Study on evolution law of particle breakage after coal adsorption of CO₂ under different uniaxial compression stress conditions

After the completion of coal seam mining, a rich fracture space is formed underground, in which there is one or more layers of residual coal, and may even include a complete unminable coal seam. This provides a new possibility for the physical storage of CO₂ ( including free and adsorbed CO₂ ) in coal mine goaf. In the process of CO₂ storage in coal mine goaf, the crushing and compaction characteristics of coal particles will change under different stress distribution conditions, which may cause permeability changes and microseismic activities in goaf reservoirs, and seriously lead to CO₂ leakage, endangering the safety of CO₂ storage. The effects of different stress conditions on permeability were revealed by comparing the particle size gradation characteristics, crushing characteristics and compaction characteristics of coal samples with and without CO₂ adsorption under different axial stress conditions. The results show that the adsorption of CO₂ has a significant effect on the particle size distribution of coal particles, and the particle size distribution of coal samples after adsorption of CO₂ is in good agreement with the Mandelbrot fractal distribution. The adsorption of CO₂ increases the crushing tendency of coal, especially when the axial pressure is small, the relative crushing rate and absolute crushing rate gradually increase with the increase of axial pressure. For the compaction process and porosity, CO₂ adsorption has a significant effect when the axial pressure is small, but this effect weakens with the increase of axial pressure. Combined with the experimental results and previous studies, the calculation method of permeability of coal and rock mass in goaf after CO₂ adsorption is proposed. The results show that the broken coal becomes more dense after CO₂ adsorption, and the adsorption influence coefficient changes nonlinearly with the increase of stress. The research results provide an important theoretical reference for the safety evaluation of CO₂ storage in coal mine goaf and the prevention and control of leakage after CO₂ injection in goaf.