Comparative Study of Oil Recovery during Miscible CO2 Injection in Carbonate Cores and Slimtube

Abstract

Abstract Three types of displacement experiments were performed using 31°API oil and CO2: (1) slimtube experiments using the DBR model Jefri (40ft long ¼ inch OD tubing uniformly-packed with 100 mesh sand), (2) a coreflood experiment at the miscible pressure and temperature determined in (1) using 6 inch by 2 inch OD carbonate core, and (3) combined slimtube-coreflood (injectant gas goes through the slimtube in (1) before entering the coreholder in (2)). The objective of the slimtube experiments was to measure the minimum miscibility pressure. The coreflood experiment was made to verify if miscibility occurred at the shorter core length. The combined coreflood-slimtube experiment was conducted to provide sufficient length and ensure miscibility of CO2 in the oil before being injected into the carbonate core. The results may be summarized as follows. First, the MMP was measured to be 1800 psig. Second, oil recovery from the carbonate coreflood experiment was 75% of oil initially-in-place (OIIP), 20% lower than that from the sand packed slimtube experiments (95% OIIP). The combined slimtube-coreflood arrangement resulted in no incremental recovery over that from the coreflood experiment. Third, decrease in oil recovery from the coreflood (compared to the slimtube experiments) was further investigated by CT scanning the core before and after the CO2 flood. It was found that because of the rock’s heterogeneity, CO2 only swept the larger pores while the smaller pores had higher residual oil saturation. The heterogeneity resulted in small scale dispersion which grows with length and will always be greater in carbonate rocks than in slimtube and sandstones. Therefore, it was concluded that extra length will not help increase recovery (miscibility). This conclusion is analogous to field applications where the distance between the injector and producer may be sufficient, yet miscible CO2 floods often result in lower than expected recoveries. Finally, the measured MMP from the slimtube was compared to nine published MMP correlations for CO2 and oil and the error was reported. It was found that Cronquist and Holm-Josendal correlations that include temperature and molecular weight of C5+ gave the closest agreement with the measured MMP.