Pore-scale study of capillary trapping mechanism during CO₂ injection in geological formations

Geological sequestration of CO₂ gas has emerged as a promising solution for reducing the amount of greenhouse gases in atmosphere. A number of continuum scale models are available to describe the transport phenomena of CO₂ sequestration. These models rely heavily on a phenomenological description of subsurface transport phenomena and the predictions can be highly uncertain. "First-principle" pore-scale models provide a better understanding of fluid displacement processes. In this work we use a Smoothed Particle Hydrodynamics (SPH) model to study pore-scale displacement and capillary trapping mechanisms of super-critical CO₂ in the subsurface. Simulations are carried out to investigate the effects of gravitational, viscous, and capillary forces on the amount of trapped CO₂ in terms of non-dimensional numbers. We found that the displacement patterns and the amount of trapped CO₂ depends mainly on Capillary and Gravity numbers. For large Gravity numbers, most of the injected CO₂ reaches the cap-rock due to gravity separation. A significant portion of CO₂ gets trapped by capillary forces when the Gravity number is small. When the Gravity number is moderately high, trapping patterns are heavily dependent on the Capillary number. If the Capillary number is very small, then capillary forces dominate the buoyancy forces and a significant fraction of injected CO₂ is trapped by the capillary forces. Conversely, if the Capillary number is high, trapping is relatively small since buoyancy dominates the capillary forces.