Chemical Transport in Partially Saturated Rocks

We have conducted single- and two-phase pulse-tracer experiments in a Berea Sandstone core using the N2/water system. X-Ray Computed tomography (CT) and Positron Emission Tomography (PET) were used to obtain the three-dimensional distribution of the nonwetting phase saturation and to image the transport of a radiotracer within the sample, respectively. Experiments were carried out at two different levels of residual saturation, so as to assess the impact of water content on solute dispersal. We observe that the solute plume is largely influenced by the level of residual gas saturation and by its spatial distribution. The latter is determined by the distribution of the nonwetting phase prior to imbibition and, ultimately, by the hysteretic behavior of the capillary pressure curve. While previous observations have unequivocally demonstrated the role of capillary pressure heterogeneity in local saturation development during drainage, our results indicate that the same is true for the subsequent imbibition and trapping process. This generates a complex interplay between saturation distribution, permeability heterogeneity, and pore-water velocity, with potential implications on the achieved rate of dissolution of the injected CO2 in geologic carbon sequestration applications.