We have performed a miscible CO2 flood study using heterogeneous carbonate cores and 31°API west Texas oil to evaluate oil recovery under four injection modes: simultaneous water and CO2 (SWACO2), water alternating CO2 (WACO2), continuous gas injection (CGI), and waterflood (WF). For WACO2 and SWACO2 injection modes, three salt concentrations are used: 0, 6, and 20 wt. %. For WF, two salt concentrations are used: 0 and 6 wt. %.
The experimental results may be summarized as follows: For injected water with 20 wt. % salt concentration, the oil recoveries for SWACO2 and WACO2 are 81 and 74 % oil initially-in-place (OIIP), respectively. For 6 wt. % salt concentration, the oil recoveries for SWACO2 and WACO2 increased to 90 and 86.5% oil initially-in-place (OIIP), respectively. With injected pure water (0 wt. %), the oil recoveries for SWACO2 and WACO2 increased to 98 and 92 % OIIP, respectively. The waterflood oil recovery remains the same at 54% for both salinity levels indicating that dissolved CO2 is the critical factor in oil displacement. Literature data suggests that CO2 solubility in water increases when salinity of the injected water is reduced. The low salinity water with CO2 appears to increase the microscopic oil displacement efficiency.