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Energy & Fuels 2008, 22, 3970–3975
Asphaltene Cake Properties Vale´rie Montel,*,† Ve´ronique Lazzeri,† Benjamin Brocart,‡,§ and Honggang Zhou‡,§ UniVersite´ de Pau et des Pays de l’Adour, LTEFC UMR 5150, BP1155, 64013 Pau Cedex, France, Total Petrochemicals France, Poˆle de Recherche et De´Veloppement Mont-Lacq, BP 47, 64170 Lacq, France, and Total, CSTJF, AVenue Larribau, 64018 Pau Cedex, France ReceiVed May 21, 2008. ReVised Manuscript ReceiVed August 18, 2008
One of the major problems encountered during the exploitation of the oil reservoir is the deposition of asphaltene in the porous rock near the wellbore. The crude oil asphaltene considerably reduces the rock permeability and, consequently, the oil production. The aim of this work is to propose a specific test to identify the relevant parameters controlling the damaging potential of asphaltene deposits. Our approach is to avoid the particle/porous media interactions to allow for the characterization of asphaltene-deposit-specific properties. To this end, a filtration process (carried out under static pressure) has been developed. First, experiments conducted on crude oil indicated that two parameters essentially characterize the asphaltene cake: the specific resistance and the delay of the cake growth depending upon the pressure drop. Overall, asphaltene cake shows intrinsic properties (specific resistances and porosities) similar to those of clay or sludge. The absolute porosity of the asphaltene deposit is high, around 88%, while permeabilities are low (10 µDarcy).
1. Introduction The phase transition of asphaltene fractions may cause well plugging or depositional problems in the surface equipment in various conditions (pressure, temperature, and composition). Motivated by such impact on production and transportation, the literature devoted to asphaltene phase behavior is huge.1 Thanks to effective solutions, such as solvent injection or use of chemicals, it may be possible to prevent plugging risks in surface facilities or in well tubing. However, when deposits form in the near-wellbore region, it makes remediation difficult. Therefore, the question arises of how to optimize the production conditions to lessen deposit accumulation in this region. What Are Asphaltenes? Asphaltenes are defined as a class of components of petroleum, which are insoluble in n-heptane and soluble in toluene.2-5 They are composed of aromatic polycyclic clusters connected with alkyl chains. Moreover, they contain some heteroatoms (N, S, and O) and metal traces (Ni, V, and Fe).6 Asphaltenes refer to a very large polydispersity of molecules of whatever size, mass, composition, and molecular conformation. Therefore, it may not be possible to propose a model based on the study of one single molecule. The molecular continuum is slightly different from one crude oil to another and from one method of extraction to another.7 Researchers agree on the fact that the asphaltene deposit is significantly different from the asphaltene fraction of oil.8-11 Indeed, the * To whom correspondence should be addressed. E-mail: valerie.montel@ etud.univ-pau.fr. † Universite ´ de Pau et des Pays de l’Adour. ‡ Total Petrochemicals France. § Total, CSTJF. (1) Merdrignac, I.; Espinat, D. Oil Gas J. 2007, 62 (1), 7–32. (2) Porte, G.; Zhou, H.; Lazzeri, V. Langmuir 2003, 19, 40–47. (3) Sirota, E. B. Energy Fuels 2007, 21 (5), 2809–2815. (4) Sirota, E. B.; Lin, M. Y. Energy Fuels 2007, 21, 2809–2815. (5) Evdokimov, I. N.; Eliseev, N. Y. Energy Fuels 2006, 20, 682–687. (6) Ruiz-Morales, Y.; Mullins, O. C. Energy Fuels 2007, 21 (1), 256– 265. (7) Speight, J. G. Oil Gas J. 2004, 59 (5), 467–488. (8) Mansoori, G. A. J. Pet. Sci. Eng. 1997, 17, 101–111.
crude oil is composed of many components (such as wax, resin, and aromatic), which co-precipitate with asphaltene. Authors have tried to separately study the contributions of the coprecipitated fractions, in particular, the characterization of the resin effect.12-14 This approach is limited because fractions are operational concepts. They cannot be considered as very precise separations. Taking these observations into account, we decided to conduct our experiments with crude oil to search global trends. A New Approach. In this work, we focus our investigation on the plugging phenomenon. At this point, it is necessary to properly define the steps leading from dispersed asphaltene particles to cake formation. Deposition hinges upon three main mechanisms: the particle in fluid transportation, particle-particle interactions, and the porous media-particle interactions. According to the size, the nature of the particles, and the structure of the porous media, one of these mechanisms prevails upon the others. Plugging conditions are specific to each reservoir and may not be generalized. However, the consequence on the flow property is always linked to a permeability decrease. The simulations need numerous tuning parameters to predict damaging potential of asphaltenes.15-19 Our approach is to avoid the particle-porous media interactions. The aim is to define specific parameters, which have influence on the intrinsic asphaltene cake properties, such as specific resistance or porosity. To reach this objective, we selected a filtration process. Understanding the mechanisms that control the filtration of a complex medium (9) Carbognani, L. Energy Fuels 2001, 15, 1013–1020. (10) Wattana, P.; Fogler, H. S.; Yen, A.; Del Carmen Garca`, M.; Carbognani, L. Energy Fuels 2005, 19, 101–110. (11) Sheu, E. Y.; Mullins, O. Energy Fuels 2004, 18, 1531–1534. (12) Andersen, S. I.; Speight, J. G. Pet. Sci. Technol. 2001, 19 (1 and 2), 1–34. (13) Spiecker, P. M.; Gawrys, K. L.; Trail, C. B.; Kilpatrick, P. K. Colloids Surf., A 2003, 220, 9–27. (14) Le´on, O.; Contreras, E.; Rogel, E.; Dambakli, G.; Espidel, J.; Acevedo, S. Energy Fuels 2001, 15, 1028–1032.
10.1021/ef8003784 CCC: $40.75 2008 American Chemical Society Published on Web 09/20/2008
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Table 1. SARA Composition of the Crude Oil (wt %) samples
saturated hydrocarbon
aromatic hydrocarbon
resin
asphaltene
oil
45.28
30.92
17
6.8
Table 2. Results (wt %) of the Elemental Analysis of the n-C7 Precipitated Asphaltenes samples
C
H
N
O
S
total
n-C7 asphaltenes
87.30
7.17
1.62
1.07
2.37
99.53
is a major challenge in many industries,20-22 for example, bioor agro-industries. In petroleum research, characterization through filtration processes are used for the study of drilling mud,23,24 emulsions,25 or aqueous clays and electrolyte suspensions.26 It has never been used for asphaltene suspensions yet. With the experiments carried out under static pressure, the flow condition favors identification of relevant parameters controlling the plugging ability of asphaltene deposits. 2. Experimental Section 2.1. Samples. Samples come from a dead crude oil, which is completely anhydrous and without any purification or distillation. It was obtained from the Total Company and was stored at ambient temperature. Samples were centrifuged under a temperature of 55 °C for 15 min at 4500 rpm to remove insoluble solid particles. Asphaltenes precipitate directly from the crude oil in a large excess of n-heptane. The solvent/oil ratio is 20:1, and the suspension concentration is 0.3 wt %. 2.2. Characterization of Oil and Asphaltene Fractions. Crude oil components are classified according to the saturated hydrocarbons, aromatic hydrocarbons, resins, and asphaltenes (SARA) separation method27 The SARA analysis of the crude oil was performed by Total using a thin-layer chromatography equipped with a flame ionization detector (TLC-FID). The compositional results are presented in Table 1. Note that the SARA asphaltene fraction could be slightly different from the n-C7 fraction. Elemental analysis of the crude oil was also carried out by Total. The carbon, hydrogen, and nitrogen contents are obtained by combustion at 1050 °C on a Nitromatic 500 analyzer. The oxygen content is determined by an indirect method. At 1120 °C, the oxygen is transformed to CO through reaction with amorphous carbon. From a consecutive reaction with copper monoxide, the gas produced is carbon dioxide. The content of CO2 is determined thanks to coulometric analysis. The sulfur content measured through combustion of the crude oil at 1320 °C on a sulfur coulometric analyzer. The elemental analysis, which was limited to the main constitutive atoms, is detailed in Table 2. The residual components are mainly metal traces. The complex chemistry of asphaltene results in numerous possibilities of interactions (van der Waals, π-bonds, etc.), which (15) Kohse, B. F.; Nghiem, L. X. SPE Tech. Pap. 89437, 2004. (16) Zekri, A. Y. J. Pet. Sci. Eng. 2004, 42, 171–182. (17) Minssieux, L.; Nabzar, L.; Chauveteau, G.; Longeron, D.; Bensalem, R. Oil Gas J. 1998, 53 (3), 313–327. (18) Kocabas, I.; Islam, M. R.; Modarress, H. J. Pet. Sci. Eng. 2000, 26, 19–30. (19) Mohsen, V.-S.; Seyyed, A. M.-D.; Mohammad, M.-Z. Fluid Phase Equilib. 2003, 206, 1–11. (20) Lee, S. A.; Fane, A. G.; Waite, T. D. EnViron. Sci. Technol. 2005, 39, 6477–6486. (21) Ould-Dris, A.; Jaffrin, M. Y.; Si-Hassen, D.; Neggaz, Y. J. Membr. Sci. 2000, 175, 267–283. (22) Soua, Z.; Larue, O.; Vorobiev, E.; Lanoiselle, J.-L. Colloids Surf., A 2006, 274, 1–10. (23) Sherwood, J. D.; Meeten, G. H. J. Pet. Sci. Eng. 1997, 18, 73–81. (24) Larsen, D. H. Pet. Eng. 1938, 42–48, 50-60. (25) Headen, T. F.; Clarke, S. M.; Perdigon, A.; Meeten, G. H.; Sherwood, J. D.; Aston, M. J. Colloids Interface Sci. 2006, 304, 562–565. (26) Li, Y.; Arguillier, J.-F.; Rosenberg, E.; Durrieu, J. Oil Gas J. 1997, 52 (2), 207–218. (27) Karlsen, D. A.; Larter, S. R. Org. Geochem. 1991, 17, 603–617.
lead to precipitation. To illustrate the precipitation of asphaltene, we pour droplets of crude oil in n-heptane at 20 °C. First, we illustrate using an optical microscope to observe the asphaltene in several conditions (aggregated and dispersed by mechanical stresses). Cryofracture of the aggregates have also been performed to observe the structure at lower scale. In addition to these observations, we used a Malvern diffractometer to measure size distributions. For illustration purposes only, below is picture taken by optical microscope (Figure 1). The aggregate (100 µm; left side of Figure 1), seems to be built with elementary grains (2 µm) stacked to each other.28,29 When the suspension is mechanically stressed, the weak bonds between the elementary grains are easily broken (right side of Figure 1). Several studies are available on this subject.30-35 It is necessary to underline that our measurements did not allow us to conclude on the distribution at fixed precipitation conditions. Experimental methods have physical limitations, which prevent observation at all scales. For diffraction, it appears that errors came from the absorbing factor36,37 and the fluorescence spectra.38 To conclude, size measurements are not completely reliable. In this way, the study of filter ability provides new pieces of information that improve the knowledge of asphaltene. We do not claim that the results are directly transposable in reservoir conditions. The target is to obtain reliable results on the plugging of asphaltene before considering a real condition system (3D problems in natural rocks). 2.3. Filtering Medium. Despite the filtering methods, which are mainly considered as a separational process using a steric selectivity, we must take into account adsorption. Adsorption of asphaltene into the membrane pores may cause a high increase in its resistivity. Therefore, the membrane resistivity will override the cake one. Consequently, the filtration medium has to be chemically neutral to avoid adsorption during the pore plugging.39,40 In this set purpose, we select polytetrafluoroethylen (PTFE) membranes manufactured by Millipore with a 5 µm membrane pore size, corresponding to the aggregate size measurements. 2.4. Apparatus. As mentioned before, the aim of this work is to identify the relevant parameters controlling the intrinsic deposit properties. To reach this objective, we have developed a specific filtration device, which is depicted in Figure 2. The pilot covers a temperature range from 20 to 110 °C and a pressure range from 1 to 30 ( 0.2 bar. The mixing vessel has a 4 L capacity. Suspension of asphaltene particles is stirred during the procedure with an anchor. We select an anchor because it is a low shear stirring system and it allows us to maintain the homogeneity of the suspension in the vessel. First, the mixing vessel is filled with n-heptane. Then, the crude oil sample is introduced with the solvent/oil ratio of 20: 1. The suspension is stirred for 1 h before filtration. The filtration works at constant pressure drop across the filter. (28) Hung, J.; Castillo, J.; Reyes, A. Energy Fuels 2005, 19 (3), 898– 904. (29) Joshi, N. B.; Mullins, O. C.; Jamaluddin, A.; Creek, J.; McFadden, J. Energy Fuels 2001, 15, 979–986. (30) Silva, K. R.; S. M., C. Braz. J. Chem. Eng. 200421, (4), 601–609. (31) Rahmani, N. H. G.; Dabros, T.; Masliyah, J. H. Chem. Eng. Sci. 2004, 59, 685–697. (32) Rahmani, N. H. G.; Dabros, T.; Masliyah, J. H. Ind. Eng. Chem. Res. 2005, 44, 75–84. (33) Pe´rez-Herna`ndez, R.; Mendoza-Anaya, D.; Mondragon-Galicia, G.; Espinosa, M. E.; Rodriguez-Lugo, V.; Lozada, M.; Arenas-Alatorre, J. Fuel 2003, 82, 977–982. (34) Hung, J.; Castillo, J.; Reyes, A. Energy Fuels 2005, 19 (3), 898– 904. (35) Marczak, W.; Dafri, D.; Modaressi, A.; Zhou, H.; Rogalski, M. Energy Fuels 2007, 21 (3), 1256–1262. (36) Buckley, J. S.; Hirasaki, G. J.; Liu, Y.; Von Drasek, S.; Wang, J.-X.; Gill, B. S. Pet. Sci. Technol. 1998, 16 (3-4), 251–285. (37) Taylor, S. D.; Czarnecki, J.; Masliyah, J. Fuel 2001, 80, 2013– 2018. (38) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. Energy Fuels 2001, 15, 972–978. (39) Xie, K.; Karan, K. Energy Fuels 2005, 19, 1252–1260. (40) Ekholm, P.; Blomberg, E.; Claesson, P.; Auflem, I. H.; Sjo¨blom, J.; Kornfeldt, A. J. Colloids Interface Sci. 2002, 247, 342–350.
3972 Energy & Fuels, Vol. 22, No. 6, 2008
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Figure 1. Asphaltene aggregates precipitated from the crude oil in n-heptane at 20 °C (left side) and dispersed asphaltene aggregates precipitated from the crude oil in n-heptane at 20 °C (right side).
Figure 2. Pilot design of the filtration process.
Filtered solution is monitored by a Malvern diffractometer 2000 on line. The aim of this apparatus is to follow the granulometry of dispersed objects and to control the filtrated size threshold. This control system has clearly showed that particles filtered were of a smaller size as the pore diameter of the filter. The particle size distribution also proves the filter integrity.
3. Results and Discussion Preliminary Tests and Results. The results exhibited here are obtained from imposed conditions of temperature and pressure drop. Our first focus is to determine if the membrane is collapsing during filtration. If this phenomenon occurs, the resistivity of the collapsed membrane will override the deposit resisitivity. Three experiments performed to check for the occurrence of collapsed membrane are plotted in Figure 3. This chart below shows the cumulate mass of filtrate as a function of time.
The slope of the curve with pure n-heptane flowing through a clean filter (9) shows the resistance of the filter. The experiment of pure n-heptane flowing through a plugged filter (0) illustrates that the membrane pores are slightly plugged with asphaltene. The first consequence is a constant slope, which is lower than the unplugged one. The filtration with asphaltene suspension flowing through a clean filter (9) depicts the effect of the cake formation. At the beginning, the slope corresponds to an unsteady state. Slope varies from the one with the pure n-heptane flow on a clean filter (O) to the one with the pure n-heptane flow on the plugged filter (0). This initial behavior is succeeded by a more significant slope decrease. This last step is explained by the cake growth. These experimental results demonstrate that the cake resistivity is non-negligible in comparison to the plugged membrane one. We tested the reproducibility of the various filtration experiments to detect artifacts. We have not given the reproduc-
Asphaltene Cake Properties
Energy & Fuels, Vol. 22, No. 6, 2008 3973
Figure 3. Time dependence (min) of filtrated mass (g) at a pressure drop of 0.5 bar for a pure n-heptane flow on a clean filter (9), a pure n-heptane flow on a plugged filter (0) (the filter was used for a filtration, and cake is removed), and an asphaltene suspension flow on a clean filter (O). Figure 5. Pressure drop on flow ratio (∆P/Q) per unit volume (∆P/ Q/V) (Pa s) as a function of time (s) at 20 °C for three pressure drops: 2 bar (O), 7 bar (4), and 15 bar (0).
Figure 4. Pressure drop on flow ratio (∆P/Q) (Pa s m-3) depending upon filtrated volume (m3) at 20 °C for three pressure drops: 2 bar (O), 7 bar (4), and 15 bar (0).
ibility tests in this work to draw clearer charts. The collected experiments at constant pressure on the same crude oil are plotted in a filtrate volume dependent upon the ratio pressure drop per instantaneous flow rate (∆P/Q). The results are shown for three pressure drops at 20 °C (Figure 4). Each curve shows a linear behavior as predicted by Darcy’s law. The Darcean flow, which is described in the following equation, specifies that the pressure drop over a given distance is proportional to the instantaneous flow rate through the porous medium and the dynamic viscosity of the fluid: ∆P )
ηz Q kΩ
we plot the evolution of each cake layer [pressure drop on flow ratio per unit volume (∆P/Q/V)] as a function of time (Figure 5). At the beginning, we observe a transient regime. The explanation of such a regime is due to the trapping of the biggest aggregates constituting the first layers on the filtering membrane. The resistance of each layer increases gradually until the cake becomes the filtering medium instead of the membrane. Then, each new layer presents stable characteristics corresponding to the cake growth. Another observation is that the cake growth is delayed when the pressure drop increases. This effect is due to the increase of shear rates. Moreover, with pressure, the inflection points of the curves are delayed. The inflection point defines the transition between the transient state and a fixed slope state. Figures 4 and 5 demonstrate that cake properties are pressuredependent, which might be due to the porosity. As pressure increases, the cake becomes more compact and resistant. This effect is enhanced by the trapping of smaller particles because of the pore size reduction. Filtration Model Application. Filtration is used as a separation process in many industries. Consequently, numerous analytical solutions for filtration are available in the literature.41-45 In this work, we choose to use the conventional filtration equations inspired from the Ruth work.46 Meeten47,48 shows that this relation describes incompressible cake as well as highly compressible cake. This filtration theory is based on Darcy’s law (see eq 1), with the filtering medium and the cake treated as resistances in series. The resistance of the filtration medium is represented by Rs (m), and the resistance of the cake is represented by Rg (m). Darcy’s law applied to filtration could be expressed as follows:
(1)
where ∆P is the pressure drop (Pa), η is the dynamic viscosity of the fluid (Pa s), z is the thickness of the cake (or filtration medium) (m), Q is the instantaneous flow (m3 s-1), Ω is the flow section (m2), and k is the permeability of the filtrating medium (m2). At Darcy’s scale, the slope is representative of the term (ηz)/ k, which increases with pressure. At this point, the change of slope could either be due to the contribution of cake growth or cake permeability decrease. To dissociate these two parameters,
(41) Pignon, F.; Magnin, A.; Piau, J.-M.; Cabane, B.; Aimar, P.; Meireles, M.; Lindner, P. J. Membr. Sci. 2000, 174, 189–204. (42) Cabane, B.; Meireles, M.; Aimar, P. Desalination 2002, 146, 155– 161. (43) Green, M. D.; Landman, K. A.; De Kretser, R. G.; Boger, D. V. Ind. Eng. Chem. Res. 1998, 37, 4152–4156. (44) Sorensen, P. B.; Moldrup, P.; Hansen, J. Chem. Eng. Sci. 1996, 51 (6), 967–979. (45) Antelmi, D.; Cabane, B.; Meireles, M.; Aimar, P. Langmuir 2001, 17, 7137–7144. (46) Ruth, B. F. Ind. Eng. Chem. 1946, 38, 564–571. (47) Sherwood, J. D.; Meeten, G. H. J. Pet. Sci. Eng. 1997, 18, 73–81. (48) Meeten, G. H. Chem. Eng. Sci. 2000, 55, 1755–1767.
3974 Energy & Fuels, Vol. 22, No. 6, 2008
∆P ) (Rs + Rg)ηum
Montel et al.
(2)
where ∆P is the pressure drop (Pa), η is the dynamic viscosity of the fluid (Pa s), and um the average speed (m s-1). The average speed expression is the following: 1 dV (3) Ω dt where Ω is the flow section (m2) and V is the flow volume of fluid in m3 that goes through the filtering medium during time t (s). The cake resistance is proportional to the dried cake mass and inversely proportional to the filter section: um )
M (4) Ω where M is the dried cake mass (kg), Ω is the flow section (m2), and R is the specific resistance (m kg-1). The dried mass of cake can be expressed as a function of the suspension volume as follows: Rg ) R
M ) VW (5) where W is the deposit cake mass per volume unit of filtrate. W is assimilated to the apparent concentration of the suspension. W is calculated from the dried mass of the cake and not from the real solid concentration to take into account the loss of particles in the filtrate. From eqs 2-4, Darcy’s law can be developed as follows:
Figure 6. Specific resistance (R) versus pressure drop (∆P) at 20 °C (0) and 55 °C (O). Table 3. Cake Porosities at 20 and 55 °C pressure drop (bar)
2
4
7
10
absolute porosity at 20 °C absolute porosity at 55 °C
0.901 0.900
0.889 0.883
0.885 0.879
0.886 0.880
Table 4. Time Corresponding to the Inflection Point at 20 and 55 °C pressure drop (bar) time (s) at 20 °C time (s) at 55 °C
2
4
7
10
52 150
72 240
90 260
108 280
(9)
The specific resistance calculated from eq 7 is an intrinsic property of the cake, which is measured in situ. The specific resistance is not porosity-dependent; thus, it is preferable to compare specific resistance rather than permeability. Figure 6 shows the specific resistance versus pressure drop at 20 and 55 °C. The specific resistances have an order of magnitude of 1014, which means 10 microdarcy for the permeabilities, and the specific resistance increases with the pressure drop. The influence of temperature is not significant, thus indicating that there is no wax crystallization. Table 3 shows the porosity versus pressure drop at 20 and 55 °C. To calculate the porosities, we measured the dry and wet mass of the deposit. The calculation takes into account all of the solvent trapped in the deposit (even if pores are not connected). The filter cake was removed from the cell, and the weight of the wet filter cake was recorded before oven drying at 65 °C for 24 h. With the density50 of the asphaltenes particles of 1.2 g/cm3, we could calculate the volume and thickness from cakes. Note that the porosities are not measured under the test conditions but under atmospheric pressure. The cake porosities do not vary with the pressure drop and remain roughly the same at 20 or 55 °C. Considering that the resistance increase with pressure is a robust experimental fact, we have calculated the compressibility index at around 0.9. This value indicates that the asphaltene cake is highly compressible. Another observation is that the cake formation is delayed when the temperature increases at a fixed pressure drop. These observations are reported thanks to the times (corresponding to the inflection points of curves plotted in Figure 5), which are detailed in Table 4. Temperature and pressure both have an impact on the formation of the first layers of the cake. However, the noticeable point is that, whatever temperature (for a fixed pressure drop),
where ε is the porosity, Fs is the solid density (kg m3), and k is the permeability (m2).
(49) Tiller, F. M.; Cleveland, T.; Lu, R. Ind. Eng. Chem. Res. 1999, 38, 590–595. (50) Rogel, E.; Carbognani, L. Energy Fuels 2003, 17, 378–386.
VW 1 dV (6) η ∆P ) Rs + R Ω Ω dt After integration, the following equation describes the filtration model for a constant pressure:
(
t)
)
Rsη RWη 2 V+ V ∆PΩ 2∆PΩ2
(7)
where Rs is the resistance of the membrane (m-1), R is the specific resistance of the cake (m kg-1), η is the cinematic viscosity of the filtrate (Pa s), ∆P is the differential pressure (Pa) through the filtration media, Ω is the flow section (m2), W is the deposit cake mass per volume unit of filtrate (kg m-3), t is the time (s), and V is the filtrate volume (m3). The increase of specific resistance as the pressure drop increases is quantified by the compressibility factor, n. It characterizes the reduction of the porosity between the solid particles of the cake, so that it is more difficult for the filtrate to flow through the cake. The variation of the specific resistance with pressure follows the law: R ) R0∆Pn
(8)
where R is the specific resistance of the filter cake (m kg-1), ∆P is the differential pressure (Pa), and n is the compressibility factor or index. Some authors49 argue for the term of compactibility that allows for no confounding of this notion with the compressibility usually found in thermodynamic science. Nevertheless, we choose to use the established terminology that is the compressibility index. The physical meaning of a high compressibility index is that flow resistance and liquid content are very sensitive to changes in contact pressure. Then, at each pressure floor, we can express the cake permeability k)
1 R(1 - ε)Fs
Asphaltene Cake Properties
it does not significantly influence the final cake properties (Figure 6). Considering the temperature data, the impact of pressure on asphaltene cake properties appears to be the key parameter. The main result is that the specific resistances increase with respect to pressure, while absolute porosities remain constant. This intriguing apparent contradiction may be explained either by the measurement of the porosity or through the porous structure of the cake. Indeed, the absolute porosities reported in Table 3 have been measured after depressurization to atmospheric pressure and not under the actual pressures of the different tests indicated in the table. When the tests are stopped and the system depressurized to atmospheric pressure, the filter cake is likely to experience expansion and porosity increase. This could explain why the obtained values are almost similar whatever the pressure of the test. The second explanation is based on the porous structure of the cake. The distribution of pore sizes is unknown and decisive in the interpretation of the results. Nevertheless, it may be reasonable to argue that the cake is comparable to a microporous/mesoporous material, which traps solvent during the filtration between precipitated asphaltenes. This closed fraction does not contribute to the flow. Only a very small number of large pores would in fact be connected. Thus, the high value of the compressibility index is due to this structure, which is comparable to the one with a double-porous medium. In this case, when the diameter of a connected pore is reduced (compaction effect), the impact on the intrinsic resistance will be high, while the absolute porosity remains constant. 4. Summary and Conclusions The aim of this work was to propose a test specifically dedicated to the characterization of parameters that may control the damaging potential of asphaltene deposits thanks to the filtration process carried out under static pressure. Using this method, we characterized the asphaltene cake developed on a membrane with a fixed pore diameter and very limited
Energy & Fuels, Vol. 22, No. 6, 2008 3975
particle-membrane interactions. Experiments carried out on crude oil indicate that two parameters essentially characterize the asphaltene cake: the specific resistance and the delay of the cake growth. Temperature and pressure increases both have a similar impact on cake growth. Increasing those two operational parameters delays the cake growth. Nevertheless, only the pressure has a significant impact on the specific resistance. The pressure drop contributes to a significant increase of the cakespecific resistance, while absolute porosities remain constant around 88%. This observation could be explained from either the way of measurements or the porous structure of the cake. Because the absolute porosities have been measured under atmospheric pressure, the filter cake could expand when the test is stopped. Thus, the absolute porosities increase and do not become representative of the test pressure. The second explanation is based on the porous structure of the cake, which could be compared to a double-porous media. Only a small quantity of pore is connected, and the high absolute porosity may be due to entrapped solvent. A study of the microporous structure of the cake is necessary to the validation of the interpretation proposed. We are actually doing this thanks to neutron-scattering diffusion and cryofracture techniques. Numerous effects, such as precipitation kinetics and asphaltene fraction composition, are also studied to widen the process applications. Acknowledgment. This research was performed at the Total Petrochemical Research Center of Lacq. Special thanks to Patrice Creux from the Laboratory of Complex Fluid for his involvement in the project. The authors also thank Didier Anglerot from the Total Petrochemical Research Center of Lacq and Patrick Bouriat from the Laboratory of Complex Fluids for very helpful dicussions. A special thanks is given to the Total Petrochemical technicians Jean-Michel Gras and Charlie Tramier for helping to develop the process and running experiments. EF8003784
