Details of In-Situ Foam Propagation Exposed With Magnetic Resonance Imaging
- F.R. Wassmuth (Petroleum Recovery Inst.) | K.A. Green (Petroleum Recovery Inst.) | L. Randall (Petroleum Recovery Inst.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2001
- Document Type
- Journal Paper
- 135 - 145
- 2001. Society of Petroleum Engineers
- 5.4.2 Gas Injection Methods, 4.3.4 Scale, 4.1.5 Processing Equipment, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.8.7 Carbonate Reservoir, 5.4 Enhanced Recovery, 4.1.2 Separation and Treating, 5.1 Reservoir Characterisation, 1.6.9 Coring, Fishing, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.3.1 Flow in Porous Media, 2.4.3 Sand/Solids Control
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This paper demonstrates the application of magnetic resonance imaging (MRI) to steady- and unsteady-state foam flow in porous media. Foam corefloods with and without surfactant preadsorption are discussed. The MRI instrument proved to be a useful tool in determining the in-situ saturation distributions during foam-flow experiments, with resolutions unmatched by other imaging techniques.
In situations where the effective foam-bubble radius is comparable to or larger than the pore-size radius of the porous medium, the foam is classified as confined foam. These types of foams are most often investigated in petroleum reservoir applications. Foam propagated at a high foam quality (high gas fractional flow) usually has thin liquid lenses separating the foam bubbles, defined as lamellae flow. As the foam quality is decreased during foam propagation, the water lenses thicken and the bubbles become more widely spaced. Hirasaki and Lawson1 related the apparent viscosity of confined foam in smooth capillaries to the sum of three contributions: slugs of liquids between bubbles, the resistance of deformation of the interface, and the surface tension gradient along the surface of the bubble. All three components depend on the number density of bubbles in the capillary, such that fine textured foam (high bubble density) generates a large apparent viscosity and vice versa. This simple foam principle has been adopted in population balance models2 used to simulate foam propagation in porous media. In these models the gas mobility is modified according to the number density of foam bubbles. It has been proved experimentally and is generally accepted that foam flow does not influence the water-phase relative permeability3,4,5 significantly in comparison to water/gas flow.
The static, repulsive forces acting between the two parallel surfaces of a thin liquid film are summed up as disjoining pressure (?). For surfactant-laden films, the disjoining pressure increases as the film thickness (h) is decreased. In thinning the film just before the point of rupturing, the rupture disjoining pressure ?rup is reached. In a porous medium, the augmented Young-Laplace equation6 determines the local capillary pressure across liquid films.
where s=the interfacial tension and Cm=the local interfacial curvature.
Thus, the capillary pressure in a flat lamella, spanning a pore in the porous medium, corresponds to Pcl=?(h). The capillary pressure in the wetting film along the pore walls is defined by the surface tension and the local interfacial curvature, Pcp=2sCm. As long as the local capillary pressure in the wetting film remains below the rupture disjoining pressure of the lamellae, local foam lamellae should be stable. Khatib et al.7 postulated that confined foam in a porous medium couldn't withstand capillary pressures above a limiting value, Pc*. Because the capillary pressure is monotonically increasing with decreasing water saturation, the water saturation during foam flow will also approach a critical minimum value, Sw*. Consider foam flowing with constant bubble size through the porous medium, under drainage conditions with increasing foam quality. The liquid saturation will decrease until Sw* is reached, corresponding to Pc*. As the foam quality is increased further, the foam texture must coarsen to keep the capillary pressure from surpassing that critical value Pc*, maintaining the water saturation at Sw*. Several foam transport models have been constructed on this premise: population balance models,2 fractional flow models,8 and relative permeability models.4
In this study, the foam-flow experiments resolved the saturation behavior in the critical capillary pressure regime for a range of foam qualities. Previous experimental studies of foam propagation have used gamma ray attenuation5 or X-ray computed tomography9 to determine the in-situ liquid saturation. With MRI it is possible to enhance the resolution of liquid saturations, during foam propagation, along the length of a cylindrical core. Enhanced resolution is generally attributed to MRI for the following reasons.
The MRI technique is sensitive to the hydrogen nuclei in the fluids only, and compensation for signal from the coreholder material or rock matrix is not necessary.
It is possible with the MRI to acquire lengthwise (axial) imaging data directly using frequency or phase encoding. In contrast, X-ray computed tomography experiments generally acquire a number of radial images and then reconstruct a lengthwise image from that data. This means that the digital resolution in the axial direction of the MRI data is generally higher than the X-ray methods (typically 128 data points vs. 16 data points).
The imaging data is acquired and stored in 32-bit integer format rather than 8-bit format, meaning that the dynamic range is larger and the ability to detect subtle differences in saturation is better.
The MRI technology does have some limitations; the sample length is generally limited to 10 cm for quantitative work, and ferromagnetic impurities must be carefully screened from the mineral matrix.
This imaging study was carried out as part of a larger research effort focused on conformance control of gas in naturally fractured carbonate reservoirs. Thus, crushed carbonate was used as core material, whereas sandpacks or sandstone cores usually are used as porous media. One benefit of the crushed carbonate is the lack of ferromagnetic ions. This minimizes the signal interference, allowing for optimum sensitivity. The packed carbonate core used in this experiment was only 10 cm in length. At this length, the whole core could be monitored at one time and the usefulness of the MRI to image foam floods could be tested by simple means. In future experiments, longer cores can be used in the foam-flow experiments, where 10-cm sections of the core are imaged at a time.
Commercially acquired 3/4-inch dolomite gravel was crushed with a swing mill. The crushed material, analyzed by X-ray diffraction, was composed of 98.1% dolomite, 1.2% quartz, and 0.7% calcite. After removal of fines, the screenings from sieve No. 45 and sieve No. 70 were used in an equal-weight ratio for packing material in the core floods.
The surfactant CD1045 supplied by Chaser Intl. (46% active) was used for all the foam experiments. This surfactant demonstrates good foaming properties in carbonate cores and in the presence of light oils.
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