Direct Observation of Coupled Geochemical and Geomechanical Impacts on Chalk Microstructure Evolution under Elevated CO2 Pressure

Abstract

Dissolution in natural porous media by injected CO2 can undermine the mechanical stability of the formation before carbon mineralization can take place. The geomechanical impact of geologic carbon storage therefore affects the structural integrity of the formation. Here, using in situ X-ray imaging, we show the coupled geochemical and geomechanical processes in natural chalk in the presence of aqueous CO2. We first measured the chalk dissolution rate in a closed, free drift system and obtained a phenomenological correlation between the rate and evolving aqueous calcium concentration. We then used this rate correlation in a segregated flow model to estimate the visual pattern of chalk microstructure dissolution. The model predicted a homogeneous pattern, which resulted from an increase in the reactive subvolume. This prediction was validated using in situ X-ray tomography. The imaging technique further revealed three typical mechanical impacts during microstructure disintegration in an imposed flow field: material compaction, fracturing, and grain relocation. These impacts differ but are strongly coupled with CO2-induced geochemical reactions and provide different types of feedback to the dissolution front migration. These observations led us to conclude that the presence of dissolved CO2 makes the migration of reactive fluid less sensitive to perturbations in the coupled geochemical and geomechanical processes.