Direct estimation of air–oil and oil–water capillary pressure and permeability relations from multi-step outflow experiments

Vadose zone description of multi-fluid flow requires measurement techniques capable of determining the capillary pressure and permeability functions of multi-fluid soil systems. Multi-step outflow experiments are presented that use a modified Tempe cell for air–water, oil (Soltrol)–water, and air–oil fluid pairs in Columbia fine sandy loam and a Lincoln sandy loam. In addition to measurements of cumulative outflow and wetting fluid pressure in outflow experiments of air displacing water or oil, pressure changes of the oil (non-wetting fluid) were measured in oil–water experiments. Results from the multi-step measurements were successfully used to directly estimate capillary pressure–saturation and wetting fluid permeability functions. The capillary pressure saturation data for each fluid pair were scaled using their interfacial tension values relative to that of air–water, thereby yielding a combined capillary pressure–saturation curve for the three measured fluid pairs. The combined relative permeability data for both soils nearly coalesce to a single curve, indicating that the relative permeability is solely a function of the porous medium properties. Furthermore, the directly estimated permeability data agreed well with permeability functions obtained using a parameter estimation approach, by which parameters were optimized from comparison of observed and numerically-simulated values of capillary pressure and cumulative outflow.