Effect of N₂ on CO₂-Foam Assisted Enhanced Oil Recovery in Sandstone Reservoir

The global need for CO₂ sequestration and associated advantages made CO₂ gas injection as one of the important enhanced oil recovery (EOR) technique. The higher solubility of CO₂ in oil, less injection pressure and miscibility makes it a favorable choice for EOR processes, but it suffers from high mobility and gravity override results in lower ultimate recovery. The high mobility and gravity override can be minimized by generating the CO₂ foam with the aid of a surfactant. However, CO₂ is unable to generate the foam/stable foam above its supercritical point (1100 psi, 31°C). These difficulties with CO₂ foam is overcome by adding N₂ in a small fraction to enhance the foam generation of CO₂ at supercritical conditions. This study shows how the addition of small quantity of N₂ helps in generating the CO₂ foam and performance of the novel CO₂/N₂ mixture foam in enhanced oil recovery.

To investigate the performance of CO₂/N₂ foam, core-flooding experiments were conducted at elevated pressure and temperature condition (higher than supercritical CO₂ – 50°C and 1500 psi) in sandstone cores. Fluorosurfactant (FS-51) was used as a foaming agent and n-decane was used as model oil in all the experiments. The selection of foam quality and N₂ fraction was optimized based on foam generation and stability tests. Every gas or foam flooding was preceded by seawater injection to simulate the behavior in the reservoir.

The results from the core-flood experiments showed that the CO₂ and CO₂/N₂ foam flooding recovered an additional 34-40% of Original Initial Oil in Place (OIIP) indicating that foam flooding succeeded in producing more oil than pure CO₂ gas injection processes. Additionally, the performance CO₂/N₂ foam injection was better than CO₂ foam injection for delaying the breakthrough.