The effect of designing parameter of WAG injection on enhancement of CO₂ trapping in heterogeneous formations: A numerical study

Abstract

Dissolution and residual trapping of CO2 injected in saline aquifers can be influenced by injection strategies applied. In this study, we focus on the water‐alternating‐gas (WAG) strategy and investigate the importance of parameters needed to design an effective WAG injection sequence, including (i) CO2 and water injection rates, (ii) WAG ratio, and (iii) number of cycles. Using TOUGH2‐ECO2N, we perform 3D numerical simulations of sequences of CO2 and water injection into a heterogeneous formation. Hysteresis in relative permeability and capillary pressure functions is considered based on the Land trapping model. Results show that to design a WAG injection in a high permeable formation, the WAG ratio and number of injection cycles are more important parameters than the CO2 and water injection rates. Increasing the total amount of water injection (i.e., decreasing the WAG ratio for given total amount of injected CO2) improves the CO2 dissolution and residual trapping. It is also shown that increasing the number of injection cycles has a negative effect on both residual and dissolution trapping as measured at the end of the injection sequence, because both the free‐phase and the dissolved CO2 plumes in the one‐cycle injection scenario reach farther distances and occupy larger reservoir volumes than in the multi‐cycle injection. This result means that while water injection following the CO2 injection improves trapping in comparison with the CO2‐only injection strategy, the WAG scheme with multiple cycles should not be chosen to enhance trapping for the scenario considered in this study. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.