Study on Desorption and Diffusion Characteristics of CO2 in Cracked Coal

Abstract

Gas migration within coal matrices is driven by concentration gradients, fundamentally representing gas diffusion and mass transfer within porous frameworks. During this process, gas molecules adsorbed on coal surfaces desorb and primarily reside in the micro-pores (<2 nm) and mesopores (2-50 nm) within the coal structure. Due to the extreme pore dimensions, gas movement is predominantly governed by intermolecular collisions and interactions with pore walls, where diffusion mechanisms dominate. To investigate the impact of CO2 phase transition-induced fracturing shock waves on gas desorption and diffusion behavior in coal samples, this study selected three coal samples: Zhangcun (ZC), Chengzhuang (CZ), and Jiulishan (JLS). We designed and conducted isothermal natural desorption experiments for gas before and after the shock wave impact. Experimental results demonstrate a positive correlation between cumulative gas desorption capacity and initial adsorption equilibrium pressure in coal samples, indicating that the desorption capacity increases with rising initial adsorption equilibrium pressure. When initial adsorption equilibrium pressure is identical, the desorption curves of impact samples consistently outperform those of original samples. Under equivalent initial adsorption equilibrium pressure, coal samples subjected to CO₂ phase transition-induced cracking exhibit significantly higher initial gas desorption rates compared to their original counterparts, with post-impact desorption curves surpassing the original curves. For identical samples, higher initial adsorption equilibrium pressure correlates with greater early-stage gas desorption rates. This establishes a clear positive correlation between initial gas desorption rates and initial adsorption equilibrium pressure in coal samples.