On the Investigation of Gravity-Assisted Inert Gas Injection Using Micromodels, Long Berea Sandstone Cores, and Computer-Assisted Tomography

Abstract

SPE Members Abstract Downward displacement of oil by inert gas injected in a reservoir either at initial oil and connate water conditions or, in a reservoir depleted by waterflooding, results in very high oil recovery efficiencies under strongly water-wet conditions in both unconsolidated and consolidated porous media respectively. Advances made in porous media respectively. Advances made in directional drilling technology and increased understanding of the mechanisms and conditions that maximize recovery efficiencies can make the inert gas injection process of oil recovery feasible for a wide variety of oil reservoirs. This paper presents experimental results and discussion of a presents experimental results and discussion of a series of experiments designed for the study of:production characteristics under immiscible inert gas driven gravity drainage conditions;microscopic mechanisms of displacements and the determination of the rate of production by gravity drainage in capillary tubes having square cross-section; andfluid distributions and oil bank formation using the x-ray computer tomography (CT) scanning facilities we recently established in our laboratory. Introduction Gas injection is being increasingly applied as a secondary or tertiary recovery process, particularly in reservoirs with a reasonable dip particularly in reservoirs with a reasonable dip angle, preferably of high permeability and containin light oil. In such reservoirs a gravity-stable injection scheme is often possible, leading to high sweep efficiencies. There are numerous projects for immiscible gas injection schemes reported in literature which have demonstrated in laboratory experiments and in field performance analysis that very high oil recoveries of residual oil are achieved if gravity drainage (the self propulsion of oil downward in a reservoir) is the dominant production mechanism. An extensive review of the literature is beyond the scope of this paper and can be found elsewhere. Pioneering studies involving the drainage of liquids from unconsolidated sands were done by Stahl et al., and Terwilliger et al. Imagine a vertical column of water-wet glass beads saturated first with brine and subsequently oil flooded to establish initial oil and connate water conditions. Next, consider the injection of air or nitrogen at the top of the column at constant pressure and the production kept at controlled flow rate such that the pore velocity of the gas-oil contact (GOC) is relatively small (e.g. 10(-2) to 10(-1) m/d). Also consider that at the production end at the bottom of the column there is production end at the bottom of the column there is a semipermeable membrane that permits the oil and the water to go through but not the gas when it eventually arrives at the producing end of the column. This experiment has been referred to as "controlled drainage of continuous oil" and has been routinely performed in our laboratory. Overall recovery efficiences were very high (90% to 99% of the oil in place, in glass bead columns of different dimensions). In earlier studies of gravity drainage very low residual oil saturations have also been reported in water-wet media. Dumore and Schols concluded that after the relatively short period in which the main oil production takes place (i.e. in the free fall production takes place (i.e. in the free fall drainage) the drainage of oil in the presence of connate water and injected gas is possibly governed by film flow of bypassed oil. Hagoort stated that gravity drainage can be an effective way of oil production. In his theoretical discussion, the production. In his theoretical discussion, the importance of relative permeabilities to oil was addressed and used th centrifuge technique in order to get the exact representation of field conditions. P. 223