Study on competitive adsorption and strain characteristic of the coal under different N₂-CH₄ ratio

In order to investigate the impact of adsorption-desorption behavior of coal seam gas on the effectiveness of nitrogen injection and displacement, experiments were conducted using N₂-CH₄ gas mixtures with varying ratios. The performance of adsorption- desorption was analysed through the coal strain,adsorption-desorption rates,and changes in pore pressure. The results show that during the adsorption stage,the gas enters the pore space of coal seam quickly at the beginning. CH₄ rapidly occupies the adsorption site,followed by gradually reduction in the adsorption rate. Enhancing the volume fraction of N 2 can improve the diffusion and adsorption of gas, thereby increasing the adsorption rate and gradually reducing the pore pressure. The coal is compressed and contracted throughout this process. However,it expands due to adsorption after continuous gas injection,and the strain value here is reduced with the increasing volume ratio of N₂. During the desorption stage,gas is swiftly expelled under the effect of pressure gradients, causing a sharp decline in pore pressure. The desorption rate first decreases and then tends to stabilize. The coal first expands axially and then continuously contracts. The strain variation is inversely related to the volume fraction of N₂.