Effect of capillary number on morphological characterizations of trapped gas bubbles: Study by using micro-tomography

Abstract

A series of capillary trapping experiments were conducted in unconsolidated porous media using high-resolution micro-tomography. Morphological characterizations of trapped gas bubbles, including residual gas saturation, bubble size distribution and interfacial area, were determined from reconstructed three-dimensional images. Different flooding flow rates, corresponding to capillary number ranges from 1.0 × 10−7 to 3.8 × 10−4, were performed to investigate the impact of flow rate on these characterizations during imbibition processes. At low capillary number, where capillary forces are dominant, the residual gas saturation is independent of capillary number and the bubble size distribution shows a universal power-law behavior predicted from percolation theory. When the capillary number increases above a critical value, the residual saturation decreases sharply, and the bubble size distribution no longer exhibits power-law behavior. As capillary number increases, the volume fraction of single-pore bubbles increases from about 16% to more than 80%. Furthermore, we explored the interfacial area of trapped bubbles, including gas–solid and gas–water (meniscus) interfacial areas, which directly affect the mass transfer process inside porous media. The specific interfacial area exhibits a linear relation with residual saturation, and the effect of particle size can be normalized by multiplying the median particle diameter. Moreover, we observed that the meniscus/total interfacial area ratio increases from 0.61 to 0.78 with an increase in capillary number, because the smaller singlet has a higher interfacial contact with the wetting phase.