Impact of reservoir mixing on recovery in enriched-gas drives above the minimum miscibility enrichment

Gas enrichment is an important variable used to optimize oil recovery in en riched-gas drives. For slimtube experiments, oil recoveries do not increase significantly with enrichments greater than the minimum miscibility enrich ment (MME). For field projects, however. the optimum enrichment required to maximize recovery on a pattern scale may be different from the MME. The op timum enrichment is likely the result of greater mixing in reservoirs than in slimtubes. In addition, 2D effects, such as channeling, gravity tonguing , and crossflow. can impact the enrichment selected. Numerical simulation is often used to model the effect of physical mixing o n oil recovery in miscible gasfloods. Unfortunately, numerical dispersion c an cloud the interpretation of the results by artificially increasing the l evel of mixing in the reservoir. This paper investigates the interplay among various mixing mechanisms, enri chment levels. and numerical dispersion. The mixing mechanisms examined are mechanical dispersion, gravity crossflow, and viscous crossflow. The U. of Texas Compositional Simulator (UTCOMP) is used to evaluate the effect of t hese mechanisms on recovery for different grid refinements, reservoir heter ogeneities, injection boundary conditions, relative permeabilities, and num erical weighting methods, including higher-order methods. The reservoir flu id used for all simulations is a 12-component oil displaced by gases enrich ed above the MME. The results show that for ID enriched gasfloods, the recovery difference be tween displacements above the MME and those at or near the MME increases si gnificantly with dispersion. The trend, however. is not monotonic and shows a maximum at a dispersivity of approximately 4 ft. The trend is independen t of relative permeabilities and gas trapping for dispersivities of less th an approximately 4 ft. For 2D enriched gasfloods with slug injection, the d ifference in recovery generally increases as dispersion and crossflow incre ase. The magnitude of the recovery differences is less than that observed f or the ID displacements. Recovery differences for 2D models are highly depe ndent on relative permeabilities and gas trapping. For water alternating ga s (WAG) injection, the differences in recovery increase slightly as dispers ion decreases. That is, the recovery difference is significantly greater wi th WAG at low level, of dispersion than with slug injection. For the cases examined, the magnitude of recovery difference varies from approximately 1 to 8% of the original oil in place (OOIP).