Mechanical damage evolution characteristics and mechanism of anthracite treated with supercritical CO₂

The effect of supercritical CO₂ will lead to the change of coal microstructure, which will lead to the damage of coal structure, thus changing the mechanical properties of coal.In order to quantitatively characterize the damage evolution law of mechanical properties of coal under the action of supercritical CO₂, a supercritical CO₂ immersion test of anthracite under dry and saturated water contents was carried out to determine the evolution law of mechanical parameters of anthracite under the action of supercritical CO₂, combined with acoustic emission test and scanning electron microscope test.The damage model and mechanism of anthracite under supercritical CO₂ were clarified.The results show that:① Supercritical CO₂ has obvious damage and deterioration effect on the compressive strength, elastic modulus and Poisson’s ratio of coal.The compressive strength and elastic modulus all decrease to varying degrees, and Poisson’s ratio shows an increasing trend.Moisture is also an important factor affecting the mechanical properties of coal.Compared with dry coal samples, the degradation effect of saturated coal samples under supercritical CO₂ is more obvious.The total degradation degree and stage degradation degree of coal samples under different test conditions show a certain time effect and non-uniformity.With the increase of soaking time, the coal damage degree gradually decreases and finally tends to a certain value.② The cumulative AE count of coal decreases to varying degrees with the increase of soaking time.The four stages of the cumulative AE curve are in good agreement with the four stages of the stress-strain curve.③ Based on the damage model of coal under different soaking times of supercritical CO₂, it can be seen from the model that the change trend of elastic modulus is negatively correlated with the change trend of Poisson’s ratio, which is consistent with the sample results.With the increase of soaking time, the damage of supercritical CO₂ to coal samples increases continuously, but the increase rate decreases gradually.④ Water+supercritical CO₂ will have a series of physical and chemical reactions with the coal sample, which will change the internal microstructure of the coal sample and increase the number of micro-cracks in the coal sample, resulting in the deterioration of the mechanical properties of the coal.The damage evolution effect of supercritical CO₂ on coal is a cumulative process from micro to macro.