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

• Residually trapped CO₂ can become remobilized under reservoir pressure depletion. • A pore network model validated by 3D X-ray imaging was used to study remobilization for different rocks. • Critical gas saturation depends on complex pore-scale connectivity rather than simple rock properties. • The reduction in CO₂ relative permeability during remobilization ranges from 60–90%. CO 2 storage in geological formations is important in the reduction of CO 2 emissions. Residual trapping – CO 2 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 CO 2 in conditions of 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 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 governing 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.