Research on microscopic oil displacement mechanism of CO2 EOR in extra-high water cut reservoirs

In China, CO2 EOR technology is mostly applied in small scale field test after water flooding. Microscopic mechanism of CO2 EOR at extra-high water cut stage is not clear to us. Based on the characteristics of residual oil after water flooding, three types of ideal core models were designed including dead-end model, shaped island model and cluster model. The microscopic experiments were carried out in high temperature high pressure micro-visualization experiment apparatus of independent research and development. The microscopic mathematic model of CO2-water-oil interaction was established. The process of CO2 dissolution and extraction during CO2 injection at extra-high water cut stage (water cut > 98%) were monitored in the experiments for the first time. The microscopic mechanism of CO2 EOR on trapped oil droplets and the influence of water barrier on miscible displacement were discussed. The results from experiments and mathematic calculation conclude that water barrier should postpone and complicate miscible process between CO2 and trapped oil. No matter how thick the water film, CO2 can diffuse through the water film into the trapped oil. The higher injection pressure is, the faster CO2 diffuses into oil, the more easily miscibility is achieved. CO2 miscible displacement contributes to oil recovery largely after water flooding. CO2 EOR might benefit from the increasing of injection pressure and the full contact time between CO2 and fluid (water and trapped oil) underground in extra-high water cut reservoit.