Modeling carbon sequestration in transition zone to optimize storage potential

Shallow aquifers are potential candidates for carbon sequestration because of their wide distribution and low injection cost. Injected CO2 in a shallow region segregates into two zones according to CO 2 phases within the initial reservoir pressure and the maximum bottom hole pressure: gas zone and transition zone. Transition zone is a better option because of favorable CO2 properties. This work investigated the CO2 storage potential in transition zone using the compositional simulator CMG-GEM and a design of experiment software iSIGHT. First, it addressed the basic characteristics of CO2 sequestration in transition zone, and then modeled CO2 injection with simultaneous water withdrawal as a necessary means for improving CO2 storage capacity. Next, it compared the difference between sequestration in transition zone and supercritical zone. Finally, it optimized the storage potential of transition zone from engineering aspects. The simulation results show that transition zone is very promising with water withdrawal. Water withdrawal greatly improves CO 2 storage capacity and injectivity. A shallow aquifer in transition zone provides considerable storage capacity, better storage volume efficiency, and a higher portion of stable CO2 than a deep aquifer in supercritical zone. High storage capacity can be obtained when the aquifer is thick with high porosity, high permeability, low salinity, medium anisotropy, and medium heterogeneity. Within 600-1000 m, storage capacity is affected primarily by thickness and porosity, then salinity. The less significant effect of depth indicates the great potential of transition zone for CO2 sequestration under favorable conditions. Pulse injection provides better storage capacity and efficiency than continuous injection and water alternating gas. A small pattern size should be considered at low rates, while a large pattern size is preferred at high rates.