Interfacial Effects in Gas-Condensate Recovery and Gas-Injection Processes

Abstract

Summary This paper evaluates the behavior of the dimensionless numbers that gauge the gas/oil interfacial tension (IFT) with respect to the other forces (viscous and buoyancy forces) involved in two-phase flow through porous media. These numbers, referred to respectively as the capillary and Bond numbers, diverge on approach to gas/oil complete miscibility, meaning that viscous and buoyancy forces become dominant over capillary forces. The divergence behaves as a power of the distance to complete miscibility as quantified, for instance, by the difference in densities between the oil and gas phases. The exponents of these power laws are "universal," whereas the prefactors vary between gas/oil systems. This allows the classification of the most common injection gases with respect to their efficiency in reducing the gas/oil IFT. This efficiency increases with the miscibility of the injected gas in the oil: supercritical CO2 is more effective in reducing IFT than off-critical CO2 or CH4, which themselves are more effective than N2. A simple procedure is then introduced to determine the wetting (or spreading) behavior of oil on a porous substrate covered with water, as often occurs in practice. The only inputs required are the composition and densities of the three coexisting phases: water, oil, and gas. When they are not measured, these quantities can be calculated by means of an appropriate equation of state. CO2 turns out to be the most effective for promoting the spreading of the oil on water, followed by CH4 and then by N2.