Experimental Study on the Generation, Stability, and Injection of CO2 Foams Stabilized by Nonionic Surfactants in Carbonate Rocks

CO 2 injection is a widely established oil recovery method; however, continuous gas injection often suffers from poor mobility control, gravitational segregation, and viscous instability, leading to reduced sweep efficiency and high gas–oil ratios. CO 2 foam injection has therefore emerged as an effective strategy to improve mobility control. Despite extensive research, systematic evaluations of how bulk foam stability relates to flow behavior in porous media under varying conditions remain limited. Thus, this study aims to develop and evaluate the formation, stability, and flow behavior of CO 2 foams using nonionic surfactants in formation water varying in salinity, temperature, and surfactant concentration. Nonionic surfactants (Ultranex NP 230 and Ultramine TA 200) were evaluated using bulk foam tests and porous-media flow experiments in carbonate rock plugs. The main results indicate that higher surfactant concentrations generate foams with greater volume and greater stability, achieving a half-life of 226 min (Ultranex 230 at 10×CMC). In addition, CO 2 alters the medium pH, which may influence foam quality and stability. Systems containing CO 2 and formation water produce more stable foams, achieving half-life times up to 84 min (Ultranex 230 at 10×CMC), compared with the corresponding system in 4% KCl, which has a half-life of 45.5 min. However, high salinity negatively affects the foam volume obtained. Increasing temperature promotes the formation of more voluminous foams, though at lower stability, reducing the foam half-life by up to 50% at some concentrations. In porous-media flow tests, foam quality also affected the flow behavior, and Ultranex NP 230 produced the most viscous foams at a foam quality of 0.6 (gas volume fraction), reaching 6.56 cP at 20×CMC in formation water. The influence of salinity strongly depends on surfactant concentration and chemical nature. Overall, these findings indicate that CO 2 foams formulated with nonionic surfactants present favorable stability and flow behavior, supporting their application as an efficient mobility-control strategy in CO 2 -EOR.