Mixed CO₂/N₂ Foam for EOR as a Novel Solution for Supercritical CO₂ Foam 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.