Quantification of capillary trapping of gas clusters using X-ray microtomography

This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://onlinelibrary.wiley.com/journal/10.1002/%28ISSN%291944-7973.A major difficulty in modeling multiphase flow in porous media is the emergence of trapped\ud phases. Our experiments demonstrate that gas can be trapped in either single-pores, multipores, or in large\ud connected networks. These large connected clusters can comprise up to eight grain volumes and can contain\ud up to 50% of the whole trapped gas volume. About 85% of the gas volume is trapped by multipore gas\ud clusters. This variety of possible trapped gas clusters of different shape and volume will lead to a better process\ud understanding of bubble-mediated mass transfer. Since multipore gas bubbles are in contact with the\ud solid surface through ultrathin adsorbed water films the interfacial area between trapped gas clusters and\ud intergranular capillary water is only about 80% of the total gas surface. We could derive a significant\ud (R²=0.98) linear relationship between the gas-water-interface and gas saturation. We found no systematic\ud dependency of the front velocity of the invading water phase in the velocity range from 0.1 to 0.6 cm/min\ud corresponding to capillary numbers from 2 x 10⁻⁷ to 10⁻⁶. Our experimental results indicate that the capillary\ud trapping mechanism is controlled by the local pore structure and local connectivity and not by thermodynamics,\ud i.e., by the minimum of the Free Energy, at least in the considered velocity range. Consistent with\ud this physical picture is our finding that the trapping frequency (= bubble-size distribution) reflects the pore\ud size distribution for the whole range of pore radii, i.e., the capillary trapping process is determined by statistics\ud and not by thermodynamics