene Precipitation Rate on Asphaltenes Content in the Oil (m). 3.3.1.1. CO2-Induced Asphaltene Precipitation. Figure 2 shows the relation-

ship between m and asphaltene molecular weight, as well as that between m and total metals content in the asphaltene. The figure shows that the rate dependence of asphaltene precipitation on asphaltene content in the oil decreases as both the asphaltene molecular weight and total metals content increase. Another set of characteristics that were evaluated was the carbonyl abundances index (CdO), as well as the propensity for aggregation of asphaltenes through hydrogen bonding. Their relationships with m are given in Figure 3, which shows that m decreases as I3435/I3050 increases but increases as CdO increases. The results for m versus I3435/I3050 and CdO show that a higher propensity for



Figure 4. Variation of asphaltene precipitation rate dependence on asphaltene content (m) with the average number of carbons per alkyl side chain of the asphaltenes molecule ((×) nNMR and (O) nGASPE) for CO2-induced asphaltene precipitation. Figure 2. Variation of asphaltene precipitation rate dependence on asphaltene content (m) with (O) asphaltene molecular weight and (4) total metals content for CO2-induced asphaltene precipitation.

Figure 3. Variation of asphaltene precipitation rate dependence on asphaltene content (m) with (O) the propensity of asphaltenes molecules for aggregation (I3435/I3050) and (4) the carbonyl abundance index (CdO) for CO2-induced asphaltene precipitation.

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 nGASPE) 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 CO2induced precipitation. 3.3.1.2. n-Heptane-Induced Asphaltene Precipitation. Figure 5 relates m to the heteroatom content and the average percentage substitution of peripheral aromatic

Figure 5. Variation of asphaltene precipitation rate dependence on asphaltene content (m) with (O) the heteroatoms content and (4) the percentage substitution of peripheral carbons of asphaltenes (As) for n-heptane-induced asphaltene precipitation.

carbon (As). The figure shows that m increases with As, indicating the detrimental effect on the n-heptaneinduced asphaltene precipitation kinetics of an increase in As. On the other hand, the figure shows that m decreases as the total heteroatom content increases. This latter result shows that an increase in heteroatom content is actually beneficial to the kinetics of n-heptane (i.e., light paraffin hydrocarbon)-induced asphaltene precipitation. Some crude oil characteristics were considered for possible direct relationships with precipitation behavior parameters. One such characteristic was the asphaltene content of the oil. According to Table 1, this content increased in the order L-O < M1-O < M2-O, whereas Table 6a shows that m decreased in the order L-O > M1-O > M2-O. A plot of the rate dependence of the asphaltene precipitation (m) on the asphaltene content


Figure 6. Variation of asphaltene precipitation rate dependence on asphaltene content (m) with (×) asphaltene content and (O) CH2/CH3 and (4) RCH2/CH3 ratios of the asphaltene molecules for n-heptane-induced asphaltene precipitation.

in the oil, and the asphaltene content, is given in Figure 6. This figure shows that m, and, therefore, the rate, decreases as the asphaltene content of the oil increases. Similar results and arguments are applicable to other oil characteristics such as total heteroatoms content and density of the crude oil. Figure 6 also presents the variation of m with the alkyl side-chain length of the asphaltene molecules (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 alkyl side chain length, the smaller the effect of m on the kinetics. This assertion was verified by making a plot of the variation of m with the degree of branching of the alkyl side chain of the asphaltene molecules (NB), as illustrated in Figure 7, which also contains relationships of m 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 m increases with NB, fa, NR, and Cb/Cbn. In the case of m versus NB, the result confirms our earlier assertion that m decreases as the amount of normal alkyl side chains (i.e., degree of straightness of the alkyl side chain) increases (see Figure 6), which, conversely, implies that m 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 relation with m indicate that m increases as the aromaticity of the asphaltenes increases. 3.3.1.3. Summary. The results on the rate dependence of the asphaltene precipitation on the asphaltene content shows that, for CO2 (i.e., polar and acidic precipitating agent)-induced asphaltene precipitation, the detrimental factor is essentially the parafinicity of the asphaltenes. In contrast, the polar indices, such as the metals content and the propensity for aggregation, are actually beneficial. On the other hand, for n-heptane (i.e., paraffinic precipitating agent)-induced asphaltene precipitation, the detrimental factor is a high aroma-

Ibrahim and Idem

Figure 7. Variation of asphaltene precipitation rate dependence on asphaltene content (m) with various asphaltene characteristics: (O) fa, aromatic carbon fraction; (×) NR, the average number of rings per molecule; (4) Cb/Cnb, the degree of condensation; and (0) NB, the degree of branching of the alkyl side chains for n-heptane-induced asphaltene precipitation.

Figure 8. Variation of asphaltene precipitation rate dependence on CO2 added (n) with (×) asphaltene content, and (O) CH2/CH3 and (4) RCH2/CH3 ratios of the asphaltene molecules.

ticity of the asphaltenes. In contrast, the beneficial factors are the heteroatom content of the asphaltenes and the asphaltenes content of the oil. The implication of the results is that the parameters that trigger asphaltene precipitation for CO2-induced precipitation are completely different from those for n-heptaneinduced precipitation. 3.3.2. Dependence of Asphaltene Precipitation Rate on Content of Flooding (i.e., Precipitating) Agent in the Oil (n). The trends for n in CO2-induced asphaltene precipitation were very similar to the trends for m in n-heptane-induced asphaltene precipitation (see Section 3.3.1.2), meaning that n correlated in the same way with all the parameters with which m correlated. Examples include Figures 8 (which shows the variation of the rate dependence of the asphaltene precipitation rate for CO2 added (n) with the asphaltene content and the CH2/CH3 and RCH2/CH3 ratios of the asphaltene molecules) and


Figure 9. Variation of asphaltene precipitation rate dependence on CO2 added (n) with various asphaltene characteristics: (O) fa, aromatic carbon fraction; (×) NR, the average number of rings per molecule; (4) Cb/Cnb, the degree of condensation; and (0) NB, the degree of branching of the alkyl side chains.


Figure 11. Variation of asphaltene precipitation rate dependence on n-heptane added (n) with (O) the propensity of asphaltenes molecules for aggregation (I3435/I3050) and (4) the carbonyl abundance index (CdO).

the asphaltene precipitation on the n-heptane added (n) with the propensity of asphaltenes molecules for aggregation (I3435/I3050) and the carbonyl abundance index (CdO)). These results clearly show that there is a huge difference in the nature of the influence of parameters, and, as such, the mechanism of precipitation between CO2-induced asphaltene precipitation and n-heptaneinduced precipitation. In our previous work,16 we proposed a possible reaction scheme for asphaltene precipitation induced by a precipitating agent such as n-heptane and CO2. In this scheme, a polymerizationtype kinetics that involve an initiation step, propagation steps, and a termination step was the most suitable. The following are the steps in the postulated reaction scheme:

As + H f As-H*

Figure 10. Variation of asphaltene precipitation rate dependence on n-heptane added (n) with (O) the asphaltene molecular weight and (4) the total metals content.

(initiation)

(17)

As-H* + H f As-H2*

(propagation) (18)

As-H2* + H f As-H3*

(propagation) (19)

As-Hn-1* + H f As-Hn*

(propagation) (20)

As-Hn* + (m - 1)H f Asm - Hn*

(propagation) (21)

9 (which shows the variation of the rate dependence of the asphaltene precipitation for CO2 added (n) with the aromatic carbon fraction (fa), the average number of rings per molecule (nNMR), the degree of condensation, and the degree of branching of the alkyl side chains). Accordingly, n for CO2-induced asphaltene precipitation increased as the aromaticity of the asphaltene increased and decreased as the heteroatom content increased. On the other hand, n for n-heptane-induced asphaltene precipitation exhibited trends with asphaltenes/oil characteristics similar to those with m for CO2-induced asphaltene precipitation (see Section 3.3.1.1). Examples include Figure 10 (which shows the variation of the rate dependence of the asphaltene precipitation on the n-heptane added (n) with the asphaltene molecular weight and the total metals content) and Figure 11 (which shows the variation of the rate dependence of

Asm-Hn* f Apm + nH

(termination) (22)

The overall stoichiometric reaction is given in reaction 23:

mAs + nH f Apm + nH

(23)

In reactions 17-23, As represents a reacting asphaltene molecule, H represents a n-heptane or CO2 molecule, Apm represents a precipitated or polycondensed asphaltene molecule, and species with asterisks (such as AsH* and As-Hn*) represent activated complexes. This reaction scheme satisfied both the rate model and the physical phenomenon that occurs during n-heptane- or CO2-induced asphaltene precipitation. This implies that it requires a stepwise addition of one n-heptane or CO2 molecule n times to destabilize the delicate balance that


Figure 12. Variation of the onset point for asphaltene precipitation (w) with (O) the asphaltenes molecular weight and (4) the total metals content for CO2-induced asphaltene precipitation.

keeps the asphaltene molecules in solution or dispersion as fine particles. After this balance is disrupted, m - 1 (i.e.,

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