Influence of Treating Temperature on Mechanical Properties of Coal under CO₂–H₂O–Coal Interactions

Grasping the influence of treating temperature on the evolution of the mechanical properties of coal under CO2–H2O–coal interactions is of great significance for evaluating reservoir stability during hot flue gas sequestration. However, the degradation mechanism of coal strength caused by CO2–H2O–coal interactions under the competitive effect of mechanical and nonmechanical degradation has rarely been mentioned. Therefore, uniaxial loading, triaxial loading, and Brazilian splitting tests on specimens of different treating temperatures coupling acoustic emission (AE) and digital image correlation (DIC) have been conducted. The experimental results indicate that the competitive effect of mechanical and nonmechanical degradation induces the failure mode of the specimens to gradually change from rapid-sudden tensile failure to progressive-slow shear failure as the treatment temperature increases, and the macroscopic failure mode of the specimens becomes more complex. In addition, SEM-EDS tests revealed that the chemical dissolution effect is significantly affected by the treatment temperature. The chemical dissolution effect is greatly weakened, as the treatment temperature is relatively high. Meanwhile, the thermal swelling of the coal matrix can generate mass fractures with larger apertures, resulting in a significant reduction in the mechanical strength of coal. The research can provide a theoretical reference for reservoir stability evaluation during the geological sequestration and gas production enhancement of hot flue gas.