An analysis of solute diffusion in rocks

Abstract

The diffusion of unreactive solutes through a fine-grained rock (Culebra Dolomite) was studied experimentally and theoretically. The measured diffusive flux is less than half that predicted from independent knowledge of the porosity and reasonable estimates of tortuosity. This low measured flux led to a review of the relationship between solute diffusion and pore geometry in rocks and sediments. We therefore examined solute transport in hypothetical pore networks where the effect of pore geometry on the solute flux is directly calculable. A conventional interpretation of pore tortuosity, as a normalized length of diffusion through a pore, must be expanded for cases where pores intersect in networks. Some important variables affecting the tortuosity are: (i) the distribution of pore sizes, (ii) the distribution of pore lengths, (iii) the number of pores which intersect at a node, and (iv) the pore shape between nodes. Furthermore, in porous materials with a preferential distribution of pore sizes and orientation, tortuosity is a tensor. For the Culebra Dolomite, the wide range of pore sizes causes the diffusive flux to vary considerably from that predicted from conventional theory. Solute diffusion through the Culebra Dolomite greatly resembles diffusion through an isotropic network of pores, once this network is assigned pore sizes similar to the rock.