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Energy & Fuels 2006, 20, 1584-1590
Study of Pressure and Temperature Effects on Asphaltene Stability in Presence of CO2 Sylvain Verdier,† Herve´ Carrier,*,‡ Simon I. Andersen,†,§ and Jean-Luc Daridon‡ Kemiteknik, Danmarks Tekniske UniVersitet, DK-2800 Kgs. Lyngby, Denmark, and Laboratoire des Fluides Complexes, UMR 5150, UniVersite´ de Pau et des Pays de l’Adour, BP 1155, 64013 PAU Cedex, France ReceiVed December 22, 2005. ReVised Manuscript ReceiVed March 22, 2006
In the prolific literature about asphaltenes, the effects of temperature and pressure on their stability are subjects of discussion. A new high-pressure cell, requiring a very small amount of sample and with wide working conditions, has been built in order to study the asphaltene phase behavior after injection of various gases and precipitants. A filtration technique is used to conclude on the effects of temperature, pressure, and composition. The precipitant used in this work is CO2. Two crude oils (from South America and the Middle East) were studied up to 383 K and 60 MPa. It was found for both oils that asphaltenes were more soluble when temperature was decreased and pressure was increased in the presence of a gas component. These effects were discussed with simple principles of thermodynamics.
Introduction Problems related to asphaltenes flocculation during oil recovery (reservoir plugging), production (deposits in production wells, generally at the depths corresponding to the bubble point), or transportation (transport of mixtures of fluids of different origins and, thus, of different compositions) are still topical questions.1 Many theories and models were developed to describe and predict asphaltene precipitation, using the FloryHuggins theory,2-7 micellization models,8,9 scaling equations,10 or advanced equations of state such as SAFT,11-13 to name a few. However, in many cases, it remains a fitting exercise and, as Porte et al. reported it,14 their capacities to predict the behavior of asphaltenic fluids are generally poor. These difficulties are partly due to the fact that asphaltenes are still defined as a solubility class:1 soluble in low-molecular* Corresponding author e-mail:
[email protected]. † Danmarks Tekniske Universitet. ‡ Universite ´ de Pau et des Pays de l’Adour. § Present address: Haldor Topsoe A/S Ny Møllevej 55, DK-6800, Kgs Lyngby, Denmark. (1) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999. (2) de Boer, R. B.; Leerloyer, K.; Eigner, M. R. P.; van Bergen, A. R. D. SPE Prod. Facil. 1995, 55-61. (3) Burke, N. E.; Hobbs, R. D.; Kashou, S. F. J. P. T. 1990, 14401446. (4) Hirschberg, A.; deJong, L. N. J.; Schipper, B. A.; Meijer, J. G. SPE J. 1984, 283-293. (5) Wang, J. X.; Buckley, J. S. Energy Fuels 2001, 15, 1004-1012. (6) Lindeloff, N.; Heidemann, R. A.; Andersen, S. I.; Stenby, E. H. Pet. Sci. Technol. 1998, 16, 307-321. (7) Andersen, S. I.; Speight, J. G. J. Pet. Sci. Eng. 1999, 22, 53-66. (8) Fahim, M. A.; Al-Sahhaf, T. A.; Elkilani, A. S. Ind. Eng. Chem. Res. 2001, 40, 2748-2756. (9) Victorov, A. I.; Firoozabad, i. A. AIChE J. 1996, 42, 1753. (10) Rassamdana, H.; Dabir, B.; Nematy, M.; Farhani, M.; Sahimi, M. AIChE J. 1996, 42, 10-22. (11) Ting, P. D.; Hirasaki, G. J.; Chapman, W. G. Pet. Sci. Technol. 2003, 21, 647-661. (12) Wu, J.; Prausnitz, J. M.; Firoozabadi, A. AIChE J. 1998, 44, 11881199. (13) Buenrostro-Gonzalez, E.; Lira-Galeana, C.; Gil-Villegas, A.; Wu, J. AIChE J. 2004, 50, 2552-2570. (14) Porte, G.; Zhou, H.; Lazzeri, V. Langmuir 2003, 19, 40-47.
weight aromatics and insoluble in low-molecular-weight alkanes. Thus, the generic word “asphaltene” gathers entities forming a continuum but belonging to fractions of distinct physical properties, and fundamental interrogations persist on their exact natures (size, molar mass, composition, and molecular arrangement) and on the laws governing their behavior, such as phenomena of aggregation and precipitation. Pressure and Temperature Effects. In particular, the influence of temperature on asphaltene stability is subject to controversy. Andersen and Birdi15 reviewed its various effects: increase in solubility with temperature,16 decrease,17,18 or an increase followed by a decrease.19 In ref 15, the authors found a minimum in solubility at 298 K, with the temperature varying between 277 and 373 K using precipitants from n-C5 to n-C8. Gonzalez et al.20 modeled the data published by Jamaluddin et al.21 with PC-SAFT. They fitted their model to the few experimental points and found a maximum in stability with respect to temperature: below 394 K; asphaltenes are more soluble when temperature is increased, and above this, the opposite effect is seen. At low temperatures, the authors explain that asphaltenes become unstable because of differences in the interaction energies between asphaltene molecules and solvent (crude oil) molecules. As temperatures increase over 427 K, they assume that the asphaltene solution demixes as a result of the large thermal expansivity of the solvent compared to that of the asphaltene. As for the effect of pressure, opinions tend to agree:22-24 if one supports the lyophilic model14 and the widespread asphalt(15) Andersen, S. I.; Birdi, K. S. Fuel Sci. Technol. Int. 1990, 8, 593615. (16) Ali, L. H.; Al-Ghannam, K. A. Fuel 1981, 60, 1043-1046. (17) Mitchell, D. L.; Speight, J. G. Fuel 1973, 52, 149-152. (18) Hotier, G.; Robin, M. ReV. Inst. Fr. Pet. 1983, 38, 101-120. (19) Rogacheva, O. V.; Gubaidullin, V. Z.; Gimaev, R. N.; Danlilyan, T. D. Colloid J. USSR 1984, 46, 715-717. (20) Gonzalez, D. L.; Ting, P. D.; Hirasaki, G. J.; Chapman W. G. Energy Fuels 2005, 19, 1230-1234. (21) Jamaluddin, A. K. M.; Joshi, N.; Iwere, F.; Gurpinar, O. Proc. SPE Int. Pet. Conf. Exhib. Mex. 2002, 427-436. (22) Lhioreau, C.; Briant, J.; Tindy, R. ReV. Inst. Fr. Pet. Ann. 1967, 22, 797-806. (23) Fotland, P. Fuel Sci. Technol. Int. 1996, 14, 3131-326.
10.1021/ef050430g CCC: $33.50 © 2006 American Chemical Society Published on Web 05/02/2006
Pressure and Temperature Effects on Asphaltene Stability
Figure 1. Onset of precipitation of three crude oils in the presence of CO2 as a function of temperature (9, oil 1; 2, oil 2; and b, oil 3) (data from Idem and Ibrahim25).
ene phase envelope, the one-phase zone is located at a higher pressure than the two-phase region (asphaltene phase + crude oil). The temperature and pressure effects of CO2 on asphaltene stability have not been extensively studied in the literature. Idem and Ibrahim25 state that, in the presence of CO2, asphaltene stability in crude oils improves with temperature for two of the three different samples they studied. However, they compare volumes at different temperatures (from 300.4 to 337.7 K) and they observe different trends. The density of CO2 varies between 882.9 and 633.1 kg/m3 at 17.2 MPa within this temperature range.26 Thus, on a mass-based scale, it is seen that asphaltenes are less stable with temperature, since less CO2 is required to initiate precipitation (Figure 1). Kokal et al.27 studied the influence of various gases on two oils and found opposite results for the CO2 injection: the Suffield oil was more stable with a temperature increase, whereas the Lindbergh oil was not. They explain these effects by the competition between temperature (affecting the solubility parameter of asphaltenes) and composition (influencing the solubility of the solvent). To quantify the impact of temperature and pressure on the asphaltene stability in the presence of a gas component as precipitant, an experimental setup based on isobaric filtration was developed in this work. The apparatus was then used to assess the stability conditions of two different stock tank oils in the presence of CO2. Asphaltene Precipitation as a Phase Transition. The nature of asphaltene precipitation is a major concern. Is it a liquid/ liquid equilibrium, a solid/liquid equilibrium, a sol-gel transition, a glass transition? It is a first- or second-order transition? Fenistein and co-workers28 explain that asphaltene flocculation is a transition between a solvated aggregation and a compaction process (compact structures were seen by radiation scattering techniques with high n-heptane contents). Asphaltene deposits are solid. Hence, is asphaltene precipitation a solid-liquid equilibrium? Precipitation is defined by the International Union of Pure and Applied Chemistry (IUPAC) as “the formation of a solid phase within a liquid phase”.29 It is indeed modeled such as one.6 Nonetheless, as Sirota states it,30 this phase separation can be seen thermodynamically as a liquid-liquid phase separation, which can be understood in the context of the solution (24) Hammami, A.; Phelps, C. H.; Monger-McClure, T.; Little, T. M. Energy Fuels 2000, 14, 14-18. (25) Idem, R. O.; Ibrahim, H. H. J. Pet. Sci. Eng. 2002, 35, 233-246. (26) NIST website: http://webbook.nist.gov/chemistry/. (27) Kokal, S. L.; Najman, J.; Sayegh, S. G.; George, A. E. J. Can. Pet. Technol. 1992, 31, 24-30. (28) Fenistein, D.; Barre, L.; Frot, D. Oil Gas Sci. Technol. 2000, 55, 123-128. (29) Clarke, J. B.; Hastie, J. W.; Kihlborg, L. H. E.; Metselaar, R.; Thackeray, M. M. Pure Appl. Chem. 1994, 66, 577-594. (30) Sirota, E. B. Energy Fuels 2005, 19, 1290-1296.
Energy & Fuels, Vol. 20, No. 4, 2006 1585
theory of molecules. The solidlike character of the asphaltenerich phase formed during precipitation is only due to the fact that asphaltenes are below their glass-transition temperature. Glass temperatures were indeed measured by differential scanning calorimeter and reported between 393 and 403 K.31 This issue is nonetheless controversial and actively debated. As for the question about first-order and second-order transitions, the classification about phase transitions is not unique. From a thermodynamic point of view, a first-order transition corresponds to a change in volume and entropy (thus, a latent heat) and a second-order transition corresponds to a change in heat capacity Cp, thermal expansivity RP, and isothermal compressibility κT. The heat of precipitation of asphaltenes, if any, is rather small, since several teams tried to measure it but with no success so far. Thus, a break in Cp, RP, and κT should be expected, as was the case, for instance, for the precipitation of poly(N-vinylcaprolactam) in water (with molar weights ranging from 21 000 to 1.5 × 106 g‚mol-1).32 Such measurements are currently tried with recombined live oils and will be presented in the future if confirmed. Le Chaˆ telier’s Principle. Effects of both pressure and temperature can be explained by Le Chaˆtelier’s principle. Henri Le Chaˆtelier stated 33 that ”eVery change of one of the factors of an equilibrium occasions a rearrangement of the system in such a direction that the factor in question experiences a change in a sense opposite to the original change.” The equilibrium we are interested in is the following one:
asphaltene (solution) a asphaltene (precipitated) (R1) Andersen and Speight7 listed two other reactions:
The aggregation: asphaltenesi + asphaltenesl a asphaltenesi+l (R2) The interaction with the resins: asphaltenes + resins a asphaltenes - resins (R3) Aggregation only takes place at low concentration (
δa - δl 11 - RP,a (δa - δl) ≈ 2T 2T
(
)
(9)
If it was possible to measure properly and accurately enough (47) Rogel, E. Energy Fuels 1997, 11, 920-925.
Both temperature and pressure effects were studied on two recombined crude oils, and the same trends were observed: asphaltenes are more soluble when temperature is decreased and when pressure is increased. This work was possible thanks to a new high-pressure cell using a filtration technique. Its main advantages are the following ones: a small sample is required (5 g), the injection of the precipitant is done at the conditions under investigation, the sample is mixed continuously, and the dead volume is almost nonexistent. The opposite effects seen in the literature can be explained by means of Le Chaˆtelier’s principle and solubility parameters. Nonetheless, this last approach is based on the assumption that no specific interactions occur between the components, which is in direct opposition to the tendency of asphaltenes to aggregate.7 Thus, this simple model and its related “fitting” parameters should be used with care and perspective. As for Le Chaˆtelier’s principle, experimental evidence about heat of precipitation and change in volume are still necessary and equilibrium (eq R2)sthe asphaltene/resin interactionsmight not be negligible as it was assumed in this work. The next steps are the injection of other gases and a calorimetric study of asphaltene precipitation. Acknowledgment. S.V. thanks the staff of the Laboratoire des Fluides Complexes for the various and fruitful stays in Pau.
Appendix 1 Let us consider a component A in equilibrium between two phases.
AR a Aβ (48) Barton, A. F. M. CRC Handbook of solubility parameters and other cohesion parameters; CRC Press: Boca Raton, FL 1991. (49) Diallo, M. S.; Strachan, A.; Faulon, J. L.; Goddard, W. A., III. Pet. Sci. Technol. 2004, 22, 877-899.
1590 Energy & Fuels, Vol. 20, No. 4, 2006
Verdier et al.
At the equilibrium, the chemical potentials are equal:
Table 2. Effect of Increasing Temperature and Pressure increase in T
µAR(T,P,nR) ) µBR(T,P,nβ) R
µR,0 A (T,P)
+ RT ln [aA(T,P,n )] ) β µβ,0 A (T,P) + RT ln [aA(T,P,n )]
(µβ,0 A (T,P)
µR,0 A (T,P))
-
(gβ,0 A (T,P)
+ RT ln
gR,0 A (T,P))
-
[
aA(T,P,nβ)
]
aA(T,P,nR)
)0
ln KA )
-
+ RT
T[sβ,0 A (T,P)
-
r f f r
increase in T hprecipitation < 0 hprecipitation > 0 VAdeposit < VAsolution VAdeposit > VAsolution
+ RT ln KA ) 0
hR,0 A (T,P)]
0 0
sR,0 A (T,P)]
|
|
∂ ln KA ∂T
|
∂ ln KA ∂T
P
) P
)
∆h0A
decrease solubility increase solubility
Pressure Effect.
|
-
RT
∆h0A 2 RT
0
-
(
∂h0,β A
+
|
∂s0,β A
c0,R p
1 0,β 1 (c - c0,R p )+ RT p R T T
|
) P
|
∂s ) -RPV ∂P T ∂s0,R A
1 R ∂T ∂T
c0,β p
∂ ln KA ∂T
]
∂h ) V(1 - RPT) ∂P T
0
) ( ) ( )
∂h0,R A
1 RT ∂T ∂T
[
R,0 R,0 ∂(sβ,0 ∂(hβ,0 ∂ ln KA A - hA ) A - sA ) 1 +T ) ∂P T RT ∂P ∂P
Furthermore, we have
1 ∂∆hA 1 ∂∆sA + ) 2 P RT ∂T R ∂T RT
∆h0A 2
increase in P
increase solubility decrease solubility
Let us investigate the pressure and temperature effects on the equilibrium constant. Temperature Effect.
∂ ln KA ∂T
increase in P
Table 3. Effect of Increasing Temperature and Pressure in Terms of Asphaltene’s Solubility
R,0 R,0 β,0 [hβ,0 A (T,P) - hA (T,P)] - T[sA (T,P) - sA (T,P)] + RT ln KA ) 0
-[hβ,0 A (T,P)
∆h0A ∆h0A ∆V0A ∆V0A
)
∆h0A 2
where Rp is the isobaric expansivity and V is the molar volume So, it becomes
|
∂ ln KA 1 β,0 R,0 R,0 ) [-Vβ,0 A (1 - Rp ) + VA (1 - Rp ) + ∂P T RT β,0 R,0 R,0 T(-Rβ,0 p VA + Rp VA )]
RT
|
- ∆V0A ∂ ln KA 1 R,0 β,0 ) [V - VA ] ) ∂P T RT A RT
∆h0A RT2
This relationship is known as the Van’t Hoff equation. If the reaction is endothermic, ∆h0A is negative, so the equilibrium shifts to its left side. If the reaction is exothermic, the equilibrium shifts to its right side.
If ∆V0A is negative (smaller molar volume in phase β than in phase R), an increase in pressure will shift the equilibrium to the right side (see Table 2).In terms of asphaltene’s solubility, the results are shown in Table 3. EF050430G
