Direct measurement of trapped gas bubbles by capillarity on the pore scale

Abstract

Abstract To utilize aquifers without proven seal structures as a CO2 storage reservoir, CO2 should be trapped by residual gas trapping and solubility trapping as well as physical trapping by cap-rock. We investigated the mechanism of the residual gas trapping from a microscopic point of view by means of micro-focused X-ray CT. First, we imaged trapped air bubbles in a Berea sandstone chip after spontaneous imbibition at atmospheric pressure. With the resolution of 6.52 m/pixel, 610 sheets of slice images of 608×608 pixels were reconstructed. Pore structure and trapped bubbles were clearly visualized. Next, distributions of trapped bubbles in Berea and Tako sandstone were imaged in core-flooding at the capillary number of 1.0×10−6. The trapped bubbles consist of two-type; one occupies the center of pore with a pore-scale size and the other has a pore-network scale size. In low porosity porous media such as sandstone, connected bubbles has high contribution to trapped gas saturation. The distributions of trapped bubbles were uniform throughout the core except for the heterogeneity due to sedimentary layers. With a packed bed of glass beads 600 μm in diameter, the effects of capillary number and the injected water volume were investigated. Even in the packed bed of glass beads, which has a high porosity of 38%, once the gas bubbles are trapped, they are stable against the water flow rate that corresponds to the capillary number of 1.0×10−4. Even at very high capillary number of 4.7×10−5, residual gas saturation is independent of volume of injected water up to 15 pore volumes. Third, distributions of the residual gas saturation in the core were measured at supercritical conditions on core scale. The effect of fracture on the residual gas saturation was also investigated. The fracture reduces the maximum CO2 saturation to about 20% at the end of CO2 injection. However, most of the CO2 were trapped after the water injection. The Land type relationship between trapped CO2 saturation and the maximum CO2 saturation was obtained for supercritical CO2.