A pore network model study of the fluid‐fluid interfacial areas measured by dynamic‐interface tracer depletion and miscible displacement water phase advective tracer methods

Abstract

Fluid‐fluid interfacial areas in porous media are of considerable interest due to the impact they have on a wide range of practical applications involving mass transfer between phases, as well as for their importance in understanding unsaturated and multiphase flow behaviors in porous media. Tracer methods provide a low‐cost experimental approach for determining interfacial areas in porous media. Although a number of different tracer methods have been developed, uncertainty remains as to exactly what areas they measure. The work presented here uses pore network model simulations to study the behavior of tracers during simulated tracer measurements for two different specific water‐phase tracer methods: the dynamic‐interface tracer depletion method and the miscible displacement tracer method. The hypothesis driving this work was that different tracer methods likely measure different areas as a result of the very different ways tracers are used. Experimental data sets for six different porous media were used to validate the model and provide comparison with model‐simulated tracer‐based area measurements. Results of the work suggest that areas measured using the dynamic‐interface tracer depletion method closely match total fluid‐fluid interfacial areas, as long as the extent of tracer depletion during the method is relatively small. However, areas measured with the miscible displacement method likely fall somewhere between capillary and total fluid‐fluid areas. Calculations conducted with realistic diffusion coefficients and film thicknesses indicate that diffusion and head‐driven flow in films are insufficient to allow significant tracer access to film area, suggesting the potential importance of other mechanisms. Calculations conducted as a part of the work suggest that Leverett estimates of maximum area formed during drainage may be closer to total areas than previously reported.