Storage CO2 in brine aquifers is considered as a promising option to reduce CO2 emissions. Many simple analytical and semi-analytical techniques have been proposed to support screening analysis and performance assessment for potential carbon sequestration sites. However, these analytical techniques have ignored the capillary trapping mechanism, which is important to the carbon sequestration in aquifers.

In this paper, a new analytical method to evaluate the saturation profile and CO2 trapped quantity was derived. First, the fractional theory (Buckley–Leverett type model) is applied to simulate the multiphase flow. And the trapping and hysteresis models are adopted to model the capillary trapping mechanism. Then, the transient-pressure solution of injection well was solved by applying the quasisteady state theory. Subsequently, this analytical method is applied to evaluate the saturation profile and CO2 trapped quantity when gas alternating water (GAW) injection (strong imbibition phenomenon). After that, we applied this new analytical method in a real case to evaluate the CO2 capillary trapped quantity and assess the saturation profile. Finally, model verification and sensitivity analysis were conducted.

The results of model verification show satisfactory match and justifies, which shows our methodology is reasonable. Furthermore, the CO2 trapped quantity can account for about 24.25% of total trapped quantity, which indicates that the capillary trapping can't be ignored for long-term storage. In addition, results of sensitive analysis show that as the residual gas saturation increases, the capillary trapping capacity increases.

This work provides a new approach to the quick assessment of the capillary trapping quantity and the location of mobile gas left during gas alternating water injection.

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