Experimental investigation on the pressure dependence of apparent gas diffusion coefficient in coal matrix under multiple potential factors

Whether the apparent diffusion coefficient D of gas in the coal matrix increases or decreases with stepwise rising pressure has long been debated. In this study, sorption kinetic data were acquired using the particle method and fitted by mUM and BM models to estimate D, with the coal pore structure characterized by various techniques. Nine potential influencing factors other than pressure were systematically investigated. Based on the concepts of overall reactions and rate-limiting steps, we deconstructed the particle method experiment, and calculated and tracked the evolution of Fick diffusion coefficient (Dbulk) and Knudsen diffusion coefficient (DK), as well as their corresponding pore size ranges at different pressure steps. In addition to reproducing the simple monotonically increasing and decreasing trends reported in previous studies, we further observed a distinct non-monotonic variation in the apparent CO2 diffusion coefficient D under specific conditions. For high-rank anthracite (YQ, vitrinite reflectance R0 = 2.35%) with large particle sizes, including 0.841–3.35 mm granular coal YQ6 and a φ13.5 × 13.5 mm single cylindrical core YQ4, tested in a pressure range of 0–2.7 MPa, the D value of CO2 first increases and then decreases with rising pressure. Specifically, for YQ6, D increases from 7.58 × 10−11 m2/s to 2.42 × 10−10 m2/s before decreasing to 2.30 × 10−10 m2/s; for YQ4, D increases from 2.11 × 10−9 m2/s to 5.66 × 10−9 m2/s and then decreases to 3.63 × 10−10 m2/s. To explain this phenomenon, we propose a qualitative conjecture of “pore-structure-regulated temporary enhancement of Knudsen diffusion”, indicating that the temporary increase in D is mainly caused by enhanced Knudsen diffusion in micropores and partial mesopores (0.6–5.5 nm, specific to this experiment). This enhancement arises jointly from the increased Knudsen diffusion coefficient and more effective pores for Knudsen diffusion, induced by pressure-driven changes in the molecular mean free path. The study reveals the unique diffusion behavior of CO2 in high-rank anthracite under specific conditions, providing a new perspective for in-depth elucidation of gas diffusion mechanisms, and offering a theoretical reference for coalbed methane recovery and CO2 geological sequestration.