Physical Property Responses of Shale Matrix and Its Surrounding Rocks to CO₂/Oxy-Coal Combustion Flue Gas Exposure: Implications for Fluid Storage Stability Assessment

To preliminarily validate the storage stability of CO₂, SO₂, and NO<i>_<i>x</i></i> within gas-bearing shale reservoirs, one representative shale and its surrounding rocks were exposed to CO₂/flue gas at 30 MPa and 353.15 K for 60 days, and changes in their physical properties were investigated. Results indicated that the CO₂/flue gas–H₂O exposure raised sample complexity via mineral dissolution and precipitation. Mineral dissolution increased micropores of the roof rock, while precipitation decreased micropores of the shale and floor rock and mesopores of all samples. Thereinto, the flue gas–H₂O exposure demonstrated higher influential degrees than those of the CO₂–H₂O exposure. Furthermore, the CO₂–H₂O exposure enlarged macropores of the surrounding rocks but narrowed those of the shale. The flue gas–H₂O exposure oppositely affects macropores as the does, which is more conducive for fluid storage. Although the CO₂–H₂O and flue gas–H₂O exposures transformed small-scale pores into medium- and large-scale pores/fractures, they reduced sample permeability by 62.51–86.43% and 65.47–90.21%, respectively. Such phenomena suggested stronger flue gas–H₂O–shale interactions and a better sealing capability of the surrounding rocks for flue gas than for CO₂. Overall, flue gas can react with shale/surrounding rocks more intensively than CO₂, making shale reservoirs a promising geologic formation for the stable storage of CO₂, SO₂, and NO<i>_<i>x</i></i>.