Meitan xuebao (Jul 2023)
Local capillary entrapment and its effect on sequestration efficiencies during impure CO2 injection into saline aquifers
Abstract
The actual saline aquifers is vertically distributed in multiple layers, with large differences in physical properties and serious heterogeneity of capillary pressure. During the sequestration of impure CO2, the capillary pressure will not only trap CO2 but also trap the accompanying impurity gas, affecting CO2 migration and sequestration. However, the entrapment of local capillary under the vertical multi-layer system is still unclear, and its impact on the sequestration efficiency of impure CO2 needs to be analyzed urgently. For this reason, stochastic statistical geological modeling and impure CO2-brine two-phase seepage model are combined to establish a vertical multi-layer impure CO2 sequestration and migration model in saline aquifers. Under the action of capillary pressure, the migration process and its plume development law are studied and the vertical migration rate of impure CO2 and the capillary pressure trapping ability are calculated quantitatively. Finally, through the sensitivity analysis, the capillary pressure, the influence of capillary pressure, heterogeneity, and impurity volume fraction on CO2 sequestration efficiency is clarified. The research results show that heterogeneous capillary pressure can effectively trap impure CO2 and increase the swept volume of impure CO2 in the lateral direction. With the increase of capillary pressure and its heterogeneity, local capillary trap volume and local capillary sequestration coefficient increase. However, the vertical migration rate of CO2 decreases significantly, and the difference between the vertical migration rate of impurity gas increases. When the capillary pressure decreases from top to bottom, the local capillary entrapment at the bottom decreases, and more impure CO2 will migrate to various areas in the upper part of the formation, which makes the local capillary sequestration coefficient of impure CO2 increase, and the residual sequestration coefficient of impure CO2 decrease. What’s more, it makes the vertical migration rate of CO2 increase and the difference between the vertical migration rate of impurity gas and impurity gas decrease. As the volume fraction of impurity gas increases, the buoyancy of impure CO2 and the vertical migration rate of impure CO2 increase. More impure CO2 migrates to the top of the formation, which makes the local capillary sequestration coefficient of impure CO2 decrease, while residual sequestration coefficient increases. Therefore, to ensure the safety of storage, impure CO2 containing few impurities should be selected as much as possible to inject into the saline aquifers with large capillary pressure, strong heterogeneity, and vertical capillary pressure increasing first and then decreasing from top to bottom.
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