Water imbibition and drainage of high rank coals in Qinshui Basin, China

Water mobility in coal plays an important role in achieving the high performance of coalbed methane (CBM) recovery from coal. To better understand this phenomenon, a systematical investigation of water imbibition and drainage was conducted experimentally using high rank coals collected from Qinshui Basin of China. The results show the shapes of the T-2 spectrum for water imbibition and drainage experiments generally distribute with the bimodal pattern with a higher left peak around 1 ms and a lower right peak around 50-100 ms. The cumulative imbibition water content ranges from 3.09 to 6.54 vol% under the 0 MPa saturated hydraulic pressure at vacuumed state, which is accounting for 58.30-91.07% of total imbibition water content, and grows exponentially with the increasing of saturated hydraulic pressure. The cumulative drainage water content ranges from 0.53 to 1.71 vol%, and residual water content varies from 3.47 to 6.73 vol%, which latter is accounting for 72.1-88.46% of the total imbibition water content. The imbibition and drainage water contents increase with the total pore volume increasing. The well-developed macropores and mesopores increase the drainage water content and rate, while the micropores mainly contribute to the residual water content. The cumulative imbibition water content is positively related to inertinite content but negatively related to ratio of vitrinite to inertinite and ash yield, whereas it is just the opposite for drainage water content and rate. As it is expected, the increasing wetting angle generally increases the drainage water content and rate.