Effect of Heptanes‐Plus Molar Distribution on CO2 Minimum Miscibility Pressure

Abstract

Holm and Josendal (1992) described the effects of C5+ molecular weight effect on MMPand developed a correlation between CO2 density at MMP and (C5‐C30)/C5+ content of oil (X). CO2 density at MMP is a function of reservoir temperature and MMP. From Holm and Josendal(1982) correlation, we can also define CO2 density as a function of X. X is affected by the C7+fractions characterization since the major part of X consist of C7+ fractions. In this study, the C7+characterization highlighted is molar distribution. Because we are using the gamma distribution model for molar distribution of C7+ fractions, X is thus affected by gamma distribution parameters (α and M7+ as we fixed the value of η) also. Indirectly, α and M7+ affects the MMP predicted. Varied α and M7+ generate new fluid compositions which are used to calculate thevalue of X. The new fluid compositions are used to estimate MMP using an EOS model. Byknowing the reservoir temperature and MMP, we can calculate CO2 density at MMP. Variedcombinations of α and M7+ can generate the same value of X, and should give the same value ofCO2 density as described in Holm and Josendal (1982) correlation.

Reservoir oils used in this study are oil A, a typical West Texas oil from San Andres formation (Negahban and Kremesec 1992) and oil B, a typical Weyburn oil (PERA confidentialreport 2010). The EOS for oil A is the AMOCO version of Redlich Kwong EOS (ARKES) and for oil Bis Peng‐Robinson EOS which was tuned by PERA. These reservoir oils and EOS data were used to estimate MMP with PhazeComp simulator for a range of temperatures. Heptanes‐plus gamma shape (α) and average molecular weight (M7+) were varied with fixed minimum molecular weight (η). Observations were not only conducted through the relationship of CO2 density at MMP and X, but also through the relationship of MMP and the gamma parameters.

The variation of M7+ gives the same trend as Holm and Josendal (1982) correlation ofCO2 density at MMP and X. On the other hand, the variation of α gives opposite trend to Holmand Josendal correlation (1982). MMP is more consistently monotonically correlated withtemperature rather than with CO2 density at MMP for given oil. MMP(T)/MMPo(T) (MMP atcertain temperature) gives the same increasing trend as M7+/M7+o or α/ αo increases for oil A and oil B.