Mechanical property alterations across coal matrix due to water-CO₂ treatments: A micro-to-nano scale experimental study

CO₂ sequestration in geological coal formations is a promising pathway to store a huge amount of CO₂ due to a strong CO₂ affinity on coal. It has been reported that CO₂ sorption can cause a strength reduction of coals. However, the mechanism-based understandings of CO₂-coal interactions induced alterations are still challenging in micro-to-nano scale. We conducted the combined grid-based nanoindentation tests and FESEM-EDS analyses to understand and quantify the mechanical property variations and distributions of coals in micro- or nano-scale due to its mineral and structural variations. We also quantified the mechanical property alterations induced by gaseous CO₂, ScCO₂, and ScCO₂-water mixture treatments at different treating durations. The combined results from the FESEM-EDS and XRD confirmed that the heterogeneity of two tested coal is prevailing at micro-to-nano scale and the corresponding mechanical properties are composition- and microstructure-dependent. The mechanical properties of coals can be altered by gaseous CO₂, ScCO₂, and ScCO₂-water mixture treatments primarily due to the organic carbon-CO₂ interactions and water hydration effects in wetted coals, or potentially due to the dissolution and precipitation of minerals occurred in fluid environment typically under the ScCO₂-water treatment condition.