Experimental study on kinetic characteristics of gas diffusion in coal under nitrogen injection

Abstract

Gas extraction after the permeability of the coal seam is enhanced is an important direction of gas control to improve the pre-drainage rate of coal seam gas and shorten the pre-drainage time. Gas injection displacement is an effective method to enhance gas extraction. The interaction mechanism between gas molecules and gas containing coal in the process of gas injection to promote desorption and the kinetic effect of gas desorption diffusion need to be further improved. In this paper, the PCTPro high-pressure adsorption instrument and Trace 1300 gas chromatograph were used to conduct gas desorption experiments on coal-containing gas at different temperatures, and the kinetic characteristics and laws of gas desorption diffusion in the process of N2 injection were analyzed. The CH4 concentration gradually decreased with the decrease of the equilibrium pressure, and the N2 concentration showed a negative correlation with the equilibrium pressure. The variation of N2 and CH4 desorption capacity at different temperatures conforms to the power function curve, and N2 and CH4 desorption capacity increases with the decrease of equilibrium pressure. The CH4 diffusion coefficient ranges from 6.555 × 10−10 m2/s to 8.388 × 10−10 m2/s. The CH4 diffusion coefficient increases with increasing temperature. The timeliness of gas diffusion during gas injection was analyzed. The dynamic correction model of coal gas diffusion under the condition of gas injection is established and verified by experimental data with high stability and accuracy. The research results are of certain significance for clarifying the mechanism of N2 injection on gas containing coal and promoting the mechanism of gas desorption, providing a theoretical basis for the field application of N2 injection in coal seam to improve CH4 extraction rate, and realizing efficient and safe gas extraction in a low permeability coal seam.