Theoretical estimation of free and entrapped nonwetting-wetting fluid interfacial areas in porous media

Fluid-fluid interfacial areas play important roles in numerous subsurface processes such as dissolution, volatilization, and adsorption. Integral expressions have been derived to estimate both entrapped (discontinuous) and free (continuous) nonwetting fluid-wetting fluid specific interfacial areas in porous media. The expressions, compatible with widely used capillary head-saturation and entrapment models, require information on capillary head-saturation relation parameters, porosity, and fluid-pair interfacial tension. In addition, information on the maximum entrapped nonwetting fluid saturation as well as the main drainage branch reversal point for water and total liquid saturations is necessary to estimate entrapped fluid interfacial areas. Implementation of the interfacial area equations in continuum-based multifluid flow simulators is straightforward since no additional parameters are needed than those required by the simulators to complete the multifluid flow computations. A limited sensitivity analysis, based on experimentally obtained parameter values, showed that imposed variations resulted in logical and consistent changes in predicted specific interfacial areas for both entrapped and free nonwetting fluid-wetting fluid systems. A direct comparison with published experimental work to test the derived expressions was limited to free air-water systems and yielded reasonable results. Such comparisons are often not possible because of the lack of information given on retention parameters, and variables used to determine nonwetting fluid entrapment. This contribution is dedicated to John W. Cary.