Effect of N2/CO2 injection and alternate injection on volume swelling/shrinkage strain of coal

To reveal the change of coal strain during N2/CO2-ECBM, four coal samples (Ro, ran = 0.42-3.23%) were pre-pared with different CH4 saturations, and the physical simulation experiment was carried out. The results show that the adsorption swelling rate decreases with increasing saturation, and the swelling rate k is CO2 > N2 > CH4. The ratio of strain induced by CO2, CH4 and N2 adsorption is about 9.52 : 4.05: 1. During N2-ECBM, the volume shrinkage strain caused by nitrogen injection increases with the increase of methane saturation, and the shrinkage percentage has the relation: medium- > low- > high-rank coal. Replace the injected N2 with CO2, the swelling strain rate decreases by an order of magnitude relative to CH4 adsorption. In the process of desorption with stepwise depressurization, the strain rate k in the N2-ECBM follows the order of test 2 < test 3 < test 4, but there is no obvious regularity in CO2-ECBM. The application of CO2-ECBM is more outstanding for high-rank coal reservoir. CO2/N2 alternate injection method can be carried out improve the injectability of CO2, and the improvement of high rank coal is obvious.