Mixed CO₂/N₂Foam for EOR as a Novel Solution for Supercritical CO₂Foam Challenges in Sandstone Reservoirs

Among the various enhanced oil recovery (EOR) processes, CO2 injection has been widely utilized for oil displacement in EOR. Unfortunately, gas injection suffers from gravity override and high mobility, which reduces the sweep efficiency and oil recovery. Foams can counter these problems by reducing gas mobility, which significantly increases the macroscopic sweep efficiency and results in higher recovery. Nevertheless, CO2 is unable to generate foam or strong foam above its supercritical conditions (for CO2, 1100 psi at 31.1 °C), and most of the reservoirs exist at higher temperatures and pressure than CO2 supercritical conditions. The formation of strong CO2 foam becomes more difficult with an increase in pressure and temperature above its supercritical conditions and exacerbated CO2-foam properties. These difficulties can be overcome by replacing a portion of CO2 with N2 because a mixture of N2 and CO2 gases can generate foam or strong foam above CO2 supercritical conditions. Although many researchers have investigated EOR by using CO2 or N2 foam separately, the performance of mixed CO2/N2 foam on EOR has not been investigated. This study provides a solution to generate CO2 foam above its supercritical conditions by replacing part of CO2 with N2 (mixed CO2/N2 foam). The mixed foam not only generates strong foam above CO2 supercritical conditions but also remarkably increases the oil recovery. This solution overcomes the difficulties associated with the formation of CO2 foam at HPHT conditions enabling the use of the CO2-foam system for effective EOR and other applications of CO2 foam such as conformance control.