WATER-MOVEMENT IN UNSATURATED POROUS-MEDIA DUE TO PORE-SIZE AND SURFACE-TENSION INDUCED CAPILLARY-PRESSURE GRADIENTS

Sharp capillary pressure gradients were generated at the interface of two sections of a horizontal column filled with wet but ''unsaturated' ' particulate material (glass beads or washed sea sand). The gradients were generated by manipulating the capillary pore size (by changing t he particle size) and/or surface tension of the wetting fluid (by addi tion of water-insoluble surfactants) in the two sections. Movement of the wetting fluid caused by these gradients was determined. Capillary pressure analysis was used to explain why certain water-insoluble surf actants displaced large amounts of water from the surfactant-containin g section into the surfactant-free section of the system. Capillary pr essure versus water content curves during desorption and sorption of w ater were developed. Hysteresis effects were found to be critical for modeling of water movement. Water movement results reported from our l aboratory and elsewhere were evaluated. The criteria for surfactant ef fectiveness (water insolubility, high equilibrium spreading pressure, and the ability to form a ''condensed solid film'') developed previous ly were explained. The equilibrium water content after flow, the water sorption branch of the capillary pressure curve, and the maximum wate r movement achievable through these methods could all be predicted thr ough understanding of the action of differential capillary pressures.