Feasibility of CO₂ sequestration in shallow transition zones

Geological CO ₂ sequestration requires suitable storage sites to store CO ₂ at a large scale. Shallow aquifers could be a viable regional solution for stationary CO ₂ 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 CO ₂ can transit from a gaseous phase into supercritical CO ₂ within the pressure range for CO ₂ 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 CO ₂ 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 CO ₂ 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 CO ₂ storage capacity are formation thickness and porosity, with reservoir heterogeneity and depth having lesser effects. Therefore, depth is of less concern when estimating CO ₂ storage potential. This work explored the transition zone as an effective new option when seeking means of sequestering CO ₂. These results not only establish parameters for CO ₂ sequestration in shallow aquifers, but also strengthen the current understanding of CO ₂ sequestration in deep aquifers.