Quantification of Displacement Mechanisms in Multicomponent Gasfloods

Local displacement efficiency in gas floods depends strongly on the minimum miscibility pressure (MMP) or minimum miscibility enrichment (MME). The values for these design parameters depend in turn on the displacement mechanisms, vaporizing, condensing, or a combination of the two known as a condensing/vaporizing (CV) drive. Characterization of the displacement mechanism, however, is currently limited to these broad categories with little reference to the degree to which a CV displacement is condensing or vaporizing. The focus of this paper, therefore, is to present a method to quantify the fraction of a multicomponent gas flood that is vaporizing or condensing as the pressure or gas enrichment is increased. The approach relies on finding key tie lines for a dispersion-free one-dimensional displacement using method of characteristic theory (MOC). We quantify the displacement mechanism for any number of oil or gas components by calculating the displacement path lengths along ruled surfaces bounded by these key tie lines. Several multicomponent fluid characterizations are considered, including a twelve-component enriched-gas flood and a thirteen-component CO₂ flood. The results show that as the pressure or enrichment is increased condensation occurs at the expense of vaporization. We also show by numerical simulations that the sensitivity of the local displacement efficiency to dispersion depends on the condensing fraction of the displacement.