Quantification of the asynchronous gas diffusivity in macro-/micropores using a Nelder-Mead simplex algorithm and its application on predicting desorption-based indexes

Abstract

Accurate and intelligent calculation of diffusion coefficient not only helps reduce greenhouse gas emissions, but also conduces to CO2 geological storage and CH4 displacement in coal seams. In this study, a kind of object-oriented software for conveniently and quickly obtaining bidisperse (i.e., macro-pore and micro-pore) diffusion coefficients and grasping main controlling factors of macroscopic desorption curves was designed based on the Nelder-Mead simplex algorithm. Furthermore, the gas desorption characteristics under different pressures were analyzed by performing conventional desorption experiments. The results show that the apparent diffusion coefficient obtained by the bulk technique displays three different trends as the pressure rises, namely a rising, falling and constant diffusion coefficient respectively. The three variation trends mainly result from the difference in deformation caused by adsorption swelling and mechanical compression. As desorption continues, the gas diffusion coefficient values in the macro-pores and micro-pores fluctuate for a short time, but increase in the long term. This is because the early stage corresponds to intense mechanical expansion and a flow boundary effect, while the later stage corresponds to an increasingly intense effect of desorption-induced shrinkage. Moreover, the time-dependent contribution law of macro-pore diffusion and micro-pore diffusion affects the reliability of outburst risk index, making △h2 more stable than K1. The conclusions provide theoretical guidance for the selection of rational gas drainage methods for diffusion-controlled coal seams.