Effect of the dynamic pore structure parameter changes on coal particle gas desorption during pulverization: Implications for estimation of underground gas loss

Abstract

The borehole drill cuttings sampling method is the predominant sampling method for Coalbed methane (CBM) content measurement in China owing to its operational simplicity and high efficiency. However, coal samples collected from field boreholes typically contain particles of mixed particle sizes. Fine pulverized coal has a high gas release rate. In the context of underground CBM content measurement, this high release rate implies that by the time the coal sample is collected and the on-site test begins, most of the gas has already been released. This makes it challenging to collect reliable desorption data in underground coal mines, which in turn affects the accuracy of lost gas volume calculation. In this study, a complete-time gas desorption measurement system was established to investigate how coal particle size affects desorption behavior. Low‑pressure N 2 /CO 2 adsorption quantitatively characterized coal’s multi‑scale pore structure. Based on this, the dual-pore series diffusion model (DPSDM) calculated dual‑pore parameters for different‑sized coal samples, revealing their rapid gas release mechanism during pulverization. Finally, we used the lost gas model to calculate lost gas for different‑sized coal samples, aiming to assess how sampling particle size affects lost gas estimation. The results indicate that the gas desorption rate, desorption ratio, and effective diffusion coefficient all increased with decreasing particle size, suggesting that fine coal particles exhibit a strong initial gas release capacity, which is unfavorable for the retention of gas desorption data. With coal pulverization, the equivalent pore length of filling pores and diffusion pores is reduced, while their equivalent number of pores is increased. Particularly for particle sizes below 0.2 mm, both parameters undergo order-of-magnitude changes, which leads to rapid gas desorption from fine coal particles. By simulating lost gas under various operating conditions based on gas desorption data, for the 1–3 mm coal samples, the lost gas estimation error calculated by the fractional diffusion model (FDM) was within 15.6%, whereas those of the conventional unipore diffusion model (UDM) and power function model (PFM) were as high as 21.95%–74.29%, demonstrating the superior applicability of the FDM for coarse coal samples. However, when the proportion of fine pulverized coal in the sample is high, none of the three models can provide reliable back-calculation of lost gas. When estimating lost gas for coal samples with different particle sizes, the estimation errors of all models increased with decreasing particle size, primarily because the majority of gas in fine coal particles is lost during the initial stage, making it difficult to accurately reconstruct the actual lost gas from the flat late-stage desorption curve alone. This study reveals the gas desorption characteristics during coal particle pulverization from the perspective of pore structure parameters, providing new insights into the determination of borehole sampling particle size and contributing significantly to improving the accuracy of CBM content measurement.