pubs.acs.org/Langmuir © 2009 American Chemical Society
Patterned Wettability of Oil and Water in Porous Media Munish Kumar and Andrew Fogden* Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra ACT 0200, Australia Received September 15, 2009. Revised Manuscript Received October 22, 2009 The microscopic wettability state of porous media, based on glass bead packings, after crude oil drainage of brine was investigated using X-ray micro-CT, white-light profilometry, and electron microscopy. Tomography revealed that the bulk residual brine occupied around 10% of void space, located in smaller pores and as pendular rings around bead contacts, in agreement with numerical simulations of drainage. The bead packing contained planar slabs of mica, quartz, and oxidized silicon wafer, which after flushing and disassembly of the pack allowed analysis of their wettability alteration due to deposition of asphaltenes from the crude oil. These substrates exhibited an overall pattern of rings with clean interiors, matching the brine pendular ring size inferred from experimental and simulated drainage, and asphaltene deposition in their exteriors, verifying the mixed wet model of oil reservoir wettability. The extent of asphaltene intrusion into ring interiors and completeness of asphaltene coverage of exteriors both increased with overall deposition tendency for the brine composition. The observed dependence on NaCl concentration and pH was consistent with expectations from DLVO and non-DLVO interactions governing brine thin film rupture and subsequent asphaltene deposition.
Introduction Multiphase flow of immiscible fluids in porous media is dictated by the boundary conditions, the physical structure of the void space, and the chemical interactions of the fluids with each other and the pore walls. The relative affinity of the two fluids for the solid surface is termed the wettability, manifested in the contact angle of the bulk fluid-fluid interface with the solid, and dictated at the molecular scale by the thin-film forces.1,2 For a porous medium filled with a wetting fluid, which is drained by a nonwetting fluid, the latter will occupy the larger pores and leave wetting fluid lining these pore walls and in smaller pores and crevices. Conversely, for imbibition of a wetting fluid into a porous medium filled with nonwetting fluid, the former will tend to displace the nonwetting fluid from pore walls and small pores and trap it as snapped-off droplets or bubbles in larger cavities.3 The fluid dynamics become more complex if the pore wall wettability is nonuniform throughout the medium, and further complicated if this heterogeneity arises from the ability of one of the liquids to alter the original wettability of walls by adsorption or deposition of components. Such situations are conceivable across a broad range of extraction, separation, or contamination processes or phenomena involving complex fluids. Thus, while the current study uses the prime example of oil reservoirs and their recovery by waterflooding,4-7 the approach and insight gained is quite general and readily transplantable. The above-mentioned drainage process, for brine-filled waterwet rock invaded by crude oil rising under hydrostatic pressure, corresponds to reservoir formation. Salathiel8 first postulated that the most polar, highest molecular weight components of the *Corresponding author. E-mail:
[email protected]. (1) Derjaguin, B. V.; Churaev, N. V. J. Colloid Interface Sci. 1978, 66, 389. (2) Hirasaki, G. J. Soc. Pet. Eng., Form. Eval. 1991, 6, 217. (3) Dullien, F. A. L. Porous Media: Fluid transport and pore structure, 2nd ed.; Academic Press: San Diego, 1992. (4) Anderson, W. G. J. Pet. Technol. 1986, 38, 1125. (5) Anderson, W. G. J. Pet. Technol. 1987, 39, 1453. (6) Morrow, N. R. J. Pet. Technol. 1990, 42, 1476. (7) Morrow, N. R.; Mason, G. Curr. Opin. Colloid Interface Sci. 2001, 6, 321. (8) Salathiel, R. A. J. Pet. Technol. 1973, 25, 1216.
4036 DOI: 10.1021/la903478q
crude oil, its asphaltenes, and resins,9,10 can rupture the thin brine films lining larger pore walls to deposit on these surfaces and render them oil-wet, while the smaller pores remain shielded by residual brine and retain their water-wetness (Figure 1). Asphaltenes are defined as the petroleum fraction insoluble in an alkane (e.g., n-heptane) but soluble in toluene. In this so-called mixed wet model, the oil- and water-wet patterning of the pore walls is determined by the applied bulk capillary pressure difference between oil and water in a twofold manner.11 This pressure sets the bulk oil-water meniscus curvature via the Laplace equation, dictating (along with pore connectivities) the size and shape of pores able to be intruded by oil. Further, the competition between capillary pressure and the brine thin film disjoining pressure dictates whether the films remain stable or collapse to directly expose the walls to the oil over these areas. The above-mentioned imbibition process relates to subsequent oil recovery by water flooding, and the mixed wet model can be applied to interpret the degree to which oil trapping by snap-off is reduced, and its displacement thus sustained, by the postulated oil-wet percolating pathways preserving its hydraulic connectivity. Theoretical work has proceeded on two fronts to formalize (a) the thermodynamics of wettability alteration2,11,12 and (b) the fluid dynamics of multiphase flow during drainage and imbibition. The latter includes analytical approaches to calculate menisci in simplified pore elements, e.g., tubes with noncircular cross-section,13 and numerical simulations on networks of tubes14 or bead packings.15 Efforts to bridge these two disciplines have given rise to mixed wet network models in which pore walls are binary patterned water-wet and oil-wet, based on an assumption of wettability alteration, with some choice of lower and higher contact angles, respectively, ascribed to brine advancement over (9) Speight, J. G. Oil Gas Sci. Technol. 2004, 59, 467. (10) Buckley, J. S. Energy Fuels 1999, 13, 328. (11) Kovscek, A. R.; Wong, H.; Radke, C. J. AIChE J. 1993, 39, 1072. (12) Buckley, J. S.; Takamura, K.; Morrow, N. R. Soc. Pet. Eng., Res. Eng. 1989, 4, 332. (13) Mason, G.; Morrow, N. R. J. Colloid Interface Sci. 1991, 141, 262. (14) Lenormand, R.; Touboul, E.; Zarcone, C. J. Fluid Mech. 1988, 189, 165. (15) Bryant, S.; Johnson, A. J. Colloid Interface Sci. 2003, 263, 572.
Published on Web 11/16/2009
Langmuir 2010, 26(6), 4036–4047
Kumar and Fogden
Figure 1. Pendular ring of residual brine in a rock crevice surrounded by oil, having ruptured the brine thin film lining the open pore walls to deposit a film of asphaltene and resin.
these areas during imbibition, and with some degree of pinning and hinging at their boundary.11,16,17 Thermodynamic theories of wettability alteration have been guided by contact angle measurements of crude oil drops on smooth planar water-wet substrates in brine to understand the compositional properties conducive to thin film rupture.12,18,19 Further, the nanoscopic asphaltene deposition over such flat substrates from immersion in crude oil has been imaged by atomic force microscopy (AFM).20-22 A larger body of literature has focused on oil-brine flow experiments in rock plugs to realistically recreate on the small scale the drainage and aging during reservoir creation and imbibition (spontaneous or forced) of brine during oil production. The bulk measures of capillary pressures, liquid volumes displaced, and permeabilities have provided substantial qualitative empirical evidence supporting the mixed wet model.4-7,23 Numerical simulations of the oil-water flow in these pore networks incorporating mixed wettability show some consistency with the experimental measures, providing some quantitative support for this model.24 At present, the simulations have little predictive power, though, and the degree of wettability alteration and advancing contact angles are often used as fitting parameters. Many studies have used other experimental techniques to shed further light on the true wettability state or the true shape and movement of oil-brine menisci in porous media. Surface-sensitive methods such as surface force apparatus,25 AFM colloidal probe,26 and quartz crystal microbalance27,28 have been applied to monitor the deposition from crude oil in the presence of brine on smooth substrates. Direct analyses of oil/brine-filled rock plugs, by cryo/environmental scanning electron microscopy29,30,23 or recently by nondestructive X-ray microcomputerized tomography (μ-CT),31,32 have been successful in characteriz(16) Ma, S.; Mason, G.; Morrow, N. R. Colloids Surf., A 1996, 117, 273. (17) Blunt, M. J. J. Pet. Sci. Eng. 1998, 20, 117. (18) Yang, S.-Y.; Hirasaki, G. J.; Basu, S.; Vaidya, R. J. Pet. Sci. Eng. 2002, 33, 203. (19) Vijapurapu, C. S.; Rao, D. N. Colloids Surf., A 2004, 241, 335. (20) Yang, S.-Y.; Hirasaki, G. J.; Basu, S.; Vaidya, R. J. Pet. Sci. Eng. 1999, 24, 63. (21) Lord, D. L.; Buckley, J. S. Colloids Surf., A 2002, 206, 531. (22) Kumar, K.; Dao, E.; Mohanty, K. K. J. Colloid Interface Sci. 2005, 289, 206. (23) Skauge, A.; Spildo, K.; Hoiland, L.; Vik, B. J. Pet. Sci. Eng. 2007, 57, 321. (24) Dixit, A. B.; Buckley, J. S.; McDougall, S. R.; Sorbie, K. S. Transport Porous Med. 2000, 40, 27. (25) Drummond, C.; Israelachvili, J. J. Pet. Sci. Eng. 2004, 45, 61. (26) Basu, S.; Sharma, M. M. J. Colloid Interface Sci. 1996, 181, 443. (27) Ekholm, P.; Blomberg, E.; Claesson, P.; Auflem, I. H.; Sjoblom, J.; Kornfeldt, A. J. Colloid Interface Sci. 2002, 247, 342. (28) Goual, L.; Horvath-Szabo, G.; Masliyah, J. H.; Xu, Z. H. Langmuir 2005, 21, 8278. (29) Robin, M. Oil Gas Sci. Technol. 2001, 56, 55. (30) Kowalewski, E.; Boassen, T.; Torsaeter, O. J. Pet. Sci. Eng. 2003, 39, 377. (31) Sarker, M. R. H.; Siddiqui, S. Soc. Pet. Eng. 2009, No. 126039. (32) Kumar, M.; Senden, T.; Knackstedt, M. A.; Latham, S. J.; Pinczewski, V.; Sok, R. M.; Sheppard, A. P.; Turner, M. L. Petrophysics 2009, 50, 311.
Langmuir 2010, 26(6), 4036–4047
Article
ing the distributions of the bulk liquids. However, generally the studies of intermolecular interactions are hampered by the instrumental constraints, demanding that much of the reality of the liquids present, their temperatures, pressures, and pore environment, be abandoned. Conversely, the porous medium imaging techniques are currently insufficiently sensitive to distinguish the surface chemistry giving rise to the observed bulk meniscus configurations. The current study is aimed at this gap in knowledge of the microscopic wettability state and the validity of the mixed wet model. It uses porous media, liquids, and conditions sufficiently realistic for the results to be relevant, yet also allowing analysis by sensitive techniques capable of resolving the surface chemistry. The experimental setup employs a porous plug based on a glass bead packing, for drainage experiments and in vivo μ-CT analysis of bulk liquid distributions, with flat mineral slabs included in the packing to enable their surface analysis in the dry state after flushing and disassembly to characterize wettability alterations by asphaltene deposition.
Materials and Methods Preparation of Porous Plugs. The beads were a soda lime glass, smooth microsphere product, GP0116 (Whitehouse Scientific, U.K.), of mean diameter 116 μm, with 90% of beads in the range 106-125 μm. The dry product was cleaned in 10 wt % NaOH solution for 7 days, then repeatedly washed with deionized water from a Millipore Milli-Q reverse osmosis system (as used throughout) and dried. The three planar silicate substrates were Muscovite mica (freshly cleaved to ∼0.1 mm thick), cleaned quartz slide (SPI Supplies, USA, 0.5 mm thick), and oxidized silicon wafer (Peregrine Semiconductor Australia, 0.7 mm thick with 3-5 nm natural oxide layer), snapped to ∼2 2 mm2 slabs. Immediately prior to plug preparation, all four components were further cleaned in an in-house-built RF water-plasma treatment unit operated at 100 W for 5 min. Each cylindrical porous plug comprising the bead pack with these substrate inclusions was assembled using two circular sintered glass frits (Robu, 5 mm diameter and 2 mm thick, with pore size 100-160 μm and porosity ∼40%), similarly plasma precleaned, as stops at the top and bottom of the pack. The frits also provided capillary continuity, thereby reducing capillary end effects common in fluid drives. Below the bottom frit, a hollow aluminum rod (5 mm outer diameter and 10 mm long) acted as a liquid injection port. The plug was prepared inside a tube of heat shrink fluoropolymer (RT-375, Farnell Electronics, Australia, 6.4 mm inner diameter and 0.3 mm wall thickness), chosen to be inert to the liquids. Beads were poured in, and substrate slabs were regularly embedded such that the pack contained three well-separated pieces of each of the three types. The fluoropolymer sleeve was then heat-shrunk using a heat gun at ∼130 °C to apply a confining pressure to seal gaps and increase bead packing density to minimize subsequent movement. Five such porous plugs, with diameter and length of approximately 7 and 20 mm, were prepared for the five flow experiments, and each was plasma cleaned once more before use. Brines and Crude Oil. Aqueous stock solutions of NaCl (AnalaR, 99.9%) were prepared at 0.01 and 1 M, denoted low (L) and high (H) salinity, with pH values ∼5.7, typical of Millipore water in equilibrium with dissolved CO2. The solutions were halved and made more strongly acidic (denoted A) by HCl addition, or close to neutral (N) by NaOH addition to counteract dissolved CO2. A fifth brine was prepared using instead the heavier 1:1 electrolyte cesium iodide (Sigma Aldrich, 99.9%) at 0.25 M to give an appropriate level of X-ray attenuation for μ-CT analysis, and without pH adjustment. The pH of each brine bath after ambient conditioning of the porous plug directly prior to flow experiments is listed in Table 1. The crude oil was from the DOI: 10.1021/la903478q
4037
Article
Kumar and Fogden
Table 1. Concentration, pH, and Short-Hand Notation of the Matrix of Five Brines notation
ionic strength (M)
pH
NaCl L_A NaCl L_N NaCl H_A NaCl H_N CsI
0.01 0.01 1 1 0.25
4.1 7.8 4.8 6.9 5.7
Minnelusa field in Wyoming, which is a reasonably representative asphaltic crude, well-studied in the literature. According to ref 33, it has density 0.9062 g cm-3, viscosity 77.2 mPa.s, n-C7 asphaltene content 9.0 wt %, and acid and base numbers 0.17 and 2.29 mg KOH/g oil, all at room temperature. Flow Experiments. Each porous plug assembly with connecting tubing was saturated in its brine under vacuum, and remained immersed in this bath for 24 h at ambient conditions to attain ionic equilibrium. On removal, the tube end of the brine-filled assembly was connected to a glass syringe containing crude oil, preheated to 60 °C, mounted in a motorized syringe pump. The oil was driven at a constant flow rate in the range (4 10-4)(5 10-4) cm3 s-1, and its drainage of brine was continued until oil breakthrough was observed at the outlet end of the plug. The tubing was then disconnected, and the plug was immersed in a sealed vessel of the crude oil held at 60 °C for 6 days to allow equilibration of oil deposition.6,33,34 After this aging, we followed literature procedures33 to dilute and remove the injected crude oil while minimizing the likelihood of destabilizing the oil or perturbing the state of its deposits on the solid surfaces by undesirable fresh precipitation or stripping. The plug was reconnected to the syringe pump for decalin (decahydronaphthalene, Sigma Aldrich, 98%) to be pushed through in an amount equivalent to 70 pore volumes (PV) until the effluent was completely clear. The plug was immersed in fresh decalin for 24 h, and reinjected with 20 PV. The decalin was subsequently displaced using 50 PV of heptane (Riedel-de-Haen, 99%) to remove all but the asphaltene component of the deposits, followed by 50 PV of water to remove salt. Finally, the plug was dried under vacuum for 24 h and dismantled for analysis. X-ray Micro-CT. To complement the analyses of the disassembled plug components outlined below, the ANU X-ray μ-CT facility35 was employed to nondestructively image in 3D the intact plug for the CsI brine sample. The plug was imaged in the dry state prior to brine infiltration and the wet state (sealed) after oil drainage of brine and aging, before solvent flushing and disassembly. In both cases, images were taken from the central 6 mm region of the plug. The reconstructed tomograms were composed of 20483 voxels at a resolution of 4.5 μm/voxel. The dry tomogram served to quantify the structure of the bead packing and its slab inclusions. Superposition of the dry and wet tomograms allowed judgment of bead movement during flow and aided in discerning the pore-scale distribution of intruded oil and residual brine, X-ray contrasted by the 0.25 M CsI. Computational analyses performed are described in the Results section. Profilometry. As AFM is often used for fine-scale imaging of crude oil components on smooth mineral substrates,20-22 some slabs were analyzed with an Asylum Research MFP-3D AFM in tapping mode in air. The alternative technology of optical profilometry employing white-light interferometry36 allows more rapid, noncontact acquisition of 3D surface maps, with angstrom vertical resolution and lateral resolution around the wavelength of light, giving a field of view on the millimeter scale across the (33) Tie, H.; Tong, Z.; Morrow, N. R. Proceedings of the Society of Core Analysts; Pau, 2003, Paper 2. (34) Wendel, D. J.; Anderson, W. G.; Meyers, J. D. Soc. Pet. Eng. Form. Eval. 1987, 2, 509. (35) Sakellariou, A.; Sawkins, T. J.; Senden, T. J.; Limaye, A. Physica A 2003, 339, 152. (36) Lee, B. S.; Strand, T. C. Appl. Opt. 1990, 29, 3784.
4038 DOI: 10.1021/la903478q
substrate. This technique was better-suited to imaging in air any crude oil deposits patterned by the bead packing on the three substrates, and became our principal quantitative tool. A Wyko NT9000 (Veeco) profilometer was used with a 20 microscope objective, 1.0 zoom, and numerical aperture of 0.4. For each substrate slab, and both of its sides for mica and quartz, a number of subareas were scanned over a 640 480 grid of pixels, at ∼0.49 μm pixel resolution, using a phase-shifting interferometry (PSI) high-magnification mode filter.37 The PSI mode operates by vibration of a piezoelectric transducer within the microscope head, which precisely shifts the interference pattern six times, each shift being captured as one data frame. We averaged 20 such frames before reconstructing the topographical map. Electron Microscopy. The oil-brine treated beads and substrate slabs were placed on double-sided adhesive carbon tabs on aluminum stubs and imaged using a field emission scanning electron microscope (FESEM, Zeiss UltraPlus Analytical) under high vacuum in secondary electron mode. As deposit height is typically on the nanometer scale, even a light platinum coating sufficed to eradicate all features, so the samples were imaged uncoated. Accordingly, the microscope was operated at low voltage (0.5 kV), small aperture (7.5 μm), low magnification, and rapid, low-resolution image acquisition, to decrease beam charging effects.
Results and Discussion X-ray Micro-CT. The image analyses of the reconstructed tomogram of the dry plug are depicted in Figure 2. Noise in the raw tomogram was reduced using anisotropic diffusion.38 The slice in Figure 2a shows five substrate slabs embedded at varying orientations in the bead pack. The packing of beads appears random, but conforms to contact the slabs over the majority of their faces. The grayscale tomogram was segmented using a converging active contours algorithm39 to distinguish beads and slabs (white) from void space (black) in Figure 2b. The segmented tomogram was partitioned into cells, each containing a single bead or slab, using a Euclidean distance transform and watershed algorithm40 to obtain size and coordination statistics of these solid elements. The void space comprises pore bodies (internal cavities formed by four or more neighboring beads or external vacancies) connected by narrower pore throats, e.g., constrictions between three contacting beads. To extract their statistics, an algorithm for topological partitioning of void space into pores and throats40 was applied to the segmented tomogram, with a representative network shown in Figure 2c and the distribution of pore and throat sizes in Figure 2d. These same procedures were also applied to a 3D subset free from slabs, and the statistical measures are summarized in Table 2. The differences between the corresponding measures with and without the slabs are slight, as expected from their low frequency in the plug and the short-range nature of these measures. Slabs have greater impact on longer-range properties such as plug permeability. For the bead pack subset, the volume fraction and coordination number of the solid phase are in line with studies of similar systems,41,42 and imply that our random bead packing is somewhat looser than a close packing. From FESEM, the beads have a low polydispersity in size and are typically spherical, but with a small population bearing a single spherical nodule or internal bubble. This leads to a slight underestimate of (37) Harasaki, A.; Schmit, J.; Wyant, J. C. Appl. Opt. 2000, 39, 2107. (38) Perona, P.; Malik, J. IEEE T. Pattern Anal. 1990, 12, 629. (39) Sheppard, A. P.; Sok, R. M.; Averdunk, H. Physica A 2004, 339, 145. (40) Sheppard, A. P.; Sok, R. M.; Averdunk, H. Proceedings of the Society of Core Analysts; Toronto, 2005, Paper 20. (41) Gotoh, K.; Finney, J. L. Nature 1974, 252, 202. (42) Aste, T.; Saadatfar, M.; Senden, T. J. Phys. Rev. E 2005, 71, No. 061302.
Langmuir 2010, 26(6), 4036–4047
Kumar and Fogden
Article
Figure 2. Sequence of analyses of the dry tomogram: (a) filtered raw grayscale slice of 4.2 3.3 mm2, showing inclusions of mica (e.g., top right), quartz (bottom left), and silicon wafer (e.g., bottom right); (b) binary segmentation; (c) network representation of pores (balls) and throats (connecting cylinders) from partitioning of void space for this cube subset of 1.3 mm length; (d) pore and throat size distributions. Table 2. Mean, with Standard Deviation, Characteristics of the Solid Phase, and Pore Network of the Dry Plug from μ-CT, Derived from the Full Tomogram or a Subset Containing Only Beads full tomogram solids
pores
bead pack subset solids
pores
volume fraction (%) 61.8 38.2 59.8 40.2 body radius (μm) 46.3 ( 11.8 22.4 ( 6.1 45.8 ( 12.4 21.9 ( 5.9 throat radius (μm) 13.9 ( 4.1 11.9 ( 4.0 13.4 ( 3.5 11.9 ( 3.8 coordination number 6.9 ( 3.8 8.1 ( 4.1 6.8 ( 2.1 7.9 ( 3.4
their body size and overestimate of its variation in Table 2. The solid-phase throat radius is that of the circular zone between contacting beads within which the surrounding air cannot be resolved, and corresponds to ∼2-3 voxels. The characteristics of the bead pack pore network are also in line with expectations, e.g., the radius of a throat between three triangularly packed spheres of radius R is (2/31/2 - 1)R = 9.0 μm for our average bead size, which lies slightly below the value in Table 2 due to the looser packing. Figure 3a shows a subset of the reconstructed tomogram of the plug in its wet state, after oil drainage of the CsI brine and aging. As expected, the majority of void space has been invaded by oil (dark regions) and the remaining brine appears as small gray regions encompassing several beads, filling smaller pores or collaring contact points. This verifies that the asphaltene deposits analyzed below after removal of the liquids and disassembly indeed pertain to substrates surrounded by bulk oil with pockets and pendular rings of residual brine. As the brine and beads exhibit similar X-ray attenuation, their segmentation would require exact registration of wet and dry tomograms to subtract the beads. However, as the bead packing is somewhat loose, it is inevitable that some relative movement of beads occurs during oil injection. Application of an image registration algorithm43 (43) Latham, S.; Varslot, T.; Sheppard, A. P. Proceedings of the Society of Core Analysts; Abu Dhabi, 2008, Paper 35.
Langmuir 2010, 26(6), 4036–4047
confirmed that some beads in the bulk underwent slight displacements, although the slabs exert a rigidifying effect, remaining fixed and also fixing the beads contacting them. This is illustrated by comparison of bead locations in Figure 3a to those in the corresponding dry subset of Figure 3b or c (temporarily ignoring the white regions painted on these). As global bead registration is not exact, the issue of whether solid regions adjacent to oil are truly exposed or sheathed by a thin brine film can neither be resolved nor inferred by assessment of uniformity of bulk meniscus curvature indicative of brine continuity via thin films. Thus, the microscopic analysis of asphaltene deposits on the substrate slabs, further supported by this tomographic evidence that such deposits are not affected by bead translation or rotation, is necessary to reveal the true wettability state. While the pore-scale distribution of residual brine cannot be distinguished exactly, its overall amount can be determined by segmenting the combination of beads and brine from oil in the wet tomogram and comparing to the solid segmentation of the dry tomogram. From these global averages, the observable residual brine is deduced to occupy approximately 10% of pore space, which is similar to residual wetting phase values reported by others.15 Further insight can be gained by performing numerical simulations of oil drainage of the brine-filled dry tomogram. The capillary drainage transform algorithm44 was applied with boundary conditions of one end face of the tomogram in contact with a reservoir of wetting fluid (brine) at pressure Pw and the other face contacting a nonwetting fluid (oil) reservoir at pressure Pnw. The capillary pressure, Pc = Pnw - Pw, is increased from zero and at each increment the Laplace equation, modified to account for pore geometry,3 is applied at each interface to test for stability. If Pc exceeds the entry threshold of a pore throat, the interface is advanced through this throat and its adjoining pore body to eventually reach a stable position at a pore throat of prohibitively small radius. At the conclusion, wetting fluid may (44) Hilpert, M.; Miller, C. T. Adv. Water Resour. 2001, 24, 243.
DOI: 10.1021/la903478q
4039
Article
Kumar and Fogden
Figure 3. (a) Subset of 1.2 1.4 mm2 of the wet tomogram, with dark regions the oil and small gray regions the brine between slightly darker gray solids. (b,c) Corresponding subset of the dry tomogram, with small white regions the likely locations of residual brine after simulated oil invasion from the top (b) or bottom (c). (d) Cumulative fraction of void space occupied by oil from simulated drainage of the brine-filled dry tomogram, from top or bottom, as a function of progressively smaller pore radii invaded.
remain in regions surrounded by the invading fluid. The residual wetting phase is likely to span the network in 3D due to the existence of connected pendular rings and/or wetting films in corners or crevices. Figure 3d shows the fraction of dry tomogram void space occupied by oil invaded from the top or bottom face in this simulation, as a function of the progressively increasing Pc, i.e., decreasing capillary radius. The two curves are almost identical and exhibit a steep initial rise, attesting to plug homogeneity. For quantitative comparison to the experimental drainage, the plug permeability (including its slabs) was estimated from the Kozeny-Carman relation45 as 3.6 10-12 m2. Using the measured plug dimensions and flow rate, and estimates of oil viscosity and oil-brine interfacial tension,33 with a brine-receding contact angle of zero, the Darcy equation predicts a capillary radius of ∼12 μm for our experimental drainage. This corresponds in Figure 3d to a residual brine content of ∼12%, in good agreement with the 10% value inferred from the wet tomogram. As further evidence, Figure 3b,c show the simulated distribution of residual brine (white) at this capillary radius, for oil invasion from the top and bottom, respectively. The distributions are similar to each other and also to the wet tomogram of the actual experiment. Surface Analysis of Planar Substrates. Initial attempts at profilometric analysis of the oil-brine treated substrate slabs using tapping-mode AFM revealed a ring-like structure of asphaltene deposits. However, their greasy nature and the sometimes steep variations across rings caused stiction and rapid contamination of the tip, with a high frequency of smearing and streaking. Further, the lateral scanning range was often too limited to encompass a full ring. The optical profilometry technique was instead used to analyze the matrix of substrates. Its images of all substrates displayed a disordered array of ring features, with the zone near the center of each lying below the (45) Bear, J. Dynamics of fluids in porous media; Dover: New York, 1972.
4040 DOI: 10.1021/la903478q
Figure 4. 3D view of asphaltene deposit pattern on the silicon wafer for L_A brine from profilometry.
average height across ring exteriors (an example of a 3D view is shown in Figure 4), in agreement with AFM. This qualitatively confirms the mixed wettability model’s premise that asphaltene deposition is hindered over substrate subregions covered by bulk pendular rings of residual brine.6,8,11 More quantitative analysis of the profilometry raw data was performed as follows. For each 313 235 μm2 image, corrected for tilt and curvature, subareas of ∼100 100 μm2 containing an asphaltene ring and its surroundings were cropped. Figure 5 shows one such cropped image on mica for its four NaCl brines. From each (x, y, z) data set, extraction of the deposit height profile necessitated subtraction of the substrate, typically deviating from flatness to a degree comparable to or greater than the deposit thickness. The inner zone of the ring over which deposition appeared nonexistent or slight was selected, also avoiding Langmuir 2010, 26(6), 4036–4047
Kumar and Fogden
Article
Figure 6. Angle-averaged profile of asphaltene deposit height on quartz for brine L_N, sampling 2-3 rings on each of the two sides of the three slabs, giving 15 rings in total.
Figure 5. Representative cropped raw height maps (brighter indicates higher) of a ring of asphaltene deposit and its immediate surroundings on mica after oil-brine treatment for the NaCl brines: (a) L_A; (b) L_N; (c) H_A; (d) H_N, with 20 μm scale bar for all.
sunken or raised damage zones (e.g., black scrapes and associated white spots in Figure 5) caused by the contacting bead, and its height was least-squares fitted to a second-order polynomial in (x, y). This adjusted height map was further thresholded to discard all z values below -2 nm (symptomatic of substrate pits or gouges) and above 20 nm (due to gouged-out substrate or occasional particle contamination). From this corrected topographic map, the profile of angle-averaged deposit height, z, as a function of lateral radial distance, r, from the ring center was calculated. This procedure was applied to around 11 rings for each substrate type and brine, e.g., Figure 6 plots the radial profile for rings sampled on quartz for the brine L_N in Table 1. From each such family of curves, their average profile was calculated. Figure 7 plots this average, together with its standard deviation, on quartz for the four NaCl brines to illustrate the typical variations. Figure 8 collects the averaged deposit profiles for all five brines on the three substrates. Profiles are generally quite flat over the cleaner inner zone of 20-30 μm radius, beyond which deposition becomes more prevalent as the ring is traversed. At even greater distances, deposition decreases or attains a plateau, with some variability and statistical uncertainty, especially for mica and quartz, depending on proximity to neighboring rings. To complement the profile information, small-scale surface roughness was determined. From six of the rings used in profiling for a substrate-brine pair, four 5 5 μm2 areas were selected within the cleaner inner zone, avoiding defects, with another four taken from just outside the peak or on the plateau. Their local plane of best fit was removed from the raw height map, and rms roughness Rq was calculated. Averages are plotted in Figure 9 for these inner and outer zones, and for cleaned “bare” substrate slabs. The bare substrates increase in Rq from mica to quartz to silicon wafer. The absolute values are underestimates as the 0.49 μm pixels miss finer features; however, they allow relative assessment of deposition. For the inner zone, even the highest Rq on mica lies below the lowest for quartz, and all quartz values lie Langmuir 2010, 26(6), 4036–4047
below the lowest on silicon wafer. Asphaltene deposits add roughness to the bare mica and quartz, but reduce roughness of the silicon wafer by partially filling gaps between its bare asperities. Figure 10 displays FESEM micrographs of deposits on silicon wafer for the NaCl brines. For each sample, the instrument settings and operation was identical, with no adjustments to brightness and contrast. While SEM techniques are not suited to imaging nanometric, low-density organic films over millimeter fields of view, the textures here are clearly distinguishable. Even at higher magnification, it is difficult though to confirm that darker regions are indeed raised above their brighter neighbors, i.e., that deposit thickness and/or coverage increases with darkness. Interpretation thus requires the support of the profilometric analysis. The following discussion of brine trends combines the findings from these two techniques. For mica and quartz, the FESEM images display similar deposition textures and trends to the silicon wafer, but with more severe beam charging for these nonconducting substrates. Low-Salinity Acidic Brine. For L_A, the two smoother substrates, mica and quartz, bear a speckled deposit of submicrometer asphaltene disk particles connected as chains and accumulated at the ring boundary (Figure 5). The deposits irregularly finger as dendritic branches toward the ring interior, but typically do not extend to the innermost zone; thus, this relatively clean subregion exhibits low Rq (Figure 9). Exteriors are sometimes covered by this same particle texture or as a more continuous accumulation, while others are quite clean. The average deposit thickness is ∼4 and 3 nm for mica and quartz (Figure 8), and owing to the absence of thick discrete deposit features, the Rq immediately external to the ring is reasonably low (Figure 9). For the silicon wafer, the ring interior is cleanest for this brine (Figure 9), i.e., its bare roughness free of blanketing deposit perseveres up to the boundary, where the profile rises most steeply of all brines (Figure 4). As Figure 8 averages over rims of slightly differing radii, it broadens and flattens this peak; the true maximum height averaged over all rings is 8.5 nm. Beyond this, the deposit film height decreases, although with large variation, as some rings decay slowly, whereas others drop rapidly. In Figure 9, Rq is greatly reduced by this partial coverage of filling deposit. The FESEM image in Figure 10 shows these sharp, dark rims surrounding clean, bright interiors, and also exhibits the variability of exteriors. Some are predominately dark (with deposit) but contain a texture of cleaner subdomains, while others bridging DOI: 10.1021/la903478q
4041
Article
Kumar and Fogden
Figure 7. Profile of asphaltene deposit height on quartz for the four NaCl brines in Table 1, averaged over all rings sampled, and with error bars giving the standard deviations.
rings at closer proximity are the inverse, i.e., clean but perforated by deposit. Low-Salinity Neutral Brine. For L_N, mica and quartz also exhibit particle aggregates, but now with an increased presence of continuous film areas or broken into rafts (Figure 5). Deposits extend further toward the inner zone, thus increasing its Rq (Figure 9). Plateau deposit height outside the rings is ∼3 and 4 nm for mica and quartz (Figure 8), with slightly lesser and greater Rq, respectively, than for L_A (Figure 9), reflecting the relatively higher frequency of continuous film (mica) and raft chunks (quartz). The silica profile (Figure 8) gives the deceptive impression of much lower deposition than for L_A, since the partial intrusion of deposits into the interior renders it impossible to identify the true zero height. The increased deposition is evidenced by the drop in Rq both inside and outside (Figure 9). FESEM confirms this greater intrusion of dark deposit eroding the bright ring boundaries (Figure 10), along with more distinctly oil-wet exteriors into which pervasion of clean subdomains is often limited to thin bright tendrils. Conversely, water-wet regions bridging close rings are less pervaded by deposits, so the distribution is more binary. High-Salinity Acidic Brine. The trends from these two brines continue for H_A. Deposition on mica and quartz predominately takes the form of continuous film or broken into larger rafts (Figure 5). Plateau thickness averages ∼4 nm for both, and exterior Rq is greater than for all other brines. Similarly, the intrusion into rings is greatest, apparent from the greatest deviation from flatness of the profiles there, especially for quartz, and the highest Rq in Figure 9. For silica, the flatness of the height profile in Figure 8 is again misleading; however, the inner and outer Rq, lowest of all brines, attest to the substantial coverage and infilling of substrate by deposit films. Its FESEM image (Figure 10) also points to stronger oil-wetness both intruding into rings and over their exteriors. The presence of clumps of rings connected by substantially water-wet bridges has decreased, i.e., the rings are more isolated by surrounding asphaltene. 4042 DOI: 10.1021/la903478q
High-Salinity Neutral Brine. H_N gives the weakest deposition of the four NaCl brines. Deposition on mica and quartz reverts to connected particles, with low ring intrusion and inner Rq. The periphery is often clearly defined (Figure 5); however, deposition outside it also has a low degree of coverage, so the profiles display a more delayed and gradual rise to a plateau of ∼1.5 and 0.5 nm on mica and quartz, in both cases with external Rq the lowest for all NaCl brines. The silica profile, now unambiguously zeroed due to the prominence of bare substrate inside (evidenced by its high Rq), exhibits a steep rise to a ring rim, for which the true average peak height is 4.2 nm. This is followed by a quite rapid decay, consistent with the external Rq being substantially greater, i.e., deposit coverage poorer, than for all other brines. FESEM also bears witness to these sharp, thin rims of dark deposit, with clean, bright substrate within and also relatively clean exteriors. Bridges between water-wet rings are scarce; instead, small rings with rims often seen between the larger rings (Figure 10) are presumably footprints of residual brine droplets. CsI Brine. The deposit distribution for this brine most resembles H_N, its closest relative in Table 1. Its profile plateaus at ∼1 nm on mica and quartz, and inner and outer Rq lie just above and below H_N for mica and quartz, respectively. On silica, the rim peak is strong for some rings and weak for others, so its height (2.2 nm peak average) is less than H_N and standard deviations are larger. The lower Rq indicate that the CsI brine gives somewhat more deposition inside and outside. FESEM images (not shown) depict circular rings with relatively clean interiors and often with a dark, thin rim, again most similar to H_N, but with more intermingling of oil-wet features in water-wet regions and vice versa, akin to L_A. The FESEM micrographs provide an estimate of the planar area fraction of silicon wafer rendered oil-wet, via segmentation of brighter (clean) pixels and darker (deposited) pixels. Strip subareas were cropped from micrograph edges to give regions of uniform background brightness, avoiding the central region with Langmuir 2010, 26(6), 4036–4047
Kumar and Fogden
Article
operation for each. From the above comparison, it can again be inferred that increasing brightness on the beads corresponds to decreasing thickness and/or coverage of deposit. Textures bear strong similarities to those on silicon wafer for the same brine in Figure 10. For the brine L_A in Figure 12, the ring boundaries are distinct, yet their frequent water-wet bridging necks are perforated by deposit and the contrast to the oil-wet exteriors is quite low, implying that coverage there is incomplete. For L_N, the clean ring interiors possess more irregular boundaries than for L_A, due to greater intrusion of dark deposit, and contrast to the exteriors is increased, implying more complete deposition. For H_A, the intrusion of deposits further isolates the inner zones and increases exterior oil-wetness. Occasional thick deposits in Figure 12 only occur for this brine. The H_N sample is an even lower contrast version of L_A, again with distinct rims of deposit bounding rings, but least distinction between their interiors and exteriors. For the beads of CsI (not shown), deposition is intermediate to H_N and L_A, as for the slabs. While these strong likenesses exist, the distributions on the beads are typically less clear, partly due to the difficulty of imaging curved surfaces. Other contributing factors (see Figure 3) include the greater spacing of rings on the planar substrate, as bead coordination is generally lower there, and drainage-induced movement of beads in the bulk leading to more indiscriminate deposition. Validity of the Mixed Wet Model. The first step in interpretation involves comparing quantitative findings from tomography and profilometry. Consider the oil-drained state of a spherical bead, radius R, contacting a flat substrate and collared by a brine pendular ring of circumference radius d and concave radius rm bridging the two solids at zero contact angle, defined in analogy to Figure 1. By simple geometry d ¼ 2ðrm RÞ1=2
ð1Þ
and the mean curvature of the film-connected pendular ring must match the reciprocal capillary radius rc dictated by the capillary pressure Pc during drainage; thus ð1=rm -1=dÞ=2 ¼ 1=rc
ð2Þ
The unknown radius rm is then given by ðR=rm Þ1=2 ¼ ½ð32R=rc þ 1Þ1=2 þ 1=4
Figure 8. Averaged profiles of asphaltene deposits for all five brines in Table 1 on the three substrates.
most beam charging. Around 30 strips from 12 micrographs for each brine were segmented by determining the break point in the grayscale histogram and calculating the percentage of pixels below this threshold. This definition is somewhat arbitrary but allows a semiquantitative comparison of the statistics in Figure 11. In line with the discussions, high salinity yields both extremes of most asphaltene coverage, at low pH, and least, at neutral pH, and the CsI brine is intermediate to H_N and L_A. Wettability of Beads and Interpretations. Figure 12 displays FESEM micrographs of the beads of the disassembled pack for the four NaCl brines, with identical instrument settings and Langmuir 2010, 26(6), 4036–4047
ð3Þ
Taking the bead diameter as its average of 116 μm and the capillary radius value of 12 μm from the Darcy equation for our experimental drainage, eqs 3 and 1 imply rm = 5.1 μm and d = 34.4 μm. While this experimental rc is consistent with the simulation in Figure 3d, for any value in the range 10-12.3 μm the simulated curve would closely match the measured residual wetting phase occupancy of 10%. Using instead this interval, rm should lie in the range 4.3-5.2 μm, giving a d range of 31.7-34.8 μm. Equivalent calculations for a pendular ring between two beads of radius R (Figure 1) implies that rm now lies in the range 3.9-4.6 μm and its radial extent on the sphere surface is 20.7-22.5 μm. Ring sizes for this sphere-sphere geometry are naturally reduced, as its meniscus can more closely approach the contact point, consistent with observations from Figures 12 and 10 that the deposit rings are smaller on the beads. This range of pendular ring radii on the flat substrate falls in the transition zone of the deposit profiles in Figure 8. For all 15 brine-substrate combinations, the residual brine-oil ring would lie on average 8 μm outside the cleaner inner zone (within which the profile is flat) and 8 μm prior to attainment of plateau or peak DOI: 10.1021/la903478q
4043
Article
Kumar and Fogden
Figure 9. Average roughness and standard deviation (error bar) over the inner zone (left) and immediate exterior (right) of the rings on the three substrates, compared to their bare values, for the five brines.
deposit thickness. These profile cutoffs are relatively independent of brine and similar for mica and quartz, while on the rougher silicon wafer, deposit onset almost coincides exactly with this meniscus position. By the same token, the results of Figure 9 and their consistency with Figure 10 indicated that fine-scale surface roughness is more sensitive to the presence of thin deposits than the averaged profiles. The true extent of thin deposits advancing on the inner zone thus increases in the order H_N, L_A, L_N, and H_A, with thickness and completeness of exterior coverage following this same hierarchy. The mixed wet model ascribes a step transition in surface chemistry from water-wet inside the residual brine pendular ring to oil-wet outside (if deposition occurs) and assumes that during oil recovery the brine injection (at decreased Pc) causes the threephase line to pin at the discontinuity before advancing at constant high contact angle over the oil-wet exterior.6,11,16,17,24 While our results generally support this model, an elaboration in which advancing angle grades from lower to its higher plateau value over the transition zone, of width increasing with deposition tendency, may serve as a better approximation. The extent to which pinning occurs during this traversal also depends on the topographical features of the pore wall and its deposit. This includes somewhat counterintuitive effects such as the brines (H_N, L_A, and CsI) 4044 DOI: 10.1021/la903478q
with the least external deposition producing the highest, sharpest deposits at the boundary, especially on the rough silicon wafer, which may best represent real rock roughness. The mixed wet model should also incorporate the reality that oil-wet exteriors typically contain a remnant texture of water-wet subdomains, accumulating from brine film rupture or ejection from adjoining rings on oil intrusion. Droplet-like residues have been frequently observed in cryo-SEM/ESEM studies.23,29,30 The current work shows that even on smooth substrates these subdomains are present on length scales from submicrometers, between partially covering asphaltene disks, to tens of micrometers, forming percolating subnetworks linking rings. Their perseverance provides an extra source of hydraulic connectivity and could be modeled by ascribing a surface water permeability to oil-wet regions, decreasing with asphaltene deposition tendency. Models incorporating an interior-exterior transition zone and surface permeabilities can also be guided by estimates of substrate coverage from FESEM (Figure 11). Salt Dependence. The second stage of interpretation aims to explain the thermodynamics of deposition in terms of the intermolecular interactions at play. The discussion is qualitative, as it invokes non-DLVO effects for which quantitative theories are unavailable, especially for complex liquids. The macroscopic Langmuir 2010, 26(6), 4036–4047
Kumar and Fogden
Article
Figure 10. Representative FESEM images of the ring pattern of asphaltene deposits on silicon wafer after crude oil-brine treatment for the NaCl brines: (a) L_A; (b) L_N; (c) H_A; (d) H_N, with 50 μm scale bar in all.
Figure 11. Average, with standard deviation error bars, of the planar area of silicon wafer judged as covered by asphaltene deposit for the five brines in Table 1.
brine films lining the solid thin under the applied capillary pressure. Their fate is decided by whether this can be balanced by the disjoining pressure Π and the Laplace interfacial curvature contribution:1,11,26 Pc ¼ Π þ 2γJ
ð4Þ
where γ and J are the oil-brine interfacial tension and mean curvature. For negligible curvature, a purely repulsive disjoining pressure isotherm yields a stable brine film, of thickness given by eq 4, while a strongly attractive isotherm leads to film rupture and oil contacting the solid. For intermediate isotherms with both repulsion and attraction, the film may be partially stable or metastable, depending on conditions. Brine composition is the key variable dictating the range of the interaction between the oil-brine and brine-solid interfaces and its sign, in conjunction with the acidic and basic ionizable surface-active asphaltenes and resins of the crude oil.12,18 If rupture occurs, the extent of Langmuir 2010, 26(6), 4036–4047
deposition of these oil components on the mineral surface also depends on asphaltene content and solvency in the oil.10,18 Film stability is usually tested by measuring contact angle of a macroscopic crude oil drop against a flat mineral substrate preequilibrated in brine. Some studies use static contact angle to classify the substrate as water-wet, intermediate, or oil-wet.25 More commonly, the drop is grown, then retracted, and described as nonadhesive or adhesive if the difference between brine advancing and receding angles is small or large.6,12,18 For these techniques, the capillary pressures are very small. Another study26 used the AFM colloidal probe technique to measure forcedistance curves for an oil-covered probe in brine, with disjoining pressure isotherms classified as stable, metastable, or unstable. Figure 13 replots the wettability or adhesion maps of these studies, showing for their specific crude oil-mineral substrate pair the boundary (chosen as the middle of the metastable region in ref 26) above and to the right of which brine thin films remain stable. The five brines from Table 1 are also shown with their symbol size reflecting the deposition hierarchy from profilometry and FESEM. Recall that our samples pertain to the aging temperature and contact time of 60 °C for 6 days and drainage Pc of 3.3 ( 0.2 kPa. Despite the variations in the curves due to the differing crude oils, silicate substrates, and methods used, film instability is always observed at low concentration and pH, as is stability at high concentration and pH. This is consistent with our findings of deposition decreasing from L_A through CsI to H_N (least) in Figure 13. For samples such as L_A, DLVO theory accounts for the instability.12 The majority of crude oils, including Minnelusa, have high base number relative to acid number,18,33 with these asphaltene basic groups protonated at low pH. Double layer attraction of the cationic oil-brine and anionic brine-solid interfaces will be quite long-ranged at this low NaCl concentration, and combined with van der Waals attraction should lead to rapid film rupture and asphaltene deposition. On increasing pH, stability should improve as the oil’s isoelectric point is passed and both interfaces become more anionic. However, we observe a small increase in deposition from L_A to L_N in Figure 13, with the latter lying near the boundaries for the DOI: 10.1021/la903478q
4045
Article
Kumar and Fogden
Figure 12. Representative FESEM images of the ring pattern of asphaltene deposition on the glass beads for brines: (a) L_A; (b) L_N; (c) H_A; (d) H_N, with 50 μm scale bar in all.
Figure 13. Stability boundaries in the space of concentration and pH of sodium 1:1 electrolyte for the listed crude oils replotted from refs 6 (Figure 5), 26 (Figure 14), and 25 (Figure 3). Brine films are unstable below and to the left of the curve, while the substrates remain predominately water-wet above and to the right of it. Triangles are the brines in our Minnelusa crude study, with symbol size increasing with deposition.
literature oils. Buckley et al.12 note that, at close proximities to the substrate in low salt, the cationicity of the brine-oil interface may persist for bulk pH values somewhat above its bulk isoelectric point. It can thus be assumed that the L_N brine-oil interface is at most weakly anionic, and any electrostatic repulsion can be overcome by van der Waals attraction, possibly aided by longerrange non-DLVO hydrophobic attraction of the mainly uncharged oil surface.26 Even in the event of less rapid film thinning than L_A, subsequent deposition for L_N may benefit from weaker attraction to the solid and weaker repulsion between asphaltene molecules or preaggregated colloids9,10 to attain a higher packing density and minimize trapping of brine pockets. Sample H_A in Figure 13 also lies close to the boundaries for other oils, yet exhibits greatest deposition. The electrostatic interfacial attraction is also weaker than for L_A, now due to a 10-fold decrease in Debye screening length, and van der Waals attraction is chiefly responsible for film rupture. Weakened interactions of the high molecular weight asphaltenes may again 4046 DOI: 10.1021/la903478q
allow greater rearrangement during deposition, reducing the incorporation of other oil components subsequently removed by the decalin and heptane flushing to leave incomplete coverage. Significantly, an increase in pH of 2 units transforms H_A with greatest deposition to H_N with least. This implies that the brine-oil interface switches from cationic to anionic, since the accentuation of cationicity for L_N no longer applies due to the strong screening. The DLVO electrostatic repulsion is presumably supported at this high salt concentration by non-DLVO hydration repulsion from the solid,2,12 probably also present for H_A but less effective there in preventing contact and attachment of asphaltene nanoparticles to the oppositely charged substrate. For a planar substrate with roughness, if the brine film immerses these features the brine-oil interface remains flat. At the opposite extreme of a thinner film conforming to substrate asperities, these raised features impart their convex curvature (J < 0) to eq 4, tending to locally destabilize the film.11 For example, in Figure 13 the stability region on mica in ref 26 is greater than on the rougher glass. In our study, the mica and rougher quartz substrates, facilitating direct comparison of profiles, exhibit similar exterior deposit thicknesses for each brine in Figure 8. As all five brines yield some degree of rupture and deposition on the smooth mica, premature nucleation on quartz asperities is not expected to contribute to final thickness of deposition. The more sensitive measure of Rq in the inner zone (Figure 9) does though suggest (from the greater difference between treated and bare substrate for quartz than for mica) that roughness could aid somewhat the ingress of asphaltene deposition into this more well protected zone. Note, however, that any differences in substrate surface charge density will also play a role.
Conclusions A novel approach to probe the microscopic wettability state in porous media relevant to oil recovery and other confined multiphase systems was developed. It used a porous plug based on a bead packing, facilitating realistic oil-brine drainage experiments and nondestructive X-ray μ-CT analysis of bulk fluid distributions, with slab inclusions enabling analysis in the dry state after disassembly to characterize wettability alterations by asphaltene deposition. White-light interferometric profilometry proved a Langmuir 2010, 26(6), 4036–4047
Kumar and Fogden
convenient tool for the latter. Results demonstrated that the mixed wet model, of location-specific asphaltene deposition via rupture of brine thin films lining pore walls in open areas unprotected by bulk brine menisci, is a good first approximation. The tomography verified that the locations of residual brine pendular rings agree with Darcy equation predictions and numerical simulations. Profilometry and FESEM showed that these meniscus locations match well the boundaries between cleaner ring interiors and deposited exteriors patterned on the substrates. Further, they evidenced that a more sophisticated model of the wettability alteration pattern would include a transition from zero to plateau deposition over a finite width across the ring boundaries, as well as a texture of cleaner subdomains permeating the deposited exteriors. For brines with a greater deposition tendency, this transition width broadens to intrude somewhat further
Langmuir 2010, 26(6), 4036–4047
Article
into the clean interiors and deposits more completely cover the exteriors. Dependence on brine concentration and pH was in line with other studies and considerations of DLVO and non-DLVO contributions taking into account both film rupture and subsequent deposition. Acknowledgment. The authors acknowledge support of an ARC Discovery Grant (A.F.) and the Digital Core Consortium member companies (ADCO, BP, Baker-Hughes, BHP Billiton Petroleum, Chevron, ExxonMobil, Japan Oil & Gas, Maersk O&G Qatar, Saudi Aramco, Schlumberger, Shell, Total). The ANU Supercomputing Facility and the Australian Partnership for Advanced Computing are thanked for allocation of computer time, as is Norman Morrow (University of Wyoming) for the crude oil sample and valuable discussions.
DOI: 10.1021/la903478q
4047
