Determination of Dispersion and Diffusion of Miscible Liquids in Porous Media by a Frequency Response Method

A new method of measuring molecular diffusion and axial dispersion during miscible displacement in porous media is described. Equal density and viscosity liquids are injected into the porous medium at a constant volumetric rate with a sinusoidal concentration of fixed frequency and amplitude at the inlet. Amplitude attenuation and phase changes are measured in situ along the direction of flow; dispersion is calculated from these frequency response measurements. Molecular diffusion is determined by stopping the flow and recording the transient decay of the concentration sine waves. Measurements were made in a consolidated sandstone core using mixtures of isopropyl alcohol and deobase. The in situ concentrations were determined at various positions along the core from changes in dielectric constant. Dispersion coefficients calculated from the standard dispersion equation were found to depend on wavelength, contrary to the basic postulate that the dispersion is frequency independent. This indicates that the standard dispersion equation is inadequate since, at a given injection rate, the dispersion coefficient will depend upon slug size. (22 refs.)