煤层气损失气含量模拟试验及结果分析

Simulation experiment and result analysis on lost gas content of coalbed methane

利用高压解吸试验装置,模拟煤芯在绳索取芯时钻孔提芯、地面暴露和密封罐中3个阶段经历的压力变化,对2种煤阶3种粒级的5个煤样,分别在高压匀快速降压和正常大气压条件下进行了解吸试验,实测了模拟损失气量,揭示了煤样在3个连续阶段甲烷气体的解吸变化规律,并将实测的模拟损失气量与美国矿业局直接法估算的损失气量进行了对比.结果表明:煤样的甲烷气体解吸量曲线呈明显的3段型,在3个不同阶段,甲烷气体的平均解吸速率差异很大,前两者分别是后者的12～20和6～8倍;煤样粒级大小对甲烷气体解吸速率影响大,同一煤层粒级分别为1～3mm煤样,30～50mm煤样和89mm柱状煤芯的煤样,甲烷气体平均解吸速率的比值约为3∶2∶1;实测的模拟损失气量比美国矿业局直接法估算的损失气量高很多,前者是后者的8～16倍,并且较小粒级的煤样,两者的差异更大.认为美国矿业局直接法估算的损失气量不准确,比实际低很多,该方法有待进一步完善.

A high pressure desorption apparatus was established. The pressure changes experienced by coal core during three stages of core recovery, exposed to atmosphere and sealed-in canister were simulated. Desorption experiments for five coal samples with three particle sizes from two rank coals were respectively performed under rapid drop pressure from approximate 8 MPa to ambient pressure. Volumes of simulated lost gas of methane were meas- ured. The desorption characteristics of methane gas from coal samples were revealed during the above three stages. Comparison between the measured volume of simulated lost gas and the volume estimated by USBM Direct method was made. The result shows that the curves shapes of methane desorption volume from coal samples all have the characteristics of three section pattern. Average desorption rates of methane in above three stages have great differences, the rates in the former two stages are almost respectively 12 -20 times and 6 -8 times as large as in the last stage. Coal particle size also has much influence on desorption rate of methane gas from coal sample. The ratios of average desorption rates of methane gas from the three distinct samples with 1 - 3 and 30 - 50 mm particle sizes and φ89 mm core size of the same coal are about 3： 2： 1. The measured volume of the simulated lost gas is much higher than one estimated by USBM Direct method, the former is 8 ~ 16 times larger than the later, and the smaller the particle size, their ratio is much more different. It is thought that the lost gas volume estimated by USMB Direct method is inaccurate, and much lower than the original one. This method needs to be modified by performing further research work.