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Energy & Fuels 2009, 23, 1230–1236
Adsorption of Petroleum Asphaltenes onto Reservoir Rock Sands Studied by Near-Infrared (NIR) Spectroscopy† R. Z. Syunyaev,*,‡ R. M. Balabin,‡ I. S. Akhatov,§ and J. O. Safieva| Physics Department of Gubkin Russian State UniVersity of Oil and Gas, 65, Leninsky Prospekt, Moscow 119991, B-296, Russia, Department of Mechanical Engineering and Center for Nanoscale Science and Engineering, North Dakota State UniVersity, 111 DolVe Hall, Post Office Box 5285, Fargo, North Dakota 58105-5285, and Emanuel Institute of Biochemical Physics RAS, 4, Kosygina Street, Moscow 119334, Russia ReceiVed July 29, 2008. ReVised Manuscript ReceiVed December 11, 2008
Asphaltenes are the most polar petroleum macromolecules. The high polarity causes their rather high superficial activity. A relatively high value of asphaltene adsorption means that this substance is responsible for many phenomena in the petroleum industry, such as well bore plugging and pipeline deposition, stabilization of water/oil emulsions, sedimentation and plugging during crude oil storage, adsorption on refining equipment, and coke formation. The knowledge of kinetic and thermodynamic parameters of adsorption opens a regulation opportunity for the capillary number and wettability. In perspective, it opens a way for physical and chemical engineering of liquid-solid interfaces in the oil industry. In this study, the adsorption parameters of petroleum asphaltenes on quartz, dolomite, and mica sands were investigated using near-infrared (NIR) spectroscopy. Each adsorbent was fractioned in its own particle size range, and the particle size was estimated. The experimental adsorption data were theoretically modeled using Langmuir adsorption. The maximal adsorbed mass density, the adsorption equilibrium constant, and adsorption/desorption rate constants were calculated. The initial stage of the colmatage mechanism for the petroleum collector based on capillary aggregation assumption is offered. Among investigated adsorbents, the highest asphaltene adsorption takes place in the case of mica.
1. Introduction Adsorption is the process of system component redistribution between a bulk mobile phase and a surface layer on a solid phase. Dissolved molecules displace molecules of solvent at the solid interface during the process of adsorption from solution. Different mechanisms of surface layer formation are possible depending upon the different nature of adsorbed components, their concentrations, morphology of the surface of the adsorbent, and its wettability. For low concentrations, the Henry law describes the adsorption process. In this case, the adsorbate concentration linearly depends upon a solute concentration. The most common physical model for a monomolecular adsorption is a Langmuir model. The Brunauer-Emmett-Teller (BET) theory is used for the more complicated case of polymolecular adsorption.1,2 Asphaltene adsorption on the rock surface of a reservoir is the special practical interest of oil production. Asphaltenes are the most high-molecular polar oil components with high surface activity.3,4 Adsorption precedes asphaltenic, resinous, † Presented at the 9th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed. Fax: +7-495-135-8895. E-mail:
[email protected]. ‡ Physics Department of Gubkin Russian State University of Oil and Gas. § North Dakota State University. | Emanuel Institute of Biochemical Physics RAS. (1) Hunter, R. J. Foundations of Colloid Science, 2nd ed.; Oxford University Press: New York, 2001. (2) Schukin, E. D.; Pertsov, A. V.; Amelina, E. A. Colloid Chemistry, 4th ed.; Vyshaya Shkola: Moscow, Russia, 2006. (3) Sayyouh, M. H.; Hemeida, A. M.; Al-Blehed, M. S.; Desouky, S. M. J. Pet. Sci. Eng. 1991, 6, 225–233.
and paraffinic formation of macroscopic deposits on the pores of the reservoir and surfaces of production equipment. Early colmatage of the reservoir may lead to a premature well shutdown.5 Surface layers of asphaltenes and resins can change rock properties. This affects the filtrational characteristics of the processes of oil and water migration in porous media.6 Adsorption of these substances determines surface hydrophobization. Gel-like film formation on the quartz surface is considered an important step of the adsorption process. These films greatly reduce oil penetrability of porous media.7 Adsorption and the following deposit formation are important problems in refinery.8,9 The filtration of fluids is described by the Darcy law6 k Q ) - × grad P η where Q is the filtration rate, k is the permeability of the petroleum collector, η is the viscosity of fluid, and grad P is the pressure gradient. (4) Syunyaev, R. Z.; Balabin, R. M. J. Dispersion Sci. Technol. 2007, 28, 419. (5) Ekholm, P.; Blomberg, E.; Claesson, P.; Auflem, I. H.; Sjo¨blom, J.; Kornfeldt, A. J. Colloid Interface Sci. 2002, 247, 342. (6) Basniev, K. S.; Kochina, I. N.; Maksimov, V. M. Underground Hydromechanics; Nedra: Moscow, Russia, 1993. (7) Adsorption of asphaltenes onto quartz and its influence to wettability. Scientific report. TatNIPI, 1998. (8) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999. (9) Syunyaev, Z. I.; Safieva, R. Z.; Syunyaev, R. Z. Petroleum Dispersed Systems; Khimiya: Moscow, Russia, 1991.
10.1021/ef8006068 CCC: $40.75 2009 American Chemical Society Published on Web 01/28/2009
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Energy & Fuels, Vol. 23, 2009 1231 Table 1. Classification of Porous Materials
macropores supercapillary D mesopores capillary D subcapillary D micropores D
> ) )
quartz > dolomite for small-sized minerals mica ) dolomite > quartz for large-sized minerals For adsorbents of the same nature, the contact area increase (particle size decrease) leads to an elevated adsorption rate, as determined by the curvature of adsorbent grains. The chemical composition and structural parameters of the surface are the main factors that control the adsorption process. Among the investigated adsorbents, mica is the most active asphaltene adsorbent. Highly concentrated asphaltene solutions contain the molecular aggregates involved in adsorption processes.12,13,17,25 This factor has no effect on adsorption kinetic parameters, because adsorption forces are governed by the Hamaker constant, which indicated the level of dispersive interaction between the molecule and the surface. The Hamaker constant value is directly proportional to the kinetic unit size.1,21 Thus, for dimers, it is higher than for monomers. At the same time, for equilibrium conditions, the speed of particle motion to the surface because of the concentration gradient created by the adsorbent surface is balanced by viscous friction force. The diffusion coefficient is inversely proportional to the kinetic unit size. Both factors cancel each other, and the adsorption kinetic parameters are independent of the kinetic unit size. This assumption, however, needs an additional detailed investigation in the future. 4.2. Parameter Estimation of Aggregative Equilibrium in Solution. Relative monomer and relative aggregate content for the chosen asphaltene concentration of 0.1 g/L was estimated (22) Diamant, H.; Andelman, D. J. Phys. Chem. 1996, 100 (32), 13732– 13742. (23) Roshchina, T. M. SEJ 1998, 2 (in Russian).
using the sequential aggregation model.26,27 According to this j on the model, the dependence of an average aggregate size N monomer concentration X was obtained j ) 2 -1 N √X For asphaltenes from the same oil, the aggregate size dependence on the concentration was measured previously by the polarizing fluorescence method,25 and this value is used in our calculations. The kinetic unit size at a concentration of 1 mg/L was measured and taken as the “monomer” size. The calculation shows the relative fraction of monomers as 0.84, dimers as 0.07, and trimers as 0.006, at the concentration of 0.1 g/L. The larger aggregate quantity is negligible. The dimerization energy is about 8.4 kJ/mol. Considering the fact that the dimer and trimer fractions are relatively small, the adsorption process at the concentration of 0.1 g/L can be treated as the monomolecular Langmuir process. Chosen adsorbents are the common minerals that compose typical petroleum collectors. The adsorption process is closely associated with asphaltene molecule diffusion in porous media under the competing conditions with toluene molecules. Toluene molecules compete with dissolved asphaltene molecules in filling the vacant adsorption sites on the surface. In previous works,12,13 the adsorption on the particle surface was carried out under the conditions of the dynamic equilibrium, while the continuous agitation of adsorbents was performed to achieve the maximal adsorption. In our work, to investigate the adsorption processes in porous media, no external agitation or mixing was applied. The adsorbent samples were confined in the permeable membrane capsules to eliminate the solid particle suspension formation, which acts as a light scatterer affecting the transmittance detection. Thus, the main factor of adsorption in our system is the masstransfer rate through the pore channels in the adsorbent. The data were collected with an assumption that the difference in the interaction between solid adsorbent particles in the air and nonpolar fluid28 is negligible. Only the structural parameters of adsorbents were taken into consideration as important factors. The obtained figures allow for the evaluation of the parameters of our model. Preliminary data do not support the mechanism of monomolecular adsorption (Figures 7 and 8). Thus far, the kinetic models correspond to the BET theoretical approach, which describes polymolecular adsorption.1,23 4.3. Concentration Oscillations. Regular deviations from the exponential curve form are observed in all measured curves at the initial stage. These deviations are small and within the limits of experimental error. However, these deviations are very regular, and a similar phenomenon was also observed earlier for resins.16 Similar concentration oscillations of a dye solution in water was measured by laser refractometry. This suggests that the adsorption is kinetically limited by diffusion. For this case, the mechanism of diffusion relaxation applies.24 The classic (24) Akhatov, I. S. Private communication, 1988. (25) Syunyaev, R. Z.; Safieva, R. Z.; Safin, R. R. J. Pet. Sci. Eng. 2000, 26, 31–39. (26) Aguilera-Mercado, B.; Herdes, C.; Murgich, J.; Muller, E. A. Energy Fuels 2006, 20, 327–338. (27) Neu, J. S.; Canizo, J. A.; Bonilla, L. L. Phys. ReV. E: Stat., Nonlinear, Soft Matter Phys. 2002, 66, 061406. (28) Israelachvili, J. N. Intermolecular and Surface Forces: With Applications to Colloidal and Biological Systems; Elsevier Science and Technology Books: London, U.K., 1992.
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Figure 9. “Adsorption pump”. The formation of concentration waves in the bulk phase. Comments are in the text.
Figure 7. Atomic force microscopy (AFM) image of adsorbed asphaltene aggregates onto the surface of mica. Adsorption from 0.1 g/L asphaltene solution in toluene; semi-contact technique in air; average height of adsorbed aggregates h ) 12 ( 2 nm.
Figure 8. Three-dimensional image of a single asphaltene aggregate adsorbed on mica. Adsorption from 0.1 g/L asphaltene solution in toluene; semi-contact technique in air.
Fick’s law can be generalized by introducing an additional relaxation term ∂C ∂j + j ) -D ∂t ∂x where j is the diffusion flow, D is the coefficient of translational diffusion, C is the concentration of the substance, and τ is the characteristic time of relaxation. The equation transforms to the partial derivative equation of the second order τ
∂2c ∂2c ∂c + )D 2 2 ∂t ∂t ∂x The solution for the equation is the superposition of two concentration time-dependent functions. One of these functions is the solution for a damped oscillation equation. For porous media neighboring the bulk phase and at a condition when adsorption time is much less than diffusion time τ
τads , τdif the mechanism of the “adsorption pump” applies. While filling porous media by solution, the adsorption process occurs faster. The result is the bulk equilibrium concentration decrease inside the porous media. Because the diffusion process goes more
slowly, discontinuity on the concentration occurs on the porous media boundary, creating the region with a decreased concentration. As a consequence, the diffusion flow emerges at this boundary. Then, the concentration jump spreads to the bulk phase while gradually decreasing its amplitude because of diffusive spreading (Figure 9). This is the damping concentration wave in the bulk phase. 4.4. Average Concentration of Asphaltene Solution in Porous Media. The average concentration of asphaltenes in porous media turns out to be higher than in a solution bulk phase of the solution. This parameter includes a pore bulk concentration and adsorbed layers. Equilibrium conditions mean that the concentration in the solution bulk phase and pore bulk phase must be the same. An increased concentration in the pore bulk phase suggests the formation of dense polymolecular layers. The process of the monomer selfassociation accelerates at the solid-liquid interface. This leads to the aggregate formation on the surface.29 AFM analysis of asphaltene adsorption on mica shows the formation of large clusters with characteristic sizes that greatly exceed the monomer size (Figures 7 and 8). The first step of the adsorption process is the filling of micropores that are associated with particle surface roughness. After the micropore saturation, the contact points of neighboring particles become centers of adsorption. These points have the maximal surface curvature. According to the Kelvin-Thomson equation, this leads to the acceleration of the monomer aggregation process. For gas adsorption on porous media, this effect is commonly known as capillary condensation.1,6 An analogous effect in solutions can be named as capillary aggregation. Adsorbed gel-like layers are formed rapidly near contact points. The formation of these layers initiates the colmatage process in porous media. Permeability k will decrease with growth of such layers, following the reduction of the effective diameter of capillaries. Quantity of these contacts in a volume unit is maximal for fine-grained fractions. The obtained data prove this statement for quartz and mica (Table 5). In the case of finegrained dolomite, the absence of this effect may be determined by a particularly different interaction type between dolomite particles in nonpolar fluids. 5. Summary Asphaltene adsorption at the surface of mineral powders, which are a framework of the oil reservoir, was studied by NIR spectroscopy. The experimental procedure of surface asphaltene concentration measurement is proposed. Adsorption parameters are evaluated using the Langmuir model of (29) Holmberg, K.; Jonsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqeous Solution, 2nd ed.; John Wiley and Sons, Ltd.: New York, 2003.
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monomolecular adsorption. Adsorption characteristics are important for the permeability coefficient estimation in the Darcy law. That is necessary for further filtration theory development. The capillary aggregation mechanism as the initial stage of colmatage is proposed. The generalized investigation of the multilayer adsorption of the aggregate using AFM is required in the future. Adsorption of asphaltenes is defined by the combined factors of a chemical composition of adsorbents and its
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polarity, its textural or structural characteristics, and solvent properties. For the future investigations, it is necessary to pay attention to the solvent effect neglected in the present study, which may change coupling forces between rock particles. Such dot contacts between grains are centers for developing capillary aggregation and a further colmatage process of porous media. EF8006068
