Fig. 1.

Residual and solubility trapping are the key trapping mechanisms that contribute to CO₂ storage capacity. (A) Shows blobs of gas immobilized by residual trapping in an experimental analog system: a glass-bead pack saturated with water. (B) Shows solubility trapping in a different analog system: a Hele–Shaw cell saturated with water, topped with a source of dense, dyed water. As in the CO₂ system, in which the brine with dissolved CO₂ is denser than the ambient brine, dissolution occurs via finger-like protrusions of dense fluid. (C) We model trapping at the large scales relevant to a nationwide analysis and account for the injection and migration of CO₂. We consider a linear arrangement of injection wells in a deep section of the aquifer (28). Initially, each well produces a radial CO₂ plume, which grows and eventually interferes with those from neighboring wells, leading to a problem that can be approximated as 2D on a vertical cross section. Trapping occurs primarily after injection, when the CO₂ migrates due to the aquifer slope and the natural head gradient. As the buoyant plume of mobile CO₂ (dark gray) rises and spreads away from the well array, residual trapping immobilizes blobs of CO₂ in its wake (light gray) (19, 29, 30), and solubility trapping shrinks the plume from below (blue) (20, 21).