Analysis of gas differential diffusion through porous media using prompt gamma activation analysis

Abstract

Accurate estimates for the molecular transport coefficients are critical to predicting the movement of gases in geological media. Here I present a novel methodology for using prompt gamma activation analysis to measure the effective diffusivity of noble gases in a porous medium. I also present a model to estimate the connectivity parameter of a soil from measurements of its saturated conductivity, macro porosity, and pore volume and pore surface fractal dimensions. Experiments with argon or xenon diffusing through a nitrogen saturated geological media were conducted. The noble gas concentration variations at its source were measured using prompt gamma activation analysis and later compared to a numerical diffusion model to estimate the effective diffusion coefficient. Numerical simulations using the estimated diffusivity and the experimental argon data produced results with a correlation parameter R² = 0.98. However, neglecting transport mechanisms other than diffusion largely under-predicted the xenon depletion rates observed during the first hours of experiment. To explain these results, a second model was developed which included the effect of pressure gradients and bulk convection that might arise from the faster molecular migration of the light species in a non-equimolar system and gravitational currents. Finally, the fractal model developed for this dissertation was used to estimate the connectivity parameters and walking fractal dimension of a group of geological samples that were previously characterized. This model successfully predicted positive connectivity factors and walking fractal dimensions between two and three for every sample analyzed.