Interrelationships between Asphaltene Precipitation Inhibitor

1038

Energy & Fuels 2004, 18, 1038-1048

Interrelationships between Asphaltene Precipitation Inhibitor Effectiveness, Asphaltenes Characteristics, and Precipitation Behavior during n-Heptane (Light Paraffin Hydrocarbon)-Induced Asphaltene Precipitation Hussam H. Ibrahim and Raphael O. Idem* Process and Petroleum Systems Engineering Laboratory, Faculty of Engineering, University of Regina, 3737 Wascana Parkway, Regina, SK, Canada S4S 0A2 Received August 21, 2003. Revised Manuscript Received April 7, 2004

Three carefully chosen chemicalssdodecylbenzenesulfonic acid (DDBSA), nonyl phenol (NP), and tolueneswere studied for their asphaltene precipitation inhibition effectiveness during lightparaffin-hydrocarbon-induced asphaltene precipitation of three Saskatchewan crude oils, as well as to evaluate possible interrelationships between their inhibition effectiveness, asphaltene precipitation behavior (in terms of kinetics and equilibrium), and crude oil/asphaltene characteristics. Results showed that asphaltene precipitation rate dependence on asphaltene content (m) was a strong function of the content of heteroatoms (nitrogen (N), sulfur (S), and oxygen (O)) of both the crude oil and asphaltenes, as well as the aromatic carbon fraction and degree of branching of the alkyl side chain of the asphaltene molecules. On the other hand, the asphaltene precipitation rate dependence on the amount of n-heptane (i.e., light paraffin hydrocarbon) added (n), the frequency factor (k0), and the activation energy for asphaltene precipitation (Ea) were strong functions of the paraffin fraction of the asphaltenes and the propensity of the asphaltene molecules for aggregation. Furthermore, the equilibrium parameter (onset point) increased as the paraffin fraction of the asphaltene molecules increased but decreased as the iron content of the oil increased. DDBSA was more effective with the least-aromatic medium oil, in terms of the kinetic parameters m and n, whereas it was more effective with the more-aromatic oil, in terms of the equilibrium parameter. A significant benefit obtained with NP and toluene was the drastic reduction of the rate constant (k), which resulted in a decrease in the overall rate of asphaltene precipitation. NP exhibited the maximum inhibition efficiency (∼10%), in terms of the onset point on the most-stable oil with the lowest iron content, and highest average number of carbons per alkyl side chain (i.e., high paraffin fraction) of the asphaltene molecules.

1. Introduction Many light and medium reservoirs are subjected to 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 other methods of flooding.3 However, flooding processes cause several changes in the flow and phase behavior of the reservoir fluids and can significantly alter the formation properties with the resultant increase in the propensity for precipitation of organic solids, mainly asphaltenes.4 Asphaltene pre* Author to whom correspondence should be addressed. Fax: (306) 585-4855. E-mail address: [email protected]. (1) Moritis, G. New Technology, Improved Economics, Boost EOR Hopes. Oil Gas J. 1996, 94, 39. (2) Saskatchewan Energy & 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.

cipitation can change the wettability of the reservoir matrix and consequently affect the flood performance.5 It can also cause formation damage and wellbore plugging, requiring expensive treatment and clean-up procedures.6-10 According to von Albretch,11 the most effective asphaltene-precipitation preventive action is reservoir pressure maintenance above the asphaltene precipita(4) 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. (5) Buckley, J. S. Asphaltene Precipitation and Crude Oil Wetting. SPE Adv. Technol. Ser. 1995, 53. (6) 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 in Villahermosa, Tabasco, Mexico, March 5-7, 1996, Paper No. SPE35342. (7) 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, Alaska, May 26-28, 1993, Paper No. SPE 26046. (8) 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, September 23-26, 1990, Paper No. SPE 20530. (9) 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.

10.1021/ef0340460 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/03/2004

n-Heptane-Induced Asphaltene Precipitation

tion 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.12-14 These products can keep the small initially formed 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, structural characteristics of asphaltenes,15,16 and the effectiveness of chemical additives as asphaltene precipitation inhibitors.17,18 However, no studies that investigate possible interrelationships between these parameters, in regard to their effect on light hydrocarbon or CO2 flooding, have been reported in the literature. Also, 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).11,19 However, because asphaltene precipitation can be considered to be a rate process,20-24 the effectiveness of the chemical additives could also be evaluated, in terms of the kinetic parameters. Therefore, it would be essential, as (10) 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. (11) 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. (12) Broaddus, G. Well- and Formation-Damage Removal with Nonacid Fluids. J. Pet. Technol. 1988, 40 (6), 685. (13) 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. (14) Acosta, A. Efecto de las Resinas en la Deposicion de Asfaltenos, Trabajo Especial de Grada. Escuela de Ingenieriay Ciencias Aplicadas, University de Oriente: Venezuela, 1981. (15) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Structural Characterization of Asphaltenes of Different Origins. Energy Fuels 1995, 9, 225. (16) Yen, T. F. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1979, 24, 901. (17) Gonzalez, G.; Middea, A. Peptization of Asphaltene by Various Oil Soluble Amphiphiles. Colloids Surf. 1991, 52, 207. (18) 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 1994a, 10, 1749. (19) Escobedo, J.; Mansoori, G. A. Viscometric Determination of the Onset of Asphaltene Flocculation: A Novel Methodology. SPE Prod. Facil. 1995, 10 (May), 115. (20) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura, M. Structure and Reactivity of Petroleum-Derived Asphaltene. Energy Fuels 1999, 13, 287. (21) Priyanto; S.; Mansoori, G. A.; Suwono, A. Structure and Properties of Micelles and Micelle Coacervates of Asphaltene Macromolecule. Presented at 2001 AIChE Annual Meeting, Session 90s Nanotools. (22) Permsukarome, P.; Chang, C.; Foggler, H. S. Kinetic Study of Asphaltene Dissolution in Amphiphile/Alkane Solutions. Ind. Eng. Chem. Res. 1997, 36, 3960. (23) 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. (24) 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 (n-Heptane) Flooding, submitted to Fuel, 2004.

Energy & Fuels, Vol. 18, No. 4, 2004 1039

well as informative, to correlate the effectiveness of the chemical additives directly 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 hydrocarbon 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 asphaltene samples that were redissolved in toluene and/or heptane.20,21 The kinetics of n-heptane-induced asphaltene dissolution has also been the subject of more recent studies.22,23 However, the kinetics of 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 involved with using the conventional techniques to evaluate the kinetics. In a recent study24 we developed a technique based on a molar programmed titration of the oil with nheptane to simulate light hydrocarbon flooding, to evaluate the kinetics of light-paraffin-hydrocarboninduced precipitation of asphaltene from three Saskatchewan crude oilssnamely L-O, M1-O, and M2-Os in which the kinetic data were fitted to an empirical power law model. This power law was of the form

rA )

( )

Ea m n β dNAp ) kO exp N N MO dNnC7 RT Ao nC7

(1)

where rA is the rate of n-heptane-induced precipitation of asphaltenes from crude oil (measured in terms of moles of asphaltene per mole of crude oil per minute), dNAp/dNnC7 represents the asphaltene precipitation as a function of n-heptane added, β is the molar programmed rate of titration or addition of n-heptane (β ) dNnC7/dt), Mo is the number of moles of crude oil charged to the sample cell, K0 is the pre-exponential constant (units are dependent on the values of m and n), Ea is the activation energy (in J/mol), R is the universal gas constant (8.314 J mol-1 K-1), NAo is the number of moles of asphaltene in the oil at any time, NnC7 is the number of moles of n-heptane in the oil at any time, m represents the rate dependence of asphaltene precipitation on asphaltene content in the oil, and n is the rate dependence of asphaltene precipitation on n-heptane content in the oil (introduced during flooding). This work enabled us to determine n-heptane (i.e., light paraffin hydrocarbon)-induced asphaltene precipitation behavior, in terms of equilibrium (i.e., the onset point of precipitation, w) and kinetic (i.e., k0, m, n, and Ea) parameters of the tertiary oil production process involving hydrocarbon flooding. Also, in a more recent work,25 we evaluated the interrelationships between asphaltene precipitation inhibitor effectiveness, asphaltene characteristics, and precipitation behavior during CO2 miscible flooding, using three carefully chosen chemicals as additives for asphaltene precipitation inhibition. In the present work, we are evaluating the inhibition effectiveness of these (25) Ibrahim, H. H.; Idem, R. O. CO2 Miscible Flooding for Three Saskatchewan Crude Oils: Interrelationships between Asphaltene Precipitation Inhibitor Effectiveness, Asphaltenes Characteristics, and Precipitation Behavior. Energy Fuels 2004, 18, 743-754.

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three chemicals during hydrocarbon flooding for three Saskatchewan crude oils and their relationships with asphaltene characteristics and precipitation behavior. The results are presented and discussed in this paper. 2. Experimental Section 2.1. Crude Oil Samples and Their Corresponding Asphaltenes. Three crude oil samples (one light oil (L-O) and two medium oils (M1-O and M2-O)) from three different reservoirs were used for this study. Their identities have been described in detail in our earlier work.24,26 Asphaltene samples were extracted from each oil sample, and the extracted samples were then thoroughly characterized. Details on the extraction procedure are given elsewhere.25 In the case of characterization, all the crude oils and their n-heptane-derived asphaltenes were characterized using a multitechnique approach including carbon (C), hydrogen (H), nitrogen (N), sulfur (S), and oxygen (O) (i.e., CHNS-O) analysis, metal content, 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) spectroscopy, as described by Ibrahim and Idem.26 2.2. Inhibitors and Stabilizers. Two nonaromatic-based surfactants or inhibitorssnamely, dodecylbenzenesulfonic acid (DDBSA) and nonyl phenol (NP)sin addition to a well-known aromatic-based solvent (toluene) 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) alkyl side-chain length, and (iv) polarity (DDBSA > NP > toluene). According to the literature,27,28 a typical range of concentrations of inhibitors used is 500-3000 ppm. We decided to use 1000 ppm for all our runs. 2.3. Effectiveness of Additives as Asphaltene Precipitation Inhibitors or Stabilizers during Hydrocarbon Flooding. The effectiveness of the chemicals as inhibitors or stabilizers for asphaltene precipitation was evaluated by comparing the asphaltene precipitation behavior during light hydrocarbon flooding for the three oils with and without chemical additives or inhibitors. 2.3.1. Equipment. Asphaltene precipitation behavior during light hydrocarbon flooding was evaluated under isothermal and isobaric conditions for both noninhibited and inhibited crude oils for the three crude oils. Light hydrocarbon flooding was simulated using a molar n-heptane programmed titration technique24 in a solid 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 preprecipitated and toluene-redissolved asphaltene samples that are usually used in most reported experiments to circumvent the problem of opaqueness of the oil samples.6,27 2.3.2. 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 (26) Ibrahim, H. H.; Idem, R. O. Structural and Molecular Characteristics of Asphaltenes from Various Saskatchewan Crude Oils, submitted to Energy Fuels, 2003. (27) Aquino-Olivos, M. A.; Buenrostro-Gonzalez, E.; Anderson, S. I.; Lira-Galeana, C. Investigations of Inhibition of Asphaltene Precipitation at High-Pressure Using Bottomhole Samples. Energy Fuels 2001, 15 (1), 236.

Ibrahim (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 n-heptane 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 n-heptane 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 n-heptane from the solvent cylinder to the PVT (sample) cell, the pressures in the lines connecting the n-heptane cylinder to the sample cell were equilibrated to avoid any backflash. The system was then opened to the backpressure 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. 2.3.3. Procedure for Stabilized Oil (i.e., with Inhibitor). In the case for inhibited oil, a known amount of inhibitor (1000 ppm) was added to the crude oil under ambient conditions. 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. 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. 2.3.4. Typical Experimental Run. Each experimental run involved a programmed addition of pressure- and temperatureequilibrated n-heptane transferred from the solvent cylinder to the crude oil sample contained in the sample cell. The contents of the sample cell were vigorously stirred at a rate of 2400 rpm, to prevent the settling of any solids formed during n-heptane titration or addition in the cell. The addition of n-heptane continued beyond 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 n-heptane 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 the amount of n-heptane added. A programmed n-heptane flow rate of 0.5 mL/min was used in the presence 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,24 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 lighthydrocarbon-induced asphaltene precipitation.

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 2.26 Characteristics in these tables include the (28) Garcia, M. C.; Carbognani, L. Asphaltene-Paraffin Structural Interactions. Effect on Crude Oil Stability. Energy Fuels 2001, 15 (5), 1021.



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

L-O

M1-O

M2-O

85.4 12.9 0.1 1.5 0.4 2.0 372.8 10.7 0.9 33.9

85.1 12.8 0.2 2.2 0.4 2.8 403.5 9.2 0.9 32.7

84.5 12.3 0.2 3.0 0.4 3.6 398.3 24.0 0.9 28.3

2.0 2.0 3.1 7.1 45.5 17.7 8.6

16.0 16.0 24.0 56.0 37.3 22.6 17.2

0.6 17.0 31.0 48.6 N/Pa N/Pa N/Pa

1.2 1.8

3.2 3.8

4.8 5.2

Not performed.

elemental composition, molecular weight, viscosity, density, specific gravity, metals content, SARA analysis, the average number of carbons per alkyl side chain (n), the methylene-to-methyl ratio (CH2/CH3), the aromatic carbon fraction (fa), and the degree of condensation (Cb/ Cbn). The equilibrium and kinetic parameters for nheptane-induced asphaltene precipitation for three non-

inhibited crude oils are summarized in Table 3A,24 whereas similar parameters for inhibited oils are given in Table 3B. The relationships given in Figures 1-11 were derived by 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 Asphaltene Content in the Oil (m). Figure 1 relates m to the heteroatom content and the average percentage substitution of peripheral aromatic carbons (As%). Figure 1 shows that m increases as As% increases, indicating the detrimental effect on n-heptane-induced asphaltene precipitation kinetics of an increase in As%. On the other hand, the figure shows that m decreases with an increase in the total heteroatom content. This latter result shows that an increase in heteroatoms 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 of such characteristics 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 3a shows that m decreased in the order L-O > M1-O > M2-O. A plot of asphaltene precipitation rate dependence on asphaltene content in the oil (m) with the asphaltenes content is given in Figure 2. This figure shows that msand, therefore, the ratesdecreases as the asphaltenes content of the oil increases. This is an interesting result for n-heptane-induced asphaltene

Table 2. Asphaltenes Characteristics Asphaltenea characteristic composition (wt %) carbon hydrogen nitrogen sulfur oxygen total heteroatoms (wt %) molecular weight (g/g-mol) metal content (µg/g) iron nickel vanadium total metals nc CdO RCH2/CH3 (by FTIR) I3435/I3050 S1H/4H CH2/CH3 fa nNMR As (%) r Cb/Cnb nGASPE NR NB C CH CH2 CH3 CH3/CH CH2/CH3 (by NMR) a

physical meaning

average number of carbon per alkyl side chain, from FTIR empirical index of carbonyl abundances, from FTIR molar ratio of CH2 and CH3 groups, from FTIR propensity for hydrogen bond formation (aggregation) ratio of intensities of aromatic C-H out-of-plane deformation with one adjacent proton to four adjacent protons methylene-to-methyl group ratio aromatic carbon fraction average number of carbons per alkyl side chain, from NMR average percentage of substitution of aromatic carbon number of substituent rings degree of condensation 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

n-Heptane-extracted asphaltenes.

L-O

M1-O

M2-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

79 100 140 319 2.69 0.48 2.34 2.10 1.40

260 290 440 990 2.58 0.35 2.46 4.33 1.20

62 240 410 712 2.97 0.36 2.62 3.6 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

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


Ibrahim

Table 3. Estimates of the Values of Kinetic and Equilibrium Parameters for n-Heptane-Induced Asphaltene Precipitation for (A) Noninhibited Crude Oils at Temperatures of 304-338 K and (B) Inhibited Crude Oils at a Temperature of 338 K Estimated Value parameter ln k0 k ) k0 exp[-(E/RT)] Ea (kJ/mol) m n onset point (mL n-heptane)

ln k m n onset point (mL n-heptane) a

inhibitor

L-O

M1-O

(A) Noninhibited Crude Oils, at 304-338 K 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 DDBSAa NPb toluene DDBSAa NPb toluene DDBSAa NPb toluene DDBSAa NPb toluene

(B) Inhibited Crude Oils, at 338 K -147.81 ( 19.63 -163.07 ( 2.37 -83.75 ( 3.54 -134.27 ( 17.79 -98.98 ( 14.92 -103.11 ( 38.79 4.38 ( 0.52 2.18 ( 0.023 2.78 ( 0.09 1.91 ( 0.18 2.76 ( 0.33 1.83 ( 0.39 75.59 ( 9.64 88.31 ( 1.27 43 ( 1.74 73.91 ( 9.61 53.91 ( 7.79 55.30 ( 20.82 30.37 25.65 29.90 26.20 30.50 25.85

M2-O 23.41 ( 6.94 4.99 × 10-17 106.83 ( 20.46 1.1 ( 0.1 39.1 ( 6.4 30.30 -14.68 ( 0.005 -177.99 ( 30.81 -146.07 ( 93.16 4.53 × 10-5 ( 6.42 × 10-5 1.86 ( 0.23 1.84 ( 0.66 -0.998 ( 0.0026 117.37 ( 20.03 90.46 ( 57.84 30.35 33.45 31.10

Dodecylbenzenesulfonic acid. b Nonyl phenol.

Figure 1. Variation of asphaltene precipitation rate dependence on asphaltene content (m) with (O) heteroatoms content and (4) percentage substitution of peripheral C atoms of asphaltenes (As%).

precipitation and represents the first attempt at confirming what influence the asphaltenes content of the oil has on the kinetics of asphaltene precipitation during light hydrocarbon flooding. Similar results and arguments are applicable to other oil characteristics such as total heteroatom content and density of the crude oil. Figure 2 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 is, the smaller the effect m has 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

Figure 2. Variation of asphaltene precipitation rate dependence on asphaltene content (m) with (×) asphaltene content and the (O) CH2/CH3 and (4) RCH2/CH3 ratios of the asphaltene molecules.

asphaltene molecules (NB), as illustrated in Figure 3, 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 as NB, fa, NR, and Cb/Cbn each increase. 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 2), 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 a general measure of the aromaticity of the asphaltenes, results of their relation with m indicate that m increases as the aromaticity of the asphaltenes increases.


Figure 3. Variation of asphaltene precipitation rate dependence on asphaltene content (m) with various asphaltene characteristics ((O) fa, aromatic carbon fraction; (×) NR, average number of rings per molecule; (4) Cb/Cbn, degree of condensation; and (0) NB, degree of branching of the alkyl side chains).


Figure 5. Variation of asphaltene precipitation rate dependence on n-heptane added (n) with (O) the propensity of the asphaltene molecules for aggregation (I3435/I3050) and (4) the carbonyl abundances index (CdO).

Figure 6. Variation of asphaltene precipitation rate dependence on n-heptane added (n) with the average number of carbons per alkyl side chain of the asphaltene molecule ((4) nNMR and (O) nGASPE). Figure 4. Variation of asphaltene precipitation rate dependence on n-heptane added (n) with (O) asphaltene molecular weight and (4) total metals content.

3.1.2. Asphaltene Precipitation Rate Dependence on n-Heptane Content in the Oil (n). Figure 4 shows the variation of n with the asphaltene molecular weight and total metals content in the asphaltene. The figure shows that the asphaltene precipitation rate dependence on n-heptane (i.e., light paraffin hydrocarbon) added to the oil (n) decreases as both the asphaltene molecular weight and the total metals content increase. Other characteristics evaluated were the carbonyl abundances index (CdO, as defined in our previous work26) and 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). Their relationships with n are given in Figure 5, which shows that n decreases as Cd O increases but increases as I3435/I3050 increases. This is in contrast to our results for CO2-induced asphaltene precipitation.25 The results for n versus I3435/I3050 and CdO show that a higher propensity for aggregation is not beneficial, in terms of the kinetics of light-paraffinhydrocarbon-induced asphaltene precipitation, whereas a higher CdO content is. On the other hand, n versus the average number of carbons per alkyl side chain (measured as nNMR and nGASPE26) in Figure 6 shows that an increase in the number of carbons in the alkyl side chain decreases n. This is beneficial to the kinetics of light-paraffinhydrocarbon-induced asphaltene precipitation. Because


Figure 7. Variation of asphaltene precipitation rate dependence on n-heptane added (n) with the number of substituent rings in the asphaltene molecule (r).

Figure 8. Variation of the frequency factor for asphaltene precipitation (ln k0) with (O) the molecular weight and (4) the total metals content of the asphaltenes.

a larger number of carbons in the alkyl side chain implies higher paraffinicity, the result shows that the higher the paraffinic content of the asphaltene, the lower the contribution of light paraffin hydrocarbon added to the oil to the rate of asphaltene precipitation. Figure 7 gives the variation of n with the number of substituent rings in the asphaltene molecule. The figure shows that n decreases as r increases. Because r is an indication of how bulky the asphaltene molecule is (i.e., how large the molecular weight is), the results confirm our earlier result (Figure 4), which showed that n increases as the molecular weight increases. 3.1.3. Pre-exponential Constant (Frequency Factor) of Asphaltene Precipitation. The variation of the frequency factor (evaluated as ln k0) with various asphaltene characteristics are presented in Figures 8-11. The characteristics are molecular weight and total metals

Ibrahim

Figure 9. Variation of the frequency factor for asphaltene precipitation (ln k0) with (O) the propensity of asphaltene molecules for aggregation (I3435/I3050) and (4) the carbonyl abundances index (CdO).

Figure 10. Variation of the frequency factor for asphaltene precipitation (ln k0) with the average number of carbons per alkyl side chain of the asphaltene molecule ((4) nNMR and (O) nGASPE).

content (Figure 8), propensity for aggregation (I3435/I3050) and carbonyl abundances index (CdO) in Figure 9, number of carbons per alkyl side chain (in terms of nNMR and nGASPE) in Figure 10, and the number of substituent rings (r) in Figure 11. The frequency factor represents the frequency of collision between asphaltene molecules and n-heptane (i.e., light paraffin hydrocarbon) molecules to induce asphaltene precipitation. Figure 8 shows that the value of ln k0 increases as both the molecular weight and total metals content of asphaltenes increase. Also, Figure 9 shows that the value of ln k0 increases as the value of I3435/I3050 increases but decreases sharply with an increase in CdO. In the case of Figure 10, the results show that the value of ln k0 decreases very sharply as the number of carbons per



Figure 11. Variation of the frequency factor for asphaltene precipitation (ln k0) with the number of substituent rings in the asphaltene molecule (r). Figure 13. Variation of the activation energy for asphaltene precipitation (Ea) with (O) the propensity of asphaltene molecules for aggregation (I3435/I3050) and (4) the carbonyl abundances index (CdO).

Figure 12. Variation of the activation energy for asphaltene precipitation (Ea) with (O) the molecular weight and (4) total metals content of the asphaltenes.

alkyl side chain increases. This is favorable, in terms of the kinetics of n-heptane-induced asphaltene precipitation. Because a larger number of carbons in the alkyl side chain implies a higher paraffin fraction, the result shows that the higher the paraffin content of the asphaltenes, the smaller the contribution of the frequency factor to the rate of light-paraffin-hydrocarboninduced asphaltene precipitation. Finally, Figure 11 shows that the value of ln k0 decreases as the number of substituent rings in the asphaltene molecules increases, similar to the trend for n versus this parameter (r). 3.1.4. Activation Energy. Figure 12 illustrates typical variations of the activation energy (Ea) with molecular weight and the total metals content of the asphaltenes. The activation energy is an indication of the temperature sensitivity of the asphaltene precipitation process. The result shows that Ea is inversely related to both the molecular weight and the total metals content. This implies that the lower the asphaltene molecular weight, the more temperature-sensitive the oil is to precipita-

tion. In other words, asphaltene precipitation from light oils is more temperature-sensitive, as compared to heavier oils. Results from this figure also imply that the presence of large amounts of metals in the asphaltenes is beneficial, in that it decreases the temperature sensitivity of the oil, in regard to asphaltene precipitation. Figure 13 shows the variation of Ea with the propensity for aggregation of asphaltene molecules through hydrogen bond formation (I3435/I3050) and the index of carbonyl abundances (CdO). The results show an inverse relationship between Ea and I3435/I3050 but a direct relationship between Ea and CdO. The figure suggests that the temperature sensitivity decreases as the propensity for asphaltene aggregation increases. In contrast, the figure illustrates that temperature sensitivity of the precipitation process increases with CdO. Figure 14 shows the variation of the activation energy with the average number of carbon per alkyl side chain obtained using GASPE technique (nGASPE), as well as that obtained using NMR spectroscopy (nNMR).26 The results show that the activation energy is directly proportional to the average number of carbons per alkyl side chain. Because a larger number of carbons per alkyl side chain indicates increased paraffinicity, the results imply that asphaltene precipitation becomes more sensitive to temperature as the asphaltene molecule becomes more paraffinic. Thus far, we have only looked at how various asphaltene and oil characteristics influence the kinetic parameters in light-paraffin-hydrocarbon-induced asphaltene precipitation. This has made it possible to identify the oil/asphaltenes characteristics that affect the kinetic parameters (m, n, ln k0, Ea) positively, in which an increase in the value of the characteristic contributes to a reduction of the rate of light-paraffinhydrocarbon-induced asphaltene precipitation, according to eq 1. We have also identified those factors that influence the kinetic parameters negatively.


Ibrahim Table 4. Effectiveness of Dodecylbenzenesulfonic Acid (DDBSA) during n-Heptane Flooding

∆m/m ∆n/n ∆k/k ∆w/w

Figure 14. Variation of the activation energy for asphaltene precipitation (Ea) with the average number of carbons per alkyl side chain of the asphaltene molecule ((4) nNMR and (O) nGASPE).

Figure 15. Variation of the onset point for asphaltene precipitation (w) with (×) the iron content in the oil and the average number of carbons per alkyl side chain ((O) nNMR and (4) nIR) of the asphaltene molecule.

3.1.5. Onset Point. The only discernible relationships between the onset point of n-heptane-induced asphaltene precipitation and oil or asphaltene characteristics were with the number of carbons per alkyl side chain of the asphaltene molecule (nNMR and nIR) and the iron content in the oil. These are illustrated in Figure 15. The figure shows that the onset point increases as the number of carbons per alkyl side chain of the asphaltene molecule increases. Because an increase in the number of carbons per alkyl side chain is a reflection of an increase in the paraffin content of the asphaltene molecule, this result implies that the stability of crude

L-O

M1-O

M2-O

-0.825 -1.094 1 -0.0675

-0.453 -0.883 1 -0.005

0.999 1.025 -8449008467 -0.002

oil toward asphaltene precipitation is improved with an increase in the paraffin content of the asphaltene molecule. Figure 15 also shows that the onset point decreases as the iron content of the oil increases. Because the presence of iron confers some degree of polarity on the oil, this result implies that the oil becomes more unstable, with respect to asphaltene precipitation, as the iron content or polarity of the oil increases. 3.2. Inhibitor Effectiveness. The effectiveness of each chemical additive to inhibit n-heptane (lightparaffin-hydrocarbon)-induced asphaltene precipitation was evaluated from Tables 3A and 3B, in terms of the fractional difference ∆i/i, as defined in eq 2:

∆i inoninhibited - iinhibited ) i inoninhibited

(2)

where i represents m, n, k, or w. In this definition, a positive value for the kinetic parameters k (k ) k0 exp[-Ea/(RT)]), 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. The converse is also true. 3.2.1. Dodecylbenzenesulfonic Acid (DDBSA). Table 4 shows the effectiveness of DDBSA on precipitation behavior of the three crude oil samples. Results show that the addition of DDBSA enhances the asphaltene content dependence of the precipitation rate (m) for the L-O and M1-O oils and reduces it for the M2-O oil. This result shows that, in regard to suppressing the effect of m, DDBSA was effective only for the least-aromatic, least-polycondensed oil (M2-O).26 On the other hand, Table 4 shows that the n-heptane dependence of the precipitation rate (n) was enhanced by 109% and 88% for the L-O and M1-O oils, respectively, but was suppressed by 102% for the M2-O oil. The other kinetic parameter considered was the rate constant (k). DDBSA was able to reduce the asphaltene precipitation rate constant by 100% for the L-O and M1-O oils, but enhanced k for the M2-O oil. In the case of the onset point (equilibrium parameter), Table 4 shows that DDBSA produced beneficial effects by increasing the onset points by 6.7%, 0.5%, and 0.2% for the L-O, M1O, and M2-O oils, respectively. One can conclude from Table 4 that DDBSA was more effective with the leastaromatic medium oil (M2-O),26 in terms of all the kinetic parameters, whereas it was more effective with the more-aromatic oil, L-O,26 in terms of the equilibrium parameter. 3.2.2. Nonyl Phenol (NP). Table 5 shows the effectiveness of nonyl phenol (NP) on asphaltene precipitation from three crude oil samples. The results show that the



Table 5. Effectiveness of Nonyl Phenol (NP) during n-Heptane Flooding


L-O

M1-O

M2-O

-0.183 -0.191 1 -0.051

-0.273 -0.576 1 -0.027

-0.691 -2.0 1 -0.104

Table 6. Effectiveness of Toluene during n-Heptane Flooding


L-O

M1-O

M2-O

-0.167 -0.493 1 -0.072

-0.22 -0.179 1 -0.0133

-0.636 -1.321 1 -0.026

addition of NP produces a detrimental effect as it enhanced the asphaltene dependence of the precipitation rate (m) for all three oils. Table 5 also shows no signs of suppressing the n-heptane content dependence of the precipitation rate (n). Instead, this parameter was enhanced by 19%, 58%, and 200% for the L-O, M1-O, and M2-O oils, respectively. In contrast, the rate constant of asphaltene precipitation was reduced by 100% for the L-O, M1-O, and M2-O oils. Thus, NP had its best kinetic effect only in terms of reducing the rate constant and, as such, the overall asphaltenes precipitation rate. In terms of the onset point (the equilibrium parameter, w), NP was effective for all three oils. 3.2.3. Toluene. Table 6 shows that the effectiveness to inhibit asphaltene precipitation from the three crude oil samples with toluene was similar to those with NP. Results show that the addition of toluene enhanced the asphaltene content dependence of the precipitation rate (m) for all three oils. Behavior that was the same as that previously reported was observed for the n-heptane dependence of precipitation (n), with an enhancement of ∼49%, ∼18%, and ∼132% for the L-O, M1-O, and M2-O oils, respectively. In contrast, other results in Table 6 indicate that the rate constant of asphaltene precipitation was reduced by 100% for the L-O, M1-O, and M2-O oils, just as observed in the case with NP. Table 6 also shows that a beneficial effect of toluene addition is also observed in terms of the onset points. These were improved by 7%, 1%, and 3% for the L-O, M1-O, and M2-O oils, respectively. On the basis of these results, it would appear that, for n-heptane-induced asphaltene precipitation, one improvement was observed in the form of delaying the onset point. The other significant benefit was obtained in terms of a drastic reduction of the rate constant, and, as such, the overall rate of asphaltene precipitation. 3.3. Interrelationship between Additive Effectiveness and Oil/Asphaltene Characteristics. It should be mentioned that not all of the inhibitor effectiveness parameters exhibited discernible relationships with oil/asphaltene characteristics. Also, not all of the inhibitors exhibited discernible patterns with precipitation behavior and oil/asphaltene characteristics. All of the chemical additives were effective, in terms of ∆w/w and ∆k/k for almost all the oils; therefore, we decided to use these parameters to evaluate the interrelationships between additive effectiveness, precipitation behavior, and oil/asphaltene characteristics. Note that the ∆w/w values, as calculated by eq 2, are negative values, as shown in Tables 4-6. However, absolute

Figure 16. Interrelationships between (4) the onset point efficiency (∆w/w), the onset point (w), and various oil/asphaltene characteristics for nonyl phenol (NP). ((O) denotes nNMR data (the number of carbons per alkyl side chain of the asphaltene molecule, as determined by NMR methods) and (+) denotes nIR data (the number of carbons per alkyl side chain of the asphaltene molecule, as determined by FTIR methods), whereas (]) denotes the iron content).

∆w/w values are used in this section, based on our definition of effectiveness for the onset point. 3.3.1. Dodecylbenzenesulfonic Acid (DDBSA). There is no discernible pattern in the variation of ∆w/w with w and the oil/asphaltene characteristics. This is attributed to the limited number of data points available to us in making the evaluation. It is expected that, with a larger number of data points, it will be possible to draw conclusions in regard to the presence or absence of definite trends. In the case of ∆k/k, DDBSA was clearly not effective for the M2-O oil, whereas it produced 100% reductions for the L-O and M1-O oils. The 100% reduction in k for the L-O and M1-O oils can be attributed to using a higher-than-necessary concentration of DDBSA. It is expected that a smaller concentration of DDBSA should enable the ranking of L-O and M1-O oils, in regard to which oil yields the best ∆k/k effect with DDBSA. It would then be possible to determine if interrelationships exist between inhibitor effectiveness, precipitation behavior, and oil/asphaltene characteristics. 3.3.2. Nonyl Phenol (NP). Figure 16 shows the interrelationship between nonyl phenol (NP) effectiveness (measured as an improvement in the onset point (∆w/ w), precipitation behavior (w), and oil/asphaltene characteristics (the number of carbons per alkyl side chain of asphaltene molecules (measured as nNMR and nIR) and iron content in the oil). Figure 16 shows that the efficiency criterion (∆w/w) increases as the oil stability (onset point) and the paraffin fraction of the asphaltene molecule each increase. However, it decreases as the iron content of the oil increases. This suggests that NP is most effective with the most-stable oil of low polarity and a low aromatic fraction of the asphaltene molecule. In the case of ∆k/k, NP produced 100% reductions for all three oils. As in the case of DDBSA, the 100% reduction in k for the L-O, M1-O, and M2-O oils can be


attributed to the use of higher-than-necessary concentration of NP. Thus, it is expected that a smaller concentration of NP would enable the ranking of L-O, M1-O, and M2-O, in regard to which oil yields the best ∆k/k effect with NP. It would then be possible to determine if interrelationships exist between inhibitor effectiveness, precipitation behavior, and oil/asphaltene characteristics. 3.3.3. Toluene. As in the case of DDBSA, there is no discernible pattern in the variation of ∆w/w with w and the oil/asphaltene characteristics. Again, this is attributed to the limited number of data points available to us in making the evaluation. It is expected that, with a larger number of data points, it will be possible to draw conclusions in regard to the presence or absence of definite trends. In the case of ∆k/k, toluene produced 100% reductions for all three oils, as in the case of NP. Also, as in case of DDBSA and NP, the 100% reduction in k for the L-O, M1-O, and M2-O oils can be attributed to the use of a higher-than-necessary concentration of toluene. Thus, it is expected that a smaller concentration of toluene would enable the ranking of L-O, M1-O and M2-O, in regard to which oil yields the best ∆k/k effect with toluene. It would then be possible to determine if interrelationships exist between inhibitor effectiveness, precipitation behavior, and oil/asphaltene characteristics. 3.3.4. Industrial Applications. The results from this work should enable the correct choice of chemical inhibitor for prevention of light-hydrocarbon-induced asphaltene precipitation by inhibition during light hydrocarbon flooding. The information needed to make this choice includes the nature of the oil (whether paraffinic or aromatic) and whether precipitation should be controlled by the kinetic parameter or the equilibrium parameter. In terms of using the kinetic parameters (m, n, and k), one must make the choice of the overriding parameter, k, because this has the controlling influence on the rate. In this case, DDBSA drops out, because it is not effective for all the oils, in terms of k. In terms of the equilibrium parameter, all three inhibitors are effective; however, NP is the choice, because its effectiveness is better and it is more environmentally benign than toluene. 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 were affected by temperature,24 whereas w was affected by pressure.29 Therefore, it is expected that inhibition effectiveness of the chemicals, in terms

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of m, n, k, and w, will also be affected by both temperature and pressure. 4. Conclusions (1) The asphaltene precipitation rate dependence on asphaltene content of the oil (m) decreased as the heteroatoms content of both the crude oil and asphaltenes increased, but increased as the aromatic carbon fraction and the degree of branching of the asphaltene molecules each increased. On the other hand, asphaltene precipitation rate dependence on the amount of n-heptane (i.e., light paraffin hydrocarbon) added (n) decreased as the paraffin fraction of the asphaltenes increased, but increased as the propensity of the asphaltene molecules for aggregation increased. The trends for the frequency factor for asphaltene precipitation (k0) were similar to those for n. (2) In contrast, the activation energy for asphaltene precipitation decreased as the propensity for aggregation increased, but increased as the paraffin fraction of the asphaltene molecules increased. Furthermore, the equilibrium parameter (the onset point) increased as the paraffin fraction of the asphaltene molecules increased, but decreased as the iron content of the oil increased. (3) Dodecylbenzenesulfonic acid (DDBSA) was more effective with the least-aromatic medium oil (M2-O), in terms of all the kinetic parameters, whereas it was more effective with the more-aromatic oil (L-O), in terms of the equilibrium parameter. A significant benefit obtained with the three chemical additives was the drastic reduction of the rate constant, and, as such, the overall rate of asphaltene precipitation. (4) Nonyl phenol (NP) exhibited its highest inhibition efficiency (∼10%), in terms of the onset point on the oil with the highest onset point (i.e., the most-stable oil) with the lowest iron content, and which also had the highest average number of carbon per alkyl side chain (nIR) in the asphaltene molecules. DDBSA and toluene did not exhibit any discernible relationships. 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. EF0340460 (29) Ramachandran, S.; Breen, P.; Ray, R. Chemical Programs Ensure Flow and Prevent Corrosion in Deepwater Facilities and Flow Lines. Baker Hughes Inc., 2000, http://www.bakerhughes.com/bakerhughes/inDepth/72k/Petrolite.pdf.

Interrelationships between Asphaltene Precipitation Inhibitor

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