SWACO2 and WACO2 Efficiency Improvement in Carbonate Cores by Lowering Water Salinity

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.