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The injection of CO2 into deep unrecoverable coal seams is one of the effective ways to achieve CO2 geological sequestration, but CO2 will be in a supercritical state under the action of high pressure and high temperature. In order to investigate the effect of supercritical CO2 action on coal reservoir structure, based on the self-developed supercritical CO2 immersion experimental system, combined with acoustic emission test system and RFPA3D numerical simulation, we studied the three coal seam thicknesses and three top and bottom lithologies of the mechanical damage characteristics and the fracture extension evolution of “Rock-Coal-Rock” (RCR) composite specimens under the action of supercritical CO2 were investigated. The results show that: ① After the action of supercritical CO2, the degradation of compressive strength and elastic modulus of the RCR composite gradually increases and decreases with the increase of coal thickness, while the degradation of compressive strength and elastic modulus are basically the same when the strength ratios of rock and coal are different and do not show large differences; ② The action of supercritical CO2 will promote the plastic damage of the coal body and intensify the transformation of the RCR composite from tensile splitting damage to shear plastic damage, and the degree of plastic damage of RCR assemblage is positively correlated with both coal thickness and rock-to-coal strength ratio; ③ The supercritical CO2 immersion promoted the RCR assemblage to enter the elastic deformation stage earlier, and the destabilization damage occurred after a more brief elastic deformation, the greater the coal thickness, the greater the influence, while the rock-to-coal strength ratio has less influence; ④ The instability potential of RCR assemblage is proportional to coal thickness and inversely proportional to rock-coal strength ratio, and the power intensity of damage is inversely proportional to coal thickness and proportional to rock-coal strength ratio; ⑤ The total energy, dissipative energy, elastic energy and surplus energy of RCR assemblage gradually decrease with the increase of coal thickness and gradually increase with the increase of rock-coal strength ratio, and the supercritical CO2 effect will cause the elastic energy ratio of RCR assemblage specimens to decrease, the dissipative energy ratio to increase and the surplus energy ratio to decreases. Combining the above research results shows that the thicker the coal seam is, the more likely it is to be destabilized, and the higher the strength of the top and bottom rock layer is, the less likely it is to be destabilized, and the dynamic strength of the seam destabilization is inversely proportional to the coal thickness and positively proportional to the rock-to-coal strength ratio. Therefore, in a certain area of stratum that meets the premise of CO2 injection and storage, the area of stratum with higher top and bottom rock strength and thinner coal seam thickness should be selected to store CO2 with higher safety. The research results can provide some theoretical reference for the safety of geological storage of CO2 injected into deep unmineable coal seams.

The mechanical properties of creep coal bodies under the influence of dynamic pressure have important engineering significance for the stability and control of remained coal pillars in the re-mining area. The strain characteristics, elastic modulus and creep rate evolution characteristics of creep process of anthracite coal were studied by carrying out the graded loading creep and loading-unloading tests under different loading rates. The energy dissipation law of creep process was characterized based on the linear energy storage law of anthracite coal, and the “hardening-damage” mechanism of creep of anthracite coal was further explained. The results show that when the loading rate is less than 0.04 mm/s, the axial strain of the anthracite specimen shows obvious transient characteristics, but the radial strain shows obvious hysteresis and creep. With the increase of the loading rate, the axial hardening and radial expansion of the specimen in the loading stage are significantly enhanced, and the creep rate decay characteristics appear more obvious. The actual yield stress of the specimen meet the linear decreasing relationship with the loading rate, and its energy storage coefficient and creep limit elastic energy show the trend of increasing and then stabilizing with the increase of loading rate. The evolution of energy dissipation rate of each stress level shows the trend of “decreasing-stabilizing-increasing”, which is corresponded to the compression-density, elasticity and yielding damage stages of the specimen, respectively. The hardening-damage mechanism of anthracite specimens was analyzed from the perspective of energy dissipation. The hardening and damage effects act on the whole creep process, and the hardening and damage effects are comparable in the elastic stage, resulting in a stable energy dissipation rate of the specimen, and the damage effect dominates after the specimen enters the yield damage stage, resulting in a rapid increase in the energy dissipation rate. The specimen hardening and damage effects gradually become significant with the increase of loading rate. When the loading rate is less than 0.04 mm/s, the specimen hardening effect is stronger than the damage effect, resulting in a rapid increase in the ultimate elastic energy of the specimen. The yield stress reduction caused by the increase of loading rate will accelerate the evolution of the damage of the specimen, when the loading rate is greater than 0.04 mm/s, the damage effect of the specimen is significantly enhanced, and the mutual constraints of hardening and damage lead the ultimate elastic energy of the specimen to be stabilized.

Remaining coal pillars in remining areas exhibit clear creep characteristics, and dynamic pressure accelerates their instability and failure. The creep and hardening characteristics of coal under dynamic pressure are of great engineering significance for the stability of remaining coal pillars in remining areas and their control. To investigate the creep and hardening characteristics of anthracite under static–dynamic coupled loading, graded loading creep tests with different loading rates were conducted. In this research, the creep strain, instantaneous elastic modulus, and creep rate of anthracite were studied under different graded loading rates. The results showed that the hardening effect of the samples manifested as an increasing instantaneous elastic modulus at the loading stage and a decreasing strain rate at the creep stage. When the graded loading rate increases from 0.01 to 0.1 mm/s, the instantaneous elastic modulus increases by 0.16–2.32 times. The sudden increase in the instantaneous elastic modulus at the failure stress level explains the instantaneous failure of the samples well. The actual yield levels corresponding to the peak instantaneous elastic modulus of the samples linearly decreased with increasing graded loading rate. The functional relationship between the graded loading rate and the elastic modulus hardening coefficient, the actual yield stress, and the strain rate decay coefficient were established, which could quantitatively describe the influence of different graded loading rates on the creep and hardening characteristics of anthracite and predict its creep damage.