Influence of viscous, gravitational, and capillary forces on DNAPL saturation

Four dense nonaqueous phase liquids (DNAPLs)-bromoform, chlorobenzene, tetrachloroethylene, and trichloroethylene-were used to investigate the influence of viscous, gravitational, and capillary forces on DNAPL saturation in a natural aquifer sand. The relative magnitudes of these forces are expressed in terms of two dimensionless groups, the Capillary Number (N-Ca), defined as the ratio of the viscous force to capillary force, and the Bond Number (N-Bo), defined as the ratio of the gravitational force to capillary force. Nondimensionalization of the equations governing two-phase flow suggests that DNAPL saturation should be a function of a linear combination of the Capillary and Bond Numbers (N-Ca/k(IV) - N-Bo), provided the permeability to water (k(IV)) in the presence of discontinuous DNAPL is considered. Experimental studies in which DNAPL saturations were measured over a range of Capillary and Bond Numbers for upward, horizontal, and downward displacement of DNAPL by water corroborate the results of the nondimensionlization. DNAPL saturations generally decreased with increasing Capillary Number and with decreasing Bond Number until N-Ca/k(IV) - N-Bo was greater than approximately 1 x 10(-5), at which point residual saturation was attained. For the DNAPLs used in this study, with adhesion tensions on the order of 26 dynes/cm and Bond Numbers ranging from 1.3 x 10(-7) to 2.4 x 10(-6), residual saturation was attained at Capillary Numbers greater than approximately 5 x 10(-5). These results provide a means of estimating the system conditions under which the DNAPLs studied achieve residual saturation in aquifer material.