Diffusion–reaction studies in low permeability shale using X-ray radiography with cesium

An X-ray radiography method for the determination of diffusion coefficients in rock has been modified to provide an estimation of diffusion–reaction parameters for the sorbing tracer, cesium (Cs+). Cesium tracer diffusion and sorption on cation exchange sites can be monitored in intact rock samples using data extracted from radiographs to plot time-series relative-concentration profiles of Cs+ as a function of transport distance. Cesium was found to be a very good tracer for these experiments because of the sensitivity of the X-ray attenuation measurements to Cs+ concentration. ve-transport modeling coupled with parameter estimation software was used to match experimental data and estimate pore diffusion coefficients for Cs+ (Dp-Cs = 7.6 × 10−11 m2/s), single-site selectivity coefficients for Cs+ exchange (log KCs+/Na+ = 1.5) and cation exchange capacity (CEC = 8.4 meq/100 g) for drill core samples of Queenston Formation shale, from the Michigan Basin in Ontario, Canada.