Direct Observation of Coupled Geochemical and Geomechanical Impacts on Chalk Microstructure Evolution under Elevated CO₂Pressure

Abstract

The dissolution of porous media in a geologic formation induced by the injection of massive amounts of 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 buoyance driven CO 2 leakage through caprocks. Here we show, with a combination of ex situ nanotomography and in situ microtomography, that the presence of dissolved 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 CO 2 makes the dissolution front less susceptible to spatial and temporal perturbations in the strongly coupled geochemical and geomechanical processes. Graphical abstract

Retraction of the dissolution front in natural porous media

“…24 When a flow field is imposed the model allows relating the spatial distribution of voxel porosity with the reactor properties. 2…”

Impact of hydro-chemical conditions on structural and hydro-mechanical properties of chalk samples during dissolution experiments

“…It also results in unstable structures that are difficult to reproduce experimentally and thus not well studied. A recent study has investigated the different dissolution patterns occurring inside chalk by injecting CO 2 at elevated pressure using in situ X-ray imaging (Yang et al, 2018). They demonstrate the high sensibility of chalk to reactive fluid compared to coupled geochemical and geomechanical processes, and show that the microstructure is highly affected by material compaction, fracturing and grain relocation.…”

Patterns of entropy production in dissolving natural porous media with flowing fluid

“…We used a previously published static tomogram of Hod chalk (HC16) from the North Sea Basin, collected with the X-ray holotomography setup at the ID22 beamline at the European Synchrotron Research Facility (ESRF), in Grenoble, France [ 31 ]. This data set offers high spatial resolution and has been shown to be most amenable to the image processing algorithms developed by our team in a few related studies [ 32 – 37 ]. The data were reconstructed from 1999 radiographs (360° rotation, 0.5 s exposure) at 100 nm voxel resolution using the holotomography reconstruction method [ 23 , 31 ] and processed as described in Bruns et al [ 35 , 36 ].…”

“…We adopted the model framework of Yang et al . [ 32 – 34 , 37 ]. The mathematical scheme is designed to treat the assemblage of greyscale voxels as a network of ideal chemical reactors.…”