Vertical Sweep Efficiency and Oil Recovery From Gas Injection in Heterogeneous Horizontal Reservoirs

Abstract

Abstract Many field tests have demonstrated that without the benefit of gravity stabilization gasflooding operations usually result in low vertical sweep efficiency due to gravity override. For this reason horizontal or low relief reservoirs, which constitute a large fraction of hydrocarbon bearing formations, are generally not considered to be good candidates for recovery schemes involving gas injection. A cross-sectional reservoir simulation study was undertaken to investigate the effect of vertical heterogeneity, viscous/gravity force ratio, and perforation thickness on oil recovery from gas injection in horizontal reservoirs. Three types of reservoir systems were considered:homogeneous,deltaic channel sands, represented by vertically decreasing layer permeabilities, anddeltaic bar sands, represented by vertically increasing layer permeabilities. The degree of reservoir heterogeneity was systematically varied and characterized by the Dykstra-Parsons coefficient. For the homogeneous reservoir, vertical sweep efficiency and oil recovery at breakthrough were found to increase with increasing gas injection rates. On the other hand, over most of the ranges of viscous/gravity force ratio and degree of heterogeneity investigated, channel sand reservoirs tended to give increasing breakthrough oil recoveries for decreasing rates of gas injection. Deltaic bar reservoirs exhibited more complex behaviour. At low heterogeneity levels these reservoirs gave higher oil recoveries at higher injection rates, and reversed this trend as the degree of heterogeneity increased. Included in the paper are gas saturation breakthrough patterns, which show the influence of geological and operating conditions on the location of uncontacted oil. Introduction Gas injection into an oil reservoir has been widely used for the purpose of increasing oil recovery. There are several reasons for this:gas is nonreacting with the reservoir rock,injectivity is higher than in the case of waterflooding, andproduction of injection gas at breakthrough is not a costly operational problem. Due to the low viscosity and density of gas, the vertical sweep efficiency of gas injection in horizontal reservoirs is reduced by viscous fingering and gravity override, respectively. Additionally, the presence of high permeability streaks in the formation gives rise to bypassing of oil by injected gas. Because of these operational problems gas injection operations have been conducted more for pressure maintenance (crestal injection in high relief reservoirs) than as a secondary recovery flood process (pattern injection in horizontal reservoirs). This tacit screening criterion, which requires gravity stabilization, excludes a large fraction of the oil bearing formations from\consideration as candidates for recovery schemes involving gas injection. P. 569^