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. (C) 1998 Elsevier Science B.V. All rights reserved.