We use confocal microscopy to directly visualize the spatial fluctuations in fluid flow through a three-dimensional porous medium. We find that the velocity magnitudes and the velocity components both along and transverse to the imposed flow direction are exponentially distributed, even with residual trapping of a second immiscible fluid. Moreover, we find pore-scale correlations in the flow that are determined by the geometry of the medium. Our results suggest that despite the considerable complexity of the pore space, fluid flow through it is not completely random.

We use confocal microscopy to directly visualize the formation and complex morphologies of trapped non-wetting fluid ganglia within a model 3D porous medium. The wetting fluid continues to flow around the ganglia after they form; this flow is characterized by a capillary number, Ca.We find that the ganglia configurations do not vary for small Ca; by contrast, as Ca is increased above a threshold value, the largest ganglia start to become mobilized and are ultimately removed from the medium. By combining our 3D visualization with measurements of the bulk transport, we show that this behavior can be quantitatively understood by balancing the viscous forces exerted on the ganglia with the pore-scale capillary forces that keep them trapped within the medium. Our work thus helps elucidate the fluid dynamics underlying the mobilization of a trapped non-wetting fluid from a 3D porous medium.

High resolution imaging of the microstructure of Fontainebleau sandstone allows a direct comparison between theoretical calculations and laboratory measurements. While porosity, pore-volume-to-surface ratio, permeability, and end point relative permeability are well predicted by our calculations, we find that electrical resistivity and wetting phase residual saturation are both overestimated. Introduction Transport in porous media is of importance in biology, chemical engineering, earth and environmental sciences, materials science, and physics [Adler, 1992; Cushman, 1990]. Because real porous media are usually highly disordered, most recent work on transport theory has been based on synthetic model systems, such as random sphere packs [Adler, 1992; Schwartz et al., 1993]. Although instructive, such studies are not easily applied to the understanding of real materials. High resolution synchrotron microtomography [Flannery et al., 1987; Kinney et al., 1993; Schwartz et al., 1994; $panne eta!., 1994 Coles et al., 1995] however, may be used to map the pore space of a real material. Here we study the geometrical and transport properties of three-dimensional tomographic reconstructions of several samples of Fontainebleau sandstone. We compare numerical calculations to laboratory measurements made on samples approximately an order of magnitude larger in linear dimension. We obtain a heirarchy of results, presented in Table 1. Geometrical properties are accurately estimated by our calculations, as is the permeability to single-phase flow. The computed electrical conductivity, on the other hand, underestimates the experimental results. Studies of immiscible displacement by a non-wetting fluid give mixed results. While our endpoint relative permeability calculation is in good •Schlumberger-Doll Research, Ridgefield, CT. agreement with the measured data, the corresponding saturation is less satisfactory. AUZERAIS ET AL.: TRANSPORT IN SANDSTONE 707 2.0 ' -1.0 ß size 56 [] size 112 ¸ size 224 ß ß •e ß ß ß ee ß ß eee ß F. M Auzerais, T. S. Ramakrishnan, and L. M. Schwartz, Schlumberger-Doll Research, Ridgefield, CT 06877 J. Dunsmuir, Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801 B. Ferr•ol, Elf Aquitaine,

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Nanofluids composed of liquid suspensions of nanoparticles may soon permit accelerated recovery of hydrocarbons from oil and gas reservoirs. Here, we present a series of flooding experiments at different capillary numbers to quantify the performance of a polymeric nanofluid compared to brine using the sintered glass-beads. A high resolution X-ray microtomography (micro-CT) was used to visualize oil and brine distribution in a sintered bead pack before and after nanofluid flooding. The results of flooding experiments showed that an additional oil recovery of approximately 15% is possible with nanofluids compared to brine at low capillary numbers and is as effective as high capillary number brine flooding. Nanofluid induced additional oil recovery decreases as we increase the capillary number, and the total oil recovered shows a marginal decrease. At first glance, these results are opposite of what one expects in the conventional EOR, where oil recovery is known to increase progressively with increasing capillary number. These results cannot be explained based on mobilization theories due to the reduced capillarity. Our results however are consistent with the mechanism of wettability alteration caused by structural disjoining pressure leading to the formation of the wetting nanofluid film between oil and substrate. Unlike brine displacement, X-ray micro-CT images show that mobilization of oil from the porous medium by a nanofluid is fairly uniform even at low capillary numbers and large-scale clusters of oil are absent. Our findings in this paper are expected to promote the understanding of EOR by nanofluids.