Laboratory Investigations of CO2 Near-miscible Application in Arbuckle Reservoir

Abstract

Abstract Carbon dioxide (CO2) is a proven enhanced oil recovery technology. However, many reservoirs are located at shallow depths or geologic conditions such that CO2 can not be injected at pressures above the MMP. CO2 injection is usually not considered as an enhanced oil recovery process in these reservoirs. When CO2 is injected below the MMP, displacement efficiency decreases as a result of the loss of miscibility. Near miscible displacement has sometimes been referred to as the process occurring between immiscible and miscible pressures, but has never been clearly defined. This paper describes laboratory study of CO2 near miscible displacement in an Arbuckle reservoir in Kansas. Phase behavior studies between CO2 and Arbuckle crude oil were carried out to define near miscible conditions at reservoir conditions. Swelling/extraction tests combined with slim-tube experiments were interpreted to identify the mass transfer mechanisms at near miscible condition. A phase behavior model was developed to match PVT data and MMP in the slim-tube experiment. Good agreement was obtained between simulated and observed data from slim-tube experiments. Core flooding tests were conducted to evaluate oil recovery at near miscible condition at which pressure varies from 1350 psi (MMP) to 1150 psi. Recovery of over 50% of the waterflood residual oil saturation was observed when CO2 was used to displace Arbuckle oil from Berea, Baker dolomite and Arbuckle dolomite cores. At near miscible conditions, extraction appears to be the primary mechanism for mass transfer between hydrocarbon components and CO2. However, the reduction of oil viscosity by a factor of five occurred when CO2 dissolved in the oil. This suggests that some of the additional oil recovery may be attributed to reduction of the mobility ratio between CO2 and resident oil.