Abstract

The effects of permeability contrast in heterogeneous porous media and the role played by pore level physics on the production characteristics of oil and water in the gravity assisted inert gas injection (GAIGl) process were studied experimentally using I) long columns of sand packs. 2) columns of glass beads, and 3) consolidated models of glass beads between glass plates for flow visualizations. The percent oil recovery efficiency in stratified(layered permeability type) porous media under water wetting and positive oil spreading coefficient conditions was found to be about 80 % of the waterflood residual oil saturation or better. No significant amount of oil is held at the boundaries of the low to high permeability zones. The main reason for this is the presence of a continuous oil film over water in the edges of pores invaded by the gas throughout the medium which permits the oil to continuously drain towards the production end at the bottom of the system. A mathematical analysis of the GAIGI process has been developed that can be used to predict the production history and oil saturation profiles during gravity drainage for both, homogeneous and layered permeability type of porous media. Excellent agreement was found between the experimentally measured oil production characteristics and those predicted theoretically.

Introduction

The gravity assisted inert gas injection process (GAIGI) or tertiary immiscible gas flooding has been the subject of three PhD studies in our laboratory (Kantzas(l988), Catalan(l992), Ayatollahi(I994?) and related publications, as well as, of others who were influenced by our pioneering work on this very promising tertiary oil recovery process(e.g., Oren et. al. (1992), Pavone(1989). Vizika(1990?). 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, namely, oil, water, and gas (Chatzis et. al(1988), Kantzas et. al(l988)). The role played by pore structure(Catalan et. aL(l994), Kantzas et aL(l993), Oren and Pinczewski(1991)), the wettability(Chatzis et. al.1988), Catalan el. aL(1994)) and 'the effects of oil spreading coefficient on oil recovery, respectively, have also been investigated for the GAIGI process(Chatzis et, al(l988), Catalan et al(1994), Vizika(1993)).

The underlying principle of the tertiary immiscible gas flooding process is the formation 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 a gas, 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.

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