Pore merging and flow focusing: Comparative study of undissolved and karstified limestone based on microtomography

Abstract

We develop methods for qualitative and quantitative assessment of pore geometry transformation within a rock as a result of karstification. We then apply these tools to characterize dissolution-induced changes in limestone samples collected from a quarry in Smerdyna (Poland), where intense epikarst development is observed, consisting of a large number of solution pipes. Partially dissolved samples collected in the immediate vicinity of the pipes were compared with undissolved samples collected 1.5 meters away. For both types of samples 26 micron resolution grayscale X-ray scans were performed. The irregular geometry of the pore space, the vast majority of which forms a single connected component, can be conveniently characterized by a local thickness function that corresponds to the diameter of the largest sphere that fits within the pore space and contains a given point. We compare the local thickness distributions of the undissolved and dissolved samples as well as a numerically generated uniformly dissolved sample. Such a comparison allows us to quantify the extent of homogeneity of the natural karstification process. The above analysis is complemented by the calculation of connectivity of the pore space as well as their flow characteristics. All of the results consistently indicate an important role of pore merging and inhomogeneous dissolution in the process of natural dissolution for the analyzed samples.