Analysis of high-dispersion tracer tests in cores containing polymer gels

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. This situation gives rise to high levels of dispersion (Pe<10) and interphase mass transfer. We show that applying the classical analytical expression for the tracer concentration in an infinite one-dimensional core introduces significant mass balance errors at low Peclet numbers, even though a good fit to the experimental data may be obtained. Next, we present a rapid test of whether mass transfer into the immobile phase is important. The test is based on the observation that 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 vs. time. If an experimental effluent history does not lie within this area, a mass transfer model must be applied. Finally, we show that at high mass transfer rates, the effluent history becomes insensitive to the volume fractions of the miscible fluid phases. Thus, it can be difficult to determine a unique set of fitting parameters for experiments in this regime. To avoid this difficulty, experiments designed to determine miscible, immobile phase volumes should be conducted at high flow rates (low residence times) or with slowly diffusing tracers.