Determination and Prediction of CO2 Minimum Miscibility Pressures (includes associated paper 8876 )

Abstract

An experimental procedure is presented for determining the CO2 minimummiscibility pressure for a reservoir oil. The procedure consists ofperforming CO2 displacements at various pressure levels in a sand-packedslim tube. A correlation is presented for predicting reservoir oil CO2minimum miscibility pressures based on reservoir temperature and bubble-point pressure. Introduction The injection of carbon dioxide (CO2) for secondary andtertiary oil recovery has received considerable attention inthe industry because of its high displacement efficiencyand relatively low cost. Miscible recovery of a reservoiroil can be achieved by CO2 displacement at a pressurelevel greater than a certain minimum. This minimumpressure is hereafter defined as the CO2 minimummiscibility pressure (MMP).The CO2 MMP is an important parameter for screeningand selecting reservoirs for CO2 injection projects. Forthe highest recovery, a candidate reservoir must becapable of withstanding an average reservoir pressure greaterthan the CO2 MMP. A knowledge of the CO2 MMP is alsoimportant when selecting a model to predict or simulate reservoir performance as a result Of CO2 injection.Methods of predicting CO2 MMP's based on reservoircomposition and reservoir temperature have beenpresented in the literature. These methods have limitationsthat can result in large discrepancies when compared withexperimentally determined values. In addition, there isno generally accepted standard method in the literature forexperimentally determining the CO2 MMP for an oil.An experimental study was undertaken to obtain abetter understanding of the effects of temperature and oilcomposition on the CO2 MMP determined for an oil. CO2MMP's were determined using the sand-packed coil (orslim-tube)method. Results of this study were used todevelop a correlation for predicting the CO2 MMP foran oil.The purpose of this paper is to present the correlation for predicting CO2 MMP's that was developed from thisstudy. Another purpose is to propose that the sand-packedcoil method be used as a standard method ofexperimentally determining the CO2 MMP for an oil. Experimental StudyExperimental Variables Two variables were considered in this study: oilcomposition and temperature. Oils were considered to consist ofthree fractions: a light fraction consisting primarily of C1and small amounts of N2 and CO2; an intermediatefraction consisting of hydrocarbons with molecular weightsbetween C2 and C6; and a heavy fraction (C7+)consisting of hydrocarbons with molecular weights equal to orgreater than normal C7. The oils discussed in this paperwere prepared by combining the molar amounts of thesefractions in varying proportions. In general, the sameC7+, fraction was recombined with different amounts ofthe light (C1 + CO2 + N2) and intermediate (C2 - C6)fractions. With the exception of one oil, the same ratio ofone component to another within each fraction wasmaintained regardless of the total amount of the fraction in anyoil. The C7 + fraction was prepared by batch distilling awest Texas 30 degrees API (0.87-g/cm) separator oil. Thisdistillation had approximately five theoretical plates and wasperformed at atmospheric pressure and 208 degrees F (98 degrees C). JPT P. 160＾