Trapping and remobilization during geological CO2 storage: A pore-scale imaging and modeling study

CO₂ storage in geological formations is important in the reduction of CO₂ emissions. Residual trapping – CO₂ immobilized by capillary forces – contributes significantly to the overall storage. Earlier findings at field conditions have indicated a delayed remobilization – a safety enhancing phenomenon – of residually trapped CO₂ under pressure depletion. The present study investigates the underlying processes of this phenomenon by means of detailed pore-level analysis. We first compare our pore network model predictions against experimental data from high-resolution 3D X-ray imaging. General agreement is found, and in both the experiment and the model, remobilization occurs at a higher saturation value – called the critical saturation (S_gc) – than the residual saturation (S_gr). A significant reduction in the relative permeability of the gas is also predicted. The model is then applied to different rocks. The results show that the S_gc is not a simple function of porosity, permeability or residual saturation. Instead, complex pore scale phenomena related to pore connectivity govern the behavior and case-specific studies are required to determine the exact value. For practical purposes, the difference between residual saturation and critical saturation is approximately between 2–4%. The reduction in gas relative permeability varies between 60–90 % compared to that for drainage with no expansion.