The downward displacement of oil by inert gas injection in a reservoir depleted by waterflooding, results in very high oil recovery efficiencies. The success of this technique stems from the continuity of the oil phase provided by the bulk films of oil in all pore space invaded by the gas under conditions of positive oil spreading coefficients and the gravity stabilized oil bank moving toward the production well. The focus of this paper is on the effect of production rate on oil recovery efficiency. Experimental results obtained with unconsolidated porous media show that the total recovery before and after gas breakthrough at high production rates is the same or better than the recovery at lower rates. The oil recovery at gas breakthrough has been found to correlate very well with the gravity number. The oil production characteristics in unconsolidated columns of glass beads of different permeabilities were found to scale with dimensionless time and gravity number as parameters. For positive spreading coefficients of oil over water in the presence of gas, the production data of oil recovery factor versus time can be used to calculate the relative permeability to oil in the gas invaded zone.
Gas injection is being increasingly applied as a secondary or tertiary oil recovery process, particularly in reservoirs with a reasonable dip angle, preferably of high permeability and containing light oil. In such reservoirs, a gravitystable injection scheme is possible, leading to high sweep efficiencies. The application of gravity assisted inert gas injection process (GAIGI) or tertiary immiscible gas flooding has been the subject of a number of studies (l-9) in our laboratory and by others (10–16) who followed our pioneeringwork. Laboratory experiments (1–9) and field performance analysis (17–18) have demonstrated that very high oil recovery of residual oil is attainable if gravity drainage (the self propulsion of oil downwards in a reservoir) is the dominant production mechanism.
A significant contribution of recent laboratory studies on tertiary immiscible gas injection schemes has been in the development of a better understanding of the pore level physics of immiscible displacements involving three immiscible fluids-oil, water and gas (4–5, 10–14) . The role played by pore structure (4,6,13) , the wettability (4,9) and the effects of oil spreading coefficient (4, 11,13,16) oil recovery, respectively, have also been investigated for the GAIGI process.
The underlying principle of the tertiary immiscible gas flooding process is the creation of a gravity stabilized oil bank from the residual oil in the course of gas invasion into pores containing residual oil and water. The most common conditions that have been investigated in depth are those corresponding to water-wet media and positive oil spreading coefficients, Under these conditions, when pores filled with waterflood residual oil are contacted by immiscible gas, such as air or nitrogen, after dented by the gas, the oil in the blobs spreads spontaneously over the water and forms bulk films of oil between the connate water and the displacing gas which is at displacement pressure (see Figure 1).