Mechanistic investigation of electrokinetically-controlled enhanced oil recovery in sandstone and carbonate reservoirs using a tailored cationic gemini surfactant

Although cationic Gemini surfactants show promise for chemical enhanced oil recovery (cEOR), the coupled influences of brine salinity and reservoir lithology on their performance remain poorly understood. This study addresses this gap by synthesizing and systematically evaluating a hexadecyl-based cationic Gemini surfactant (GEM 16-2 ) in sandstone and carbonate systems under varying salinity and temperature conditions. Successful synthesis was verified via FTIR and NMR, while TGA/DTG indicated thermal stability up to 220 °C. GEM 16-2 formed nanoscale aggregates (≈ 9.31 nm hydrodynamic diameter) and exhibited excellent halotolerance, remaining phase-stable in NaCl brines up to 22.5 wt% at 45 °C. At a concentration of 25 PPM (near its critical micelle concentration), GEM 16-2 reduced oil–water interfacial tension (IFT)to 0.85 mN/m. The IFT-salinity relationship followed a U-shaped trend with a minimum at ⁓4500 ppm Na 2 SO 4 , whereas rising temperature monotonically decreased IFT up to 70 °C. Thermodynamic profiling confirmed spontaneous micellization and adsorption, the latter being energetically favored. Zeta potential, adsorption isotherms, and contact angle dynamics revealed that lithology-dependent performance is driven by electrostatic interactions. Specifically, adding 2000 ppm Na 2 SO 4 screened electrostatic attraction in sandstone, reducing surfactant adsorption from 0.75 to 0.64 mg/g. Conversely, in carbonate, sulfate ions shifted the surface charge negatively, promoting surfactant uptake from 0.11 to 0.53 mg/g. Consequently, water contact angles decreased to 43 o (sandstone) and 65 o (carbonate), altering wettability. Surfactant performance in porous media was found to be lithology-dependent; specifically, ultimate spontaneous imbibition recoveries reached 24.74% and 20.85% OOIP at ≈ 2000 ppm Na₂SO₄ for sandstone and ≈ 4000 ppm Na₂SO₄ for carbonate, respectively, corresponding to improvements of 9.44 and 9.05% points over seawater. These findings demonstrate that optimal salinity for cationic Gemini-mediated cEOR is governed by lithology-specific surface-charge modulation rather than IFT minimization alone.