Multicomponent mass transfer across water films during hydrocarbon gas injection

Understanding the dynamics of pore-scale multicomponent gas and oil mass transfer across water films during hydrocarbon gas injection in petroleum reservoirs is important in the design of tertiary oil recovery schemes at the field scale. The water films prevent oil and gas coming into direct contact and, for miscible gas injection, delay the onset of miscibility. We use a pore-scale model to describe the diffusion-controlled mass transfer through the water films. The following different processes are found: (i) rapid oil swelling which results in short times needed to rupture the water films shielding the oil, (ii) slow oil swelling resulting in very long water film rupture times and (iii) both oil swelling and oil shrinking. The rate of oil recovery and the way it is recovered (either by rupturing the water film or being vaporized into the displacing gas) is critically dependent upon the oil and gas compositions, the oil droplet size and the water film thickness. We show cases at the pore scale where the time for gas and oil to be brought into direct contact and reach equilibrium in the presence of water films during miscible gas displacement is so long that water-blocking will adversely affect oil recovery at the field scale.