Three- and Four-Phase Flow Compositional Simulations of CO2/NGL EOR

摘要

CO_2 mixed with NGL is being evaluated as a method toenhance viscous oil recovery from Schrader Bluff and otheroil reservoirs at Milne Point, Alaska. The sequestration ofCO_2 is a secondary objective of these proposed projects.Mixtures of CO_2 and NGL with the crude oil show a largethree-phase liquid-liquid-gas region, so three and four-phaseflow may occur in the reservoir when water is alternated withthe miscible injectant. A compositional EOS simulator hasbeen used to simulate the oil recovery for both three-phase andfour-phase flow cases. The objective was to understand howimportant it is to simulate four-phase flow rather than use athree-phase flow approximation. Two methods were used tomodel three and four-phase relative permeabilities from two-phaserelative permeabilities as well as to evaluate thesensitivity of the results to the relative permeabilityparameters. A two-dimensional vertical cross-section of thereservoir was modeled with a stochastic permeability field toapproximate heterogeneities. These simulations clearly showthat under these conditions, four-phase flow is significant.Four-phase flow occurs over a significant part of the reservoirand affects both the sweep efficiency and the injection andproduction rates when the wells are pressure constrained andthe production pressure corresponds to the three-phase regionof the phase diagram. The results are also very sensitive to therelative permeability parameters regardless of which relativepermeability model is assumed. The project life is particularlysensitive to the relative permeability and is longer for four-phaseflow than predicted by a three-phase flowapproximation because the relative permeability of each phaseis lower when four phases are modeled.