摘要：In order to describe displacement effect on coalbed methane (CBM) by CO_2 and N_2, the paper takes displacement and replacement efficiency as evaluation parameters. There are 5 gases being taken for indoor displacement experiment on coal samples adsorbed CH4, i.e. CO_2, N_2 and three mixed gases with different ratio (CO_2：N_2=1:1, CO_2:N_2=1:4 and CO_2: N_2=1:9) under 1.5MPa, 2.5MPa, 3.5MPa, 4.5MPa and 5.5Mpa controlled gas injection pressure. Following rules are obtained from the experiments: (1) In terms of displacement efficiency, as the injection pressure increases, displacement efficiency of different gases will increase first and then decrease. (2) In terms of replacement efficiency, replacement efficiency of N_2 shows slowly decrease as injection pressure increase, while replacement efficiency of CO_2 shows increase at first and then decrease as injection pressure increase. (3) Taking coal samples saturated with CH4 under 2.5MPa pressure as an example, the best displacement pressure shall be 2.5-3.5Mpa; (4) When gas injection pressure is relatively low (lower than 2.0 MPa), the displacement and replacement efficiency of N_2 is higher than that of CO_2, however, under relatively high pressure (higher than 2.0 MPa), the displacement and replacement efficiency of N_2 is lower than that of CO_2. (5) For a mixed gas with certain mix ratio, for example (CO_2: N_2=1:1), replacement of CO_2 and displacement of N_2 will produce synergistic effect under a certain pressure range (2.5-3.5MPa).Its average displacement efficiency is 86.14%, average replacement is 30.61%, which is higher than average CO_2 displacement efficiency 83.06% and average N_2 displacement efficiency 83.39%; It is lower than average CO_2 replacement efficiency 34.92%, but higher than average N_2 replacement efficiency 20.78%.
