Experimental insights into the adsorption-desorption of CH4/N2 and induced strain for medium-rank coals

Abstract

To support the understanding of the effect of N2-enhanced coalbed methane (N2-ECBM) application on medium-rank coals, four coal samples from Shanxi Province were selected to test the porosity, mechanical properties and strain induced by binary gas adsorption-desorption. The results show that the desorption hysteresis is weakened with increasing N2 fraction and is strengthened with increasing CH4 fraction, indicating that increasing the N2 fraction can reduce the selectivity of coal to CH4. The critical desorption pressure of a mixed gas increases in a high N2 fraction environment, promoting the desorption of CH4 and enhancing its recovery. This research clarifies the anisotropic strain characteristics induced by the adsorption-desorption of CH4/N2 and provides reliable data for anisotropic permeability numerical simulations in the N2-ECBM process. Swelling and shrinkage volume strain are negative-exponentially related to the N2 fraction, and the volume shrinkage strain is close to the volume swelling strain given a N2 fraction of 50%. There exists a polynomial relationship between the volume swelling/shrinking strain and adsorption capacity, which has a potential predictive effect on the volume strain variation in coal during N2-ECBM. The analysis of the adsorption volume swelling strain rate and desorption volume shrinkage strain rate indicates that the N2 fraction and micropore volume are the main factors controlling volume deformation. The volume strain decreases with increasing elastic modulus, but nitrogen injection weakens the effect of the elastic modulus on the strain. In this study, the desorption percentage increased and the volume swelling strain decreased with increasing nitrogen fraction. Nitrogen injection and sealing have a positive effect on methane desorption and the optimal N2 fraction should be greater than 50%. This work provides a reference for improving coalbed methane production.