Direct observation of coupled geochemical and geomechanical impacts on chalk microstructural evolution under elevated CO2 pressure. Part I

TL;DR

This study combines advanced imaging techniques to observe how dissolved CO2 affects chalk's microstructure and mechanical stability, revealing homogeneous dissolution and complex geomechanical responses that influence CO2 storage safety.

Contribution

It provides the first direct observation of coupled geochemical and geomechanical effects on chalk microstructure under elevated CO2 pressure using nanotomography and microtomography.

Findings

Dissolved CO2 causes homogeneous chalk dissolution.

Dissolution induces compaction, fracturing, and grain movement.

Dissolution front becomes less sensitive to perturbations.

Abstract

The dissolution of porous media in a geologic formation induced by the injection of massive amounts of CO2 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 CO2 leakage through caprocks. Here we show, with a combination of ex situ nanotomography and in situ microtomography, that the presence of dissolved CO2 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 CO2 makes the dissolution front less susceptible to spatial and temporal perturbations in the strongly coupled geochemical and geomechanical processes.