Feasibility of CO2 sequestration in shallow transition zones

Geological CO2 sequestration requires suitable storage sites to store CO2 at a large scale. Shallow aquifers could be a viable regional solution for stationary CO2 emitters because of their broad distribution, considerable capacity, and the potential low drilling cost associated with the shallow depths. This study introduces a shallow transition zone in which CO2 can transit from a gaseous phase into supercritical CO2 within the pressure range for CO2 injection; thus, considerable storage capacity is expected. The characteristics and major influencing factors of shallow transition zone storage were compared with those of deep storage to determine the feasibility of CO2 sequestration in shallow transition zones. The transition zone was found to provide a comparable storage capacity, greater storage efficiency, and a higher proportion of stable CO2 than a deep aquifer. The depth of this transition zone can be as shallow as 537 m for a warm basin and 656 m for a cold basin. At a depth of 600–1000 m, the primary factors affecting CO2 storage capacity are formation thickness and porosity, with reservoir heterogeneity and depth having lesser effects. Therefore, depth is of less concern when estimating CO2 storage potential. This work explored the transition zone as an effective new option when seeking means of sequestering CO2. These results not only establish parameters for CO2 sequestration in shallow aquifers, but also strengthen the current understanding of CO2 sequestration in deep aquifers. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.