Dispersion and Tortuosity in Sandstones

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers Inc. Abstract Linear dispersion and mass transfer coefficients were measured for Bandera, Berea, Cottage Grove, Noxie and Torpedo sandstones ranging in permeability from 12 to 420 md. Experimental data were permeability from 12 to 420 md. Experimental data were obtained from analysis of effluent concentration changes resulting from a step increase of the influent tracer concentration. Brine and a tritium-labeled brine solution were used as tracers. Coefficients of dispersion were calculated by comparing the experimental data with solutions of the diffusion equation by three methods:analytical solution of the diffusivity equation,a graphical method, andnumerical, finite difference, solution. The three methods yielded equivalent results. Tailing of the effluent response curves was interpreted by introducing two additional parameters to the diffusivity equation: (1) the immobile parameters to the diffusivity equation:the immobile fluid fraction of the porous medium determined independently by capillary pressure measurement anda mass transfer coefficient to account for tracer interaction with the immobile fluid fraction. The Berea sandstone exhibited the sharpest displacement front and the least amount of tailing, making it the best selection for experiments in which steep displacement fronts are encountered, such as miscible displacement. Molecular diffusion measurements were used to calculate tortuosity factors for each of the five sandstones using two pairs of gases: hydrogenair and hydrogen-nitrogen. These results are equivalent to tortuosities independently determined by electrical conductivity measurements. The coefficient of dispersion and the tortuosity of sandstones are shown to be excellent parameters for characterization of geologic materials for the study of fluid flow properties. Introduction Knowledge of the flow patterns of solutes in flowing streams through porous geologic materials is important in applying the technology for miscible phase displacement for recovery of petroleum and injection of mobility- and wettability-control agents. Kaufman and Orlob also emphasized the use of tracers for analysis of saline water intrusion into fresh water aquifers and evaluation of proposals for the underground storage of radioactive materials.