Direct observation of coupled geochemical and geomechanical impacts on chalk microstructural evolution under elevated $\mathrm{CO_{2}}$ pressure. Part I

Abstract

The dissolution of porous media in a geologic formation induced by the injection of massive amounts of $\mathrm{CO_{2}}$ can undermine the mechanical stability of the formation structure before carbon mineralization takes place. The geomechanical impact of geologic carbon storage is therefore closely related to the structural sustainability of the chosen reservoir as well as the probability of buoyancy driven $\mathrm{CO_{2}}$ leakage through caprocks. Here we show, with a combination of ex situ nanotomography and in situ microtomography, that the presence of dissolved $\mathrm{CO_{2}}$ in water produces a homogeneous dissolution pattern in natural chalk microstructure. This pattern stems from a greater apparent solubility of chalk and therefore a greater reactive subvolume in a sample. When a porous medium dissolves homogeneously in an imposed flow field, three geomechanical effects were observed: material compaction, fracturing and grain relocation. These phenomena demonstrated distinct feedbacks to the migration of the dissolution front and severely complicated the infiltration instability problem. We conclude that the presence of dissolved $\mathrm{CO_{2}}$ makes the dissolution front less susceptible to spatial and temporal perturbations in the strongly coupled geochemical and geomechanical processes