This paper was prepared for the Improved Oil Recovery Symposium of the Society of Petroleum Engineers of AIME, to be held in Tulsa, Okla., April 22–24, 1974. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon requested to the Editor of the appropriate journal, provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussions may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract Displacement of a 36 degrees API West Texas oil from watered-out cores with carbon dioxide was investigated at reservoir conditions of 90 degrees F and 1250 psig. Phase behavior of the CO2-West Texas oil system was also investigated. The objectives of the study were (1) to determine the oil recovery efficiency, and (2) to improve understanding of the oil recovery mechanisms, especially in relation to the phase behavior. The phase behavior studies indicate that CO2 efficiently swells the oil and forms two equilibrium liquid phases with the oil. Significant amounts of light and intermediate hydrocarbons are shown to be extracted into the CO2-rich liquid phase from the oil phase. The CO2 displacement studies conducted on watered-out cores indicate that a continuous CO2 drive can achieve a maximum recovery of 69 percent of residual oil from a 6-ft Berea core and 66 percent from a 20-ft sand-packed core. The minimum CO2 slug size required in a water-propelled CO2 drive for maximum recovery is 0.42 pore volume for the Berea core and 0.27 pore volume for the sandpacked core. In both cases, the required slug sizes are slightly greater than residual oil saturations. An analysis of the results of the phase behavior and displacement studies indicates that CO2 swelling and CO2 extraction of oil are among the dominant mechanisms responsible for recovering residual oil. Introduction As oil becomes more difficult to find and expensive to produce, more attention is being given to methods for recovery of the residual oil still present in reservoirs already waterflooded. Because of its availability and relatively low cost in some areas, carbon dioxide as a flooding medium is attracting renewed interest. A number of laboratory investigations on CO2 flooding processes for oil recovery have been reported in the literature. Several of these reports are primarily concerned with CO2 injection into high oil saturation cores. With regard to work on CO2 recovery of oil at conditions of high water saturation, it is only recently that two papers have been published. Shelton and Schneider reported results on first-contact and multiple-contact miscible displacement in secondary and tertiary recovery, and Kumar and Von Gonten presented data on oil recovery efficiency with CO2 injection into watered-out cores. It is known that when CO2 is mixed with a number of hydrocarbons, the phenomenon of liquid-liquid equilibrium is observed.