The Influence of O2 Contamination on MMP and Core Flood Performance in Miscible and Immiscible CO2 WAG

Abstract For a given oil and reservoir temperature, the CO2 Minimum Miscibility Pressure (MMP) is largely affected by the CO2 purity. A commonly found contamination gas in CO2 EOR is O2, the existence of which will influence the MMP and thus influence the CO2 Water Alternating Gas (WAG) performance. Slim tube tests and core flood experiments are conducted in order to study the influence of O2 on the MMP and CO2 WAG performance. Both experiments utilize crude oil from South Slattery Field in Wyoming. The mole concentration of O2 in CO2 gas varies from 0 mol% to 5 and 10 mol% and the experimental temperature is set at 57 °C to mimic the reservoir condition. In the slim tube tests, the oil recoveries at gas breakthrough are used to interpret the MMP. All core flood experiments utilize Berea sandstone cores and synthetic brines. A pressure of 20% higher than the pure CO2 MMP is used to ensure miscible condition and a pressure of 50% of the pure CO2 MMP is used for the immiscible study. WAG is injected in the tertiary recovery mode and its parameters include a WAG ratio of 1:1, a half cycle slug size of 0.1 PV, and a total slug size of 2.0 PV. WAG performance, i.e., percent oil recovery, tertiary recovery factor, and CO2 utilization factor are determined. Slim tube results show that the existence of O2 increases the CO2 MMP significantly. Five mol percent of O2 in CO2 gas increases its MMP by 20.88%, while 10 mol% of O2 increases it by 61.92%. Core flood results show that the existence of O2 in CO2 gas adversely influence the WAG performance. For miscible CO2 WAG flooding, the higher the O2 content, the lower the incremental recovery and tertiary recovery factor (TRF). The same results are also found in immiscible CO2 WAG with less significant effects. Similar studies have been mainly focused on N2 and CH4, while the influence of O2 has never been experimentally investigated. This study is essential for better understanding the influence of CO2 contamination on WAG performance, which is a major concern in CO2 EOR utilizing unpurified CO2 gas sources.