Experimental investigation of internal gas distribution characteristics in loaded water-bearing coal during N2 displacement

To investigate the impact of residual moisture in coal bodies on gas distribution during gas injection following hydraulic permeability enhancement measures, N 2 displacement experiments were conducted using large-sized water-bearing coal samples. This revealed the evolution characteristics of gas pressure and CO 2 concentration within coal bodies under varying moisture contents and injection pressures. Results indicate that during gas injection, coal body pressure exhibits a trend of rapid initial increase followed by gradual decrease and eventual stabilization. Pressure response velocity diminishes with increasing distance from the injection port. CO 2 concentration undergoes a phased change characterized by rapid initial decline followed by slow reduction, displaying significant spatial heterogeneity. Vertically, it exhibits a layered distribution pattern with higher concentrations in the lower section, lower concentrations in the middle section, and intermediate concentrations in the upper section. Under identical moisture content conditions, increasing injection pressure enhances gas diffusion and flow capacity. This leads to an overall rise in coal body gas pressure, accelerated pressure changes rates, easier CO 2 expulsion, and faster concentration decline, thereby shortening displacement completion time. However, it also increases internal gas pressure and CO 2 concentration gradients within the coal body. At constant injection pressure, increased coal moisture content impedes gas diffusion and migration. This leads to elevated pressures in all directions within the coal body, slowed pressure change rates, reduced CO 2 expulsion, decreased concentration decline rates, and exacerbated concentration distribution heterogeneity, significantly diminishing displacement efficiency. Based on the aforementioned principles, this study proposes an integrated “injection-displacement-extraction” optimization scheme tailored for deep coalbeds containing water. It recommends employing zone-specific regulation, dynamic pressure adjustment, and coordinated drainage-injection processes, complemented by methane pressure and concentration monitoring, to enhance the overall efficiency and uniformity of methane displacement.