Simulation study of miscible gas injection for gas enriched above the minimum miscibility enrichment

Miscible gas injection mobilizes residual oil to improve the displacement efficiency of oil recovery processes. Large density contrasts and an unfavorable mobility ratio between the displacing (injected gas) and displaced fluid (reservoir oil) leads to poor volumetric sweep efficiency of gas floods. Water-alternating-gas injection (WAG) is commonly used to improve volumetric sweep efficiency in heterogeneous reservoirs. WAG reduces density and mobility contrasts, and increases mixing of injected and reservoir fluids. Little has been reported in the literature, however, on the effectiveness of WAG processes where the gas is enriched above the minimum miscibility enrichment (MME). The amount of induced mixing is also relatively unknown, partially because the true dispersion level remains unknown. This research examines optimization of WAG processes for enriched gas floods above the MME, particularly as a primary recovery method. Compositional simulations of x-z cross-sections are used to quantify the effects of WAG parameters, heterogeneity, and level of enrichment on local displacement efficiency and sweep efficiency. Stochastic permeability fields with different vertical to horizontal permeability ratios are used to represent the impact of heterogeneity on recovery from miscible WAG floods. The results of this study show that over-enrichment decreases the optimum number of WAG cycles and the amount of gas required to be injected per cycle to maximize recovery. A moderate level of heterogeneity increases WAG recovery. Reservoirs with high permeability layers at the bottom benefit most from enriched WAG. This research also explores the use of compositional core data in the form of stripping ratios to estimate the level of dispersion at field scale. Results from 1-D simulation of continuous slug injection along with analytical gas injection theory are used to determine an appropriate definition of stripping ratio for certain gas injection processes. The stripping ratio calculated with an intermediate component as the lightest component in the oil not present in the injected gas, best estimates the displacement mechanism. Horizontal and vertical sections from 2-D simulations are used to analyze the effect of viscous and gravity crossflow on interpretation of stripping ratio. Resaturation of gas swept regions is significant even for low vertical permeability and a high level of heterogeneity, and complicates the interpretation of stripping ratio. Nevertheless, the magnitude of stripping ratios in resaturated regions can be used to qualitatively estimate the level of dispersion at the field scale