An Analytical Approach for Evaluating the Capillary Trapping During CO2 Storage in Aquifers

Abstract Geological storage of CO2 in saline formations is employed as a key emissions reduction method in the global effort to mitigate against climate change. There has been significantly development of numerical and simple analytical techniques to support screening analysis and performance assessment for potential carbon sequestration sites. However, these techiniques had ignored the capillary trapping mechanism which has been shown as the fastest mode of trapping. In this work, we focused on the capillary trapping phenomenon occurring during the gas alternating water (GAW) injection and developed a new analytical method to predict the potential of capillary storage. Firstly, based on the fractional theory and considering the effect of mutual solubility between CO2 and aqueous phase, the saturation profile of two-phase flow is acquired. And the drainage and imbibition phenomenon are applied to simulate the capillary trapping mechanism. Then, accoring to the saturation profile, the analytical method for estimating the capillary trapping capacity of GAW is proposed. Finally, this new analytical model is applied in a real case to estimate the capillary trapping capacity, and the sensitivity analysis of some important parameters is also conducted. The results of this work show that the capillary trapping of subsequent gas injection occupied about 5.77%, which could not be ignored in estimating the capillary trapping potential. In addition, results of sensitive analysis show that higher residual gas saturation would lead to higher capillary trapping capacity. And higher retardation factor would lead to higher capillary trapping capacity of subsequent gas and water injection, which illustrated that the mutual solubility between water and gas is important to the capillary storage potential. This work further developed and improved the accuracy of our previous method in estimating the capillary trapping capacity. And it also provides a new idea for evaluating the potential of capillary sequestration.