TIME FOR A NEW DIMENSION IN Pc-S SPACE

TUCK, David M., david.tuck@srs.gov, Savannah River Technology Center, Westinghouse Savannah River Company, Aiken, SC 29808

Surface chemistry researchers have long known that surface and interfacial tensions are time-dependent for complex fluids, especially when they contain minor surface active components. NAPLs are nearly always complex solutions, even in simplified laboratory experiments where dyes are used to aid visualizing fluid distributions (Tuck et al. 1996; Tuck et al. 1998). Schroth et al. (1995) suggested that interfacial tension time-dependence might effect capillary pressure - saturation relationships. Tuck et al. (1998) proposed a mechanism for this effect. My objective is to examine the effects of that mechanism.

The proposed mechanism is based on an analogy between pore-scale advancement of fluid-fluid interfaces through a pore body and the dynamic drop-volume method for interfacial tension measurement. The time-dependent nature of interfacial tension arises because the interface is expanding. Expansion requires bringing fluid from the interior of the advancing phase into the fluid-fluid interface. Randomly oriented surface-active molecules (surfactants) in the bulk advancing phase cannot diffuse to and adsorb into the interface instantly. Diffusion and adsorption cause the surface concentration of surfactants to increase with time. Interfacial tension will thus be time-dependent because the surface concentration of surfactants, i.e., the cause of interfacial tension reduction, is time dependent.

The mechanism was tested using a simple two-dimensional cubic pore-scale model consisting of a total of 49 pores. The results indicate that fluid saturation is a complex function of "hydrostatic" capillary pressure and time. Time dependence enters through the "surface chemistry" portion of capillary pressure, where interfacial tension and contact angle become functions of time. Time-dependent behavior of interfacial tension and contact angle was assumed to be that of a 0.5 g/L Sudan IVdyed-tetrachloroethylene- water-glass system (Tuck et al. 1998). DNAPL penetration occurred at hydrostatic pressure differences below the capillary entry pressure. The saturation surface assumes roughly a partial cylindrical shape with the line of minimum saturation occurring at the "hydrostatic" capillary entry pressure. At that pressure both the hydrostatic and the surface chemical components of capillary pressure play approximately equal roles. At lower hydrostatic pressure differences the surface chemical kinetics dominate the system, while at higher differences the hydrostatic pressure dominates.