ABSTRACT
Minimum miscibility pressure (MMP) is an important parameter for screening and selecting reservoirs for miscible gas injection projects. This paper presents a new correlation for the enriched-gas process that can be used to calculate the miscible slug compositions which consist of methane and mixtures of intermediate molecular weight hydrocarbons such as ethane to butane. The miscibility correlation was developed from MMP predictions of four reservoir fluids and several enriched-gas mixtures using the Peng-Robinson equation-of-state model. The correlation can be used to predict MMP for a fixed slug composition, or predict methane concentration for a given operating pressure. The correlation was tested with published data and showed an average prediction error of ± 2.5 mol% methane.
The miscibility conditions for two reservoirs and several enriched-gas mixtures were also determined in this study using slim tube apparatus. The miscibility in the slim tube tests was judged from the change of phases in the miscible transition zone rather than arbitrarily choosing oil recovery at the 90% level. A photosensor was used to aid the determination of miscibility. This experimental technique has proved to be very consistent. The results of these tests were compared with the new correlation and showed an average prediction error on MMP of less than ± 4%.
In general, either a higher pressure or a higher concentration of C2 to C4 hydrocarbons in the injection gas is required for achieving miscibility as (1) reservoir fluid C5+ molecular weight increases, (2) average molecular weight of C2 to C4 components in the injection gas decreases, or (3) reservoir temperature increases.
