Solid–fluid affinity and its effects on hydraulic conductivity in natural porous media

We present the results of a comprehensive experimental study to investigate the variations in measured absolute permeabilities to incompressible fluids in sandstone and carbonate rock samples under varying surface affinity (i.e., hydrophilic, weakly hydrophobic, and hydrophobic) conditions. Surface affinity on rock pore walls was established by aging the core samples with carefully selected chemical solutions. The efficacy of the aging process was verified through contact angle measurements on rock substrates. Thereafter, coreflooding experiments were conducted, and the results revealed that the measured absolute permeability of a porous medium to a given fluid was directly influenced by the extent of rock affinity to that fluid. Specifically, the measured absolute permeability of the rock samples to a less-wetting fluid was consistently greater than that to the more-wetting fluid, with conductance enhancements ranging up to approximately 50%. Additionally, the results indicated that the impact of rock affinity to fluids on the measured hydraulic conductivity was more pronounced in cores with smaller pore sizes and more angular pore shapes. We observed an inverse logarithmic correlation between the hydraulic conductivity enhancement for the less-wetting fluid and the rock's absolute permeability to the more-wetting fluid. To further bridge the gap between classical slip models in simplified pore geometries and the more pronounced permeability variations observed in experiments, we performed pore-scale flow simulations in different pore structures. The simulation results showed that even a moderate slip length can considerably enhance the hydraulic conductivity of the actual pore element, owing to its intricate geometric and topological characteristics.