Miscible Displacement of Heavy West Sak Crude by Solvents in Slim Tube

ABSTRACT In enriched gas drives, for correct interpretation of slim tube displacement (STD) tests and for determination of minimum miscibility pressure (MMP) or minimum enrichment (ME) required to achieve dynamic miscibility, it is extremely important to identify the correct mechanism of misciblity development. Traditional, pseudo-ternary diagram construction and limiting tie-line method or the criteria of very high (90-95% plus) ultimate oil recoveries or the criteria of breakover point in ultimate oil recovery versus pressure diagram in STD tests, used to determine MMP or ME are not always valid and can lead to solvents designed either too rich or too lean. In this study, STD tests supported by equation-of-state (EOS) predictions were used to evaluate the ability of various solvents such as CO2, n-butane and various mixtures of Prudhoe Bay natural gas (PBG) and natural gas liquids (NGL), to miscibly displace heavy, asphaltic West Sak crude. Results indicate that for enriched gas drives, the development of dynamic miscibility occurs via simultaneous vaporizing and condensing mechanisms. The solvent minimum enrichments for this dual mechanism were obtained from the solvent-oil, pressure-composition isotherms, compositional path in multicontact test (MCT) calculations and the methane spike disappearance phenomena in STD tests and were compared to those determined by condensing type, pseudo-ternary diagram construction method. STD test results indicate that the ultimate oil recoveries, even for first contact miscible (FCM) solvent were considerably lower due to asphaltene precipitation. Asphaltene tests were conducted for various solvent-West Sak crude mixtures to determine the amount of precipitation and it's effect on oil composition. STD results and EOS predictions indicate that CO2 was unable to develop dynamic miscibility with West Sak crude at reservoir pressure and temperature conditions and the process mechanism for CO2 drive is simultaneous vaporizing-condensing drive.