Surfactant-Enhanced interlayer mass transfer during CO2 flooding in heterogeneous Reservoirs: Mechanisms and influencing factors

Abstract

Surfactants can enhance CO₂ flooding efficiency by reducing the minimum miscible pressure (MMP). However, in heterogeneous reservoirs, where CO₂ preferentially channels through the high-permeability layer (HPL), the mobilization characteristics and EOR mechanisms of surfactant-assisted recovery in such reserviors remain unclear. In this study, dual-layer cores with the permeability ratio (PR) were constructed. Real-time nuclear magnetic resonance (NMR) was used monitoring of oil migration, aiming to elucidate the mobilization characteristics and the mechanisms by which surfactants enhance sweep efficiency. Results showed that surfactant addition significantly improved oil recovery, particularly in the the low-permeability layer (LPL), where mobilization degree increased by 13.1%, 1.5 times higher than that of the HPL. Static CO₂ contact experiments further revealed that surfactants enhanced the diffusion of LPL oil into CO₂, thereby promoting interlayer mass transfer and improving LPL’s oil mobilization. Moreover, above the minimum miscible pressure, the increment of the LPL mobilization degree after surfactant addition increased steadily with pressure, indicating that increased pressure benefits the surfactant-enhanced diffusion between CO₂ and oil. Compared to continuous injection, intermittent injection further enhanced LPL’s mobilization degree. It provided longer CO₂–oil contact time, facilitated surfactant accumulation at the interface, thereby strengthening interlayer mass transfer dominated by diffusion. By clarifying the mechanism of surfactant-enhanced interlayer mass transfer and the influence patterns of different factors, the application potential of surfactants in CO₂ flooding for heterogeneous tight reservoirs can be enhanced.