Critical assessment of CO2 trapping capacity under gravity segregation with capillary heterogeneity

Abstract

CO2 trapping mechanisms in geologic sequestration have been typically categorized as structural, residual, dissolution, and mineral trapping. However, a storage analysis with fine-scale heterogeneity requires an investigation of newly suggested mechanism, capillary trapping. In addition, CO2 injection process, in which gravity force is relatively strong compared to viscous force, leads to severe gravity segregation and poor contact volumes. This study covers an integrated assessment of CO2 trapping associated with gravity number in heterogeneous formations. Numerical simulations have been undertaken to evaluate the CO2 trapping efficiency. The effects of formation properties on various CO2 trapping mechanisms are studied with different injection scenarios. Leverett J-function is applied to take account of capillary heterogeneity. Inclusion of capillary trapping enables considerable amount of CO2 to be secured by local capillary barriers. To examine the effects of CO2 gravity override, gravity number, a ratio of gravity to viscous forces, has been introduced. The amount of trapped CO2 and leakage are quantified with the gravity number determined by average permeability, ratio of vertical to horizontal permeability, and injection rate. Since the overall values of capillary entry pressure are higher in the formation with lower permeability, the larger portion of the lower aquifer can act as capillary barriers, which lead to a large accumulation of CO2 by capillary trapping. Low ratio of vertical to horizontal permeability enables the CO2 plume to spread out more laterally, thus enhancing the CO2 trapping capacity. Under low injection rate, the injection pressure does not have significant impact on the fluids flow and gravity force is dominant. It induces the gas to flow upward shortly after injection, which results in an accelerated CO2 leakage. As the injection rate is higher, stronger viscous force allows the gas to migrate mainly in the horizontal direction. It causes the gas to contact a larger area with brine, so that the amount of trapped CO2 increases. The portion of capillary trapping reduces from 51% to 17% as the gravity number increases. By residual trapping, the immobilized CO2 are 13% and 28% for high and low gravity numbers, respectively. In comparison with residual and capillary trapping, the dissolution of CO2 is insensitive to the parameters associated with gravity number. For the successful evaluation of CO2 trapping capacity in heterogeneous domain, the results have proven the importance of capillary trapping which accounts for the largest portion of CO2 immobile phase. Integrated assessment of CO2 migration with gravity number is particularly useful to improve the accuracy of CO2 leakage estimation. The workflow will provide simple parameter sensitivity to aid engineers in optimizing CO2 storage design.