Adsorption results in a phase transition, which is driven by pressure. It is shown that the transition of free-phase CO2 to adsorbed CO2 in pore space is favored thermodynamically as it decreases the Gibbs free energy of the system. Because of the higher density of adsorbed phase, the density of the free-phase CO2 becomes lower, thereby decreasing the overpressure. The adsorption process is much faster than dissolution, which enables reduction of overpressure before CO2 dissolution takes significant effect.