Analysis of oil recovery efficiency based on nuclear magnetic resonance in porous media under the action of electric field: insights from microstructure and pore scale analysis

Carbonate reservoirs, critical to global hydrocarbon resources, face development challenges due to heterogeneous multi-scale pore structures, resulting in severe water channeling and low recovery rates (<30 %) during conventional waterflooding. While direct current (DC) electric field-assisted oil displacement offers efficient, cost-effective, and eco-friendly potential, its microscopic mechanisms remain underexplored. This study combines multi-scale pore characterization and core flooding experiments to systematically evaluate electric field effects on multiphase flow in heterogeneous reservoirs, emphasizing pore-scale electro-osmosis–electrophoresis synergy. Analyses of a calcite-dominated carbonate formation (95 % calcite, inter/intragranular porosity, poor connectivity) using scanning electron microscope (SEM), thin-section petrography, mercury intrusion, and nuclear magnetic resonance spectroscopy (NMR) revealed limited conventional waterflooding performance (25 %–28 % recovery), primarily mobilizing macropores (>100 μm). Applying a 20V DC electric field increased recovery by 10.6 %, with an optimized “post-water-free electric drive” strategy adding 7.57 % incremental recovery. Even in long heterogeneous cores, a sustained 4.7 % recovery gain demonstrated field applicability. NMR confirmed enhanced oil mobilization in mesopores (10–100 μm), expanding accessible pore networks. Kinetic analysis identified dual mechanisms: Optimal displacement pressure gradients (0.5–0.6 MPa/cm) stabilized injection pressure, suppressed water channeling, and delayed water breakthrough; Intensified electro-osmosis promoted ion-directed migration, dynamically stabilizing pressure fields while reducing water consumption by ∼10 %. These processes synergistically improved displacement efficiency across pore scales. The study demonstrates DC electric fields effectively regulate multi-scale pore utilization and optimize seepage field distribution, providing mechanistic insights and engineering guidelines for carbonate reservoir development. By enhancing recovery efficiency while reducing water use and injection energy requirements, this approach demonstrates potential for low-carbon hydrocarbon recovery, supporting sustainable energy transitions.