Enhanced Oil Recovery by Inert Gas Injection

Abstract

Abstract Downward displacement of oil by inert gas injected at the top of the formation using horizontal wells, has the potential of becoming an efficient method of oil recovery for reservoirs either at initial oil conditions or for reservoirs depleted by waterflooding. Advances in horizontal well drilling technology can make this method of oil recovery feasible for a wide variety of oil reservoirs. This paper presents experimental results of nitrogen or air injection into strongly water-wet unconsolidated and consolidated samples. Production characteristics are discussed, displacement mechanisms are formulated and the applicability of this method on the field scale is examined. Introduction Gravity forces have been considered for a long time to be a factor that should be minimized in oil recovery processes. However, recent laboratory studies have shown that very high yields of petroleum production can be obtained when the oil recovery process is gravity assisted. New directional and horizontal drilling techniques can assist in improving oil recovery. There are many projects reported in the literature that relate reservoir behaviour and gravity phenomena. Emphasis has been given to gravity drainage, i.e. the self propulsion of oil downward in the reservoir rock. The first gravity drainage studies were performed by Stahl et al. Air was used to displace various liquids from a column containing Wilcox Sand. They reported results showing the dependence of liquid saturation on column height at both equilibrium and dynamic conditions. They also showed a temperature dependence of the drainage rates. Lewis gave an extensive review of the general aspects of gravity drainage and discussed conditions that favor this process. He also reported some field studies. Terwilliger et al. performed gravity drainage experiments on a silica sand using brine and gas. The main difference between the work of Terwilliger et al. and the one of Stahl et al. is the fact that Terwilliger et al had continuous production of he wetting phase. Stahl et al. stopped the experiment intermittently to take samples for saturation determinations. The results of Terwilliger et al. are matched by the Buckley-Leverett approach while the results of Stahl et al. are not. Marx described a method to predict the complete gravity drainage characteristics of long columns from centrifuge drainage measurements on reconstituted core samples. Gravity segregation has been studied by Craig et al. using scale up criteria, Templeton et al. in glass bead systems, Cook in a mathematical model, Gardner et al. in miscible systems, Slobod and Howlett in laboratory studies of miscible displacement in vertical unconsolidated porous media, and most gamma-ray camera. Spivak used a reservoir simulator to examine the effect of gravity segregation in two-phase displacement processes. Dumore and Schols found that when oil is displaced by gas at connate water conditions and at very low capillary numbers, the residual oil saturations are very low. They concluded that the rate of drainage of oil in the presence of connate water and gas is possibly governed by film flow of bypassed oil after the relatively short period in which the main oil production takes place. Recently, the importance of gravity drainage in oil production has been stressed by Hagoort, while its effect on steam flooding has been demonstrated theoretically and experimentally by several investigators. The above studies indicate that there is potential of using a gravity assisted inert gas injection scheme as a method of Enhanced Oil Recovery. P. 653^