Effective permeability to water in a petroleum column was estimated from routinely-measured mercury injection capillary pressure data of reservoir rocks, by applying a well-known reservoir engineering formula. The gradient of hydraulic potential across the column for a simplified petroleum trap model was then calculated to assess the realistic magnitude of hydrodynamic effects on capillary seal capacity under abnormally pressured conditions. The estimated effective permeability to water in the column ranges from 10–2 to 10–3 mD, even at several hundreds of metres above the free water level where it must become minimal. Since this figure is significantly larger than permeability of typical seals, it results in a negligible gradient of hydraulic potential in the column. The calculation, though including inherent uncertainties, indicates that hydrodynamics play a minor role on capillary sealing of typical petroleum traps, providing the reservoirs are water-wet, their tortuosity does not change with water saturation, and their anisotropy is not significantly large.