Propagation of tracers through cores containing an immobile fluid phase

The analysis of a tracer experiment requires care when one of the fluid phases within the porous medium is immobile but miscible with the tracer carrier fluid. High levels of dispersion (Pe < 10) are commonly encountered in this situation. Applying the classical analytical expression for the tracer concentration in an infinite core is straightforward and convenient but introduces significant mass balance errors at low Peclet numbers. Theoretical effluent concentration histories from a finite core in the absence of interphase mass transfer span a well-defined and relatively limited area on a plot of concentration versus time. Comparison of experimental effluent histories with this family of curves provides a rapid test of whether mass transfer into the immobile phase is important. At high mass transfer rates the effluent history becomes insensitive to the volume fractions of the fluid phases. Thus it can be difficult to determine a unique set of fitting parameters. To avoid this difficulty, experiments should be conducted at high flow rates (low residence times) or with slowly diffusing tracers.