Effects of Impurities on Minimum Miscibility Pressures and Minimum Enrichment Levels for CO2 and Rich-Gas Displacements

Multicontact miscible displacement processes are becoming increasingly popular as a means of recovering secondary and tertiary oil reserves in the U.S. and Canada. Economics of multicontact miscible flooding are governed to some extent by the availability of large sources of high purity CO2 or suitable liquefied petroleum gas (LPG) streams. This is because achievement of miscibility depends on the solvent composition as well as the system temperature and pressure. Atypical components in a CO2 or solvent stream therefore may increase the required pressure or enrichment levels for achievement of miscibility. Several papers have been published discussing the pressure (for CO2) and composition (for rich gas) levels required for miscible displacement. The potential CO2 supply could be increased if complicated cleanup procedures for injected and produced fluids were not required. For engineering studies it is important that CO2 streams containing H2S and hydrocarbons be evaluated for their miscible flooding potential. It is also important to evaluate the effects of CO2 and C5+ components in rich gas mixtures to determine whether they can be used to reduce calculated enrichment levels for solvent systems. This paper presents results of studies using mixtures of CO2, H2S, and C1, CO2-LPG, and rich gas solvents containing CO2 or C5 to displace oil miscibly in slimtube experiments. The purpose of this work is to show the effects of various components on pressure and compositions required for miscibility. As expected, the changes in CO2 miscibility pressure are direct functions of temperature. It is reported that the addition of H2S and C2+ hydrocarbons lowers the miscibility pressure for CO2, whereas the presence of C1 in a CO2 solvent increases it. More important, the results give a quantitative measure of the degree of reduction/elevation in miscibility pressure to be expected with impure CO2 streams. The paper also presents similar results from displacements with typical rich gas solvents mixed with CO2 and/or C5. It is reported that CO2 increases the minimum enrichment required, while a heavier hydrocarbon component actually can reduce anticipated enrichment levels. Introduction The use of miscible gas flooding as an improved oil recovery technique is increasing rapidly. CO2 in particular is being tested in at least 16 pilot or fieldwide floods. Multicontact miscible (CO2 and rich gas) processes are pressure and/or composition dependent - i.e., a certain pressure is required before a gas of given composition can miscibly displace a given crude oil. The pressure level required for mlticontact miscibility is therefore an important control variable. Control of this pressure can help to increase the number of economically feasible miscible projects. Of more importance, perhaps, is the ability to select a CO2 or LPG stream of less than 100% purity with the assurance that some minor concentration of H2, N2, C1, H2S, or C2 through C4 will not affect the performance adversely. Benham,1 Rutherford,2 and Jacobson,3 by monitoring recovery from slim-tube or core tests, have looked at ways in which C1, C2, C3, or C4 concentrations affect miscibility pressure. This research was important for understanding the compositional relationships between solvent and oil in the hydrocarbon multicontact miscible process. Jacobson also studied effects of H2S on displacement efficiencies of a C1 drive gas. Generally, these papers considered rather large concentrations of hydrocarbons, H2S, or CO2 in C1. The effects of N2 or C1 contaminants on the pressure required for CO2 flooding have been discussed most recently by Graue and Zana.4