Experimental study on the influence of coal-based CO₂ geological storage on roof fracture conductivity

Coal-based CO₂ geological storage is an important way to reduce greenhouse gas emissions, but there is also a safety risk of underground CO₂ leakage. In order to evaluate the safety of coal-based CO₂ geological storage, the roof samples of No.3 coal seam in Hudi Coal Mine, southern Qinshui Basin were collected, and the changes of artificial fracture morphology, whole rock mineral composition and CO₂ conductivity of columnar samples in "CO₂-H₂O-Rock" reaction were studied by simulation experiments. The results show that calcite vein dissolution, secondary mineral filling and external effective stress jointly affect the fracture conductivity. For the low permeability samples with original permeability of 0.016×10^–3μm², calcite vein dissolution leads to the increase of permeability in the early stage of the experiment. As the reaction proceeds, the fracture is closed under the guidance of effective stress, and the permeability increases first and then decreases. For the high permeability samples with original permeability of 3.785×10^–3μm², H₂CO₃ continuously dissolves feldspar and other minerals on the fracture wall, producing kaolinite and other secondary minerals to fill the fracture, so that the permeability continuous to decrease. In the process of CO₂ geological storage, high injection pressure leads to artificial roof fracture. After the completion of CO₂ injection construction, the fracture conductivity decreases rapidly due to the filling of secondary minerals and the increase of effective stress, therefore, the risk of long-term leakage of CO₂ in coal along the roof fracture is relatively low.