Experimental evidence for reaction-induced fracturing during CO2 mineralisation of basalt

Recent laboratory and field studies have indicated that in-situ mineralisation of CO₂ within basalt formations offers the potential for secure storage of large volumes of anthropogenic CO₂. Here, we present the results of x-ray microtomographic imaging and fluid chemistry of a long-term operando experiment simulating the geological reservoir environment of engineered CO₂ mineralisation.

We induced CO₂ mineralisation within a mm-sized core of picrite at 170°C, 1.65 MPa fluid pressure and constant fluid flow conditions. The CO₂ mineralisation reaction is documented in a time-resolved dataset of 3-dimensional x-ray microtomography images. We have determined the chemical, physical, and mineralogical changes in the sample over the course of the experiment through produced fluid chemistry and post-mortem analyses.

Our results document the development of an interlinked network of new fracture porosity which permeates the entire rock volume. We find that a bulk porosity increase of up to 2% has occurred, providing the first quantification of fracture generation during engineered CO₂ mineralisation in a laboratory under realistic reservoir conditions.

Our data show that the generated pore space is frequently infilled with carbonate minerals. We find that magnesite is the dominant precipitated phase, with dolomite and a range of Ca-Mg carbonates also being observed, totalling up to 1.56 vol% of the sample. We also find evidence for the formation of oxyhydroxides and clays, but no indication of serpentinization.

Our results imply that a self-sustaining coupled chemical-mechanical-hydraulic process is occurring due to the formation of a reactive surface area in the picrite sample during CO₂ mineralisation. The quantification of this process, which our results provide, will be useful for the accurate forward modelling of reservoir capacities, particularly those with limited permeability or volumetric extent.