3-PHASE FLOW AND GRAVITY DRAINAGE IN POROUS-MEDIA

We present a theoretical and experimental treatment of three-phase flo w in water-wet porous media from the molecular level upwards. Many thr ee-phase systems in polluted soil and oil reservoirs have a positive i nitial spreading coefficient, which means that oil spontaneously sprea ds as a layer between water and gas. We compute the thickness and stab ility of this oil layer and show that appreciable recovery of oil by d rainage only occurs when the oil layer occupies crevices or roughness in the pore space. We then analyze the distribution of oil, water and gas in vertical equilibrium for a spreading system, which is governed by alpha = gamma(ow) (rho(o) - rho(g))/gamma(go)(rho(w) - rho(o)), whe re gamma(ow) and gamma(go) are the oil/water and gas/oil interfacial t ensions respectively, and rho(g), rho(o) and rho(w) are the gas, oil a nd water densities respectively. If alpha > 1, there is a height above the oil/water contact, beyond which connected oil only exists as a mo lecular film, with a negligible saturation. This height is independent of the structure of the porous medium. When alpha < 1, large quantiti es of oil remain in the pore space and gravity drainage is not efficie nt. If the initial spreading coefficient is negative, oil can be trapp ed and the recovery is also poor. We performed gravity drainage experi ments in sand columns and capillary tubes which confirmed our predicti ons.