IFT–Wettability Interplay and Force-Regime Transition in Chemical EOR: A Mechanistic SI/FI Diagnostic Approach

Despite the crucial role of enhanced oil recovery (EOR) in meeting rising energy demands, interpretation of chemical EOR (CEOR) performance is limited by reliance on interfacial tension (IFT) and contact angle (CA) data. Most studies correlate IFT and CA with core flooding results, masking the contribution of capillary and gravity forces. The study introduces a force-isolation workflow that distinguishes capillary-, viscous-, and gravity-governed recovery by combining spontaneous imbibition (SI) and forced imbibition (FI) experiments conducted on clay-rich Bandera Gray sandstone cores. The key novelty is a modified Amott cell that applies controlled hydraulic pressure heads (10–30 cm) during the FI experiment, which allows measuring the lowest pressure to trigger oil displacement in every fluid system. Four injection systems—high-salinity (HS) brine, low-salinity (LS) brine, an anionic surfactant (sodium lauryl sulfate, SLS), and a cationic surfactant (cetyltrimethylammonium bromide, CTAB)—are selected to induce interfacial conditions. SLS generates high capillary pressure and a near-zero contact angle, promoting capillary-assisted viscous FI at the lowest pressure threshold, achieving 41% SI recovery and an SI/FI ratio of ~80%. By contrast, CTAB produces a low IFT but insufficient capillary pressure, resulting in gravity-driven SI with 5% recovery and viscous-dominated FI, with an SI/FI ratio of ~9%. The brine systems exhibit viscous-assisted capillary recovery, requiring the highest entry pressure. The SI/FI ratio classifies the force regimes, confirming that capillary support governs mobility control under SI and FI conditions. This force-regime approach provides a more reliable basis for CEOR formulation design than IFT and CA screening alone.