Impact of Reservoir Mixing on Recovery in Enriched-Gas Drives Above the Minimum Miscibility Enrichment

摘要

Gas enrichment is an important variable to optimize oilrecovery in enriched-gas drives. For slimtube experiments, oilrecoveries do not increase significantly with enrichmentsgreater than the MME. For field projects, however, theoptimum enrichment may be different from the MME. Theoptimum enrichment is likely the result of greater mixing inreservoirs than in slimtubes. In addition, 2-D effects such aschanneling, gravity tonguing, and crossflow can impact theenrichment selected.Numerical simulation is often used to model the effect ofphysical mixing on oil recovery in miscible-gas floods.Unfortunately, numerical dispersion can cloud theinterpretation of the results by artificially increasing the levelof mixing in the reservoir.This paper investigates the interplay between variousmixing mechanisms, enrichment level, and numericaldispersion. The mixing mechanisms examined are mechanicaldispersion, gravity crossflow, and viscous crossflow.UTCOMP is used to evaluate the effect of these mechanismson recovery for different grid refinements, reservoirheterogeneities, injection boundary conditions, relativepermeabilities, and numerical weighting methods includinghigher-order methods. For all simulations, the reservoir fluidused is a twelve-component oil displaced by gases enrichedabove the MME.The results show that for 1-D enriched-gas floods, therecovery difference for enrichments above the MMEincreases significantly with dispersion. The trend, however, isnot monotonic and shows a maximum at a dispersivity ofabout 3 ft. Furthermore, the trend is independent of relativepermeabilities and gas trapping for dispersivities less thanabout 3 ft, i.e. range of possible reservoir dispersivities. For 2-D enriched gas floods with slug injection, the difference inrecovery generally increases as dispersion and crossflowincrease. The magnitude of the recovery differences is lessthan for the 1-D displacements. Recovery differences for 2-Dare highly dependent on relative permeabilities and gastrapping. For water alternating gas (WAG) injection, thedifferences in recovery increase slightly with as dispersiondecreases. That is, the recovery difference is significantlygreater with WAG at low levels of dispersion than with sluginjection. For the cases examined, the magnitude of recoverydifference varies from about 1 to 8 percent of the original oilin-place (OOIP).