Shear Behavior of Anthracite Weakened by Supercritical Carbon Dioxide

Understanding the shear behaviors of coal affected by supercritical carbon dioxide (ScCO2) treatment is crucial to ensuring safe carbon sequestration in deep coal seams. This study presents direct shear tests on cubic anthracite samples (100 x 100 x 100 mm) after ScCO2 treatment. Both continuous and cyclic ScCO2 treatments with varying soaking times of 7 and 14 days were considered. The results indicate that ScCO2 treatment leads to a decrease in the shear strength, shear stiffness, and shear dilation of coal. The shear strength and shear stiffness decreased significantly by 22 and 42% after 14 days of continuous treatment and by 29 and 34% after 14 days of cyclic treatment. With the increase of the soaking time from 7 to 14 days, the shear dilation of coal decreased and shear contraction became more pronounced, demonstrating significant shear post-peak plastic behavior. There is a strong correlation between shear stress and acoustic emission (AE) parameters during the shearing. The AE events of large energy mainly concentrated in the middle of coal, well indicating the occurrence of the shear bond. After ScCO2 treatment, the cumulative ring-down count increased, while the cumulative AE energy decreased. The brittleness index and AE energy growth rate can well describe the shear damage and brittle-plastic transformation of coal after ScCO2 treatment with a positive correlation between them. The ScCO2 treatment reduced the brittleness of coal, with a longer soaking time leading to lower brittleness. The study found that cyclic ScCO2 treatment had a more significant effect than continuous ScCO2 treatment in inducing stronger plasticity, which was well-correlated with the significant shear contraction of the coal.