The effect of WACO2 ratio on CO2 geo-sequestration efficiency in homogeneous reservoirs

Various factors such as reservoir temperature, wettability, caprock properties, vertical to horizontal permeability ratio, salinity, reservoir heterogeneity, injection well configuration affect the CO2 geo-sequestration efficiency. Furthermore, it was previously investigated that CO2 storage efficiency can be improved by using water alternating CO2 (WACO(2)) technology. However, the effect of the WACO(2) ratio (the ratio of the total amount of injected CO2 to the total amount of injected water) on CO2 storage efficiency has not been addressed adequately. Thus, in this paper, a 3D homogeneous reservoir simulation model has been developed to study the impact of the WACO(2) ratio on CO2 mobility and CO2 trapping capacity using five different WACO(2) ratios (i.e. 3, 2, 1, 1/2, and 1/3). For all WACO(2) ratios tested, 9000 kton (kt) of CO2 were injected during 3 CO2 injection cycles (2 years each) and at an injection rate of 1500 kt per year. Each CO2 injection cycle was followed by a 2 years water injection cycle with injection rates of 500 kt/year, 750 kt/year, 1500 kt/year, 3000 kt/year, and 4500 kt/year for the 3, 2, 1, 1/2, and 1/3 WACO(2) ratios, respectively. Then, this 12 years WACO(2) injection period was followed by a 100 years post-injection period. Our results clearly indicate, after 100 years post-injection period, that the WACO(2) ratio has an important effect on the CO2 migration distance, CO2 mobility and CO2 trapping capacity. The results demonstrate that lower WACO(2) ratio leads to reduce the vertical CO2 plume migration and CO2 mobility. Furthermore, low WACO(2) ratio enhances the capacities of capillary and solubility trapping mechanisms. Thus, we conclude that WACO(2) has a significant impact on the geo-sequestration efficiency and less WACO(2) ratios are preferable. (C) 2018 The Authors. Published by Elsevier Ltd.