Permeability of CO_2 and He in Yangquan coal matrix-plug saturated by CH_4

The CO2 permeability of Yangquan coal matrix-plug(φ 6×13 mm) saturated by CH4 was studied by carrying out CO2 breakthrough tests at 40 ℃.The similar tests were performed with inert gas He for comparison.The breakthrough tests were performed in a self-confining flow cell equipped with an online thermal conductivity detector for continually monitoring the signal of CO2 or He.The imbibition of gas by coal and the influence of the pore gas pressure were investigated.It shows that unlike He there is significant residual CO2 left in the coal matrix even after CO2 was flushed by CH4 for 36 h.It reveals that coal has stronger imbibition ability for CO2,which leads to the difference in its breakthrough time for different CH4 flushing time.No notable residual He is found during the breakthrough tests.He permeability is significantly higher than that of CO2.The obtained permeability coefficients of the coal plug with CO2 or He at the steady state are closely the same for the repeat tests.It implies that no significant structure change involved during the tests.The permeability coefficients varied with the pore gas pressure due to the existence of the influences of the pore wall,the swelling of coal and the adsorption boundary layer.