煤层注氮模拟实验中的置换-驱替效应及其转化机制分析

为了研究注N_2促排煤层瓦斯过程中驱替和置换效应及其主导作用,利用自研的煤层注气实验装置,进行了单轴应力、分层预压成型条件下煤层注N_2促排瓦斯的模拟实验。实验结果表明:驱替和置换效应始终贯穿整个注气过程,共同作用将煤体中瓦斯排出。在注气初期14 min内,置换解吸效应起主导作用,注入的N_2由于被吸附或充填于煤层孔裂隙等自由空间而被全部滞留煤中,表现为出口并无N_2排出,而煤体中的CH_4则大量解吸排出。在14~200 min注气实验时间内,置换作用的主导地位将逐渐丧失,开始进入置换作用减弱和驱替作用增强的转换阶段;在注气时间大于400 min的后期,处于置换和驱替相互作用、彼此平衡的时期,但置驱总效率处于较低水平。根据注气400 min(6.67 h)后注气促排效率较低的实验结果,建议井下煤层注气时间控制在8 h工作制的一个小班以内为宜。

Analysis of replacement-displacement effect and its change mechanism in simulation experiment of nitrogen injection into coal seam

To study the displacement and replacement effect during gas injection into coal seam for displacing methane，and the leading function between two effects，a simulation experiment of gas injection into coal seam was conducted under the condition of one dimensional stress and layer pre-press formed coal seam by using self-developed experiment device.The results indicate that the displacement effect and replacement effect is always occurring throughout the whole process of gas injection.Under their combined action，the methane is discharged.At the early stage within 14 min of gas injection，the replacement effect plays a leading role.The injected nitrogen is all detained in coal sample because of the adsorption by coal or filling with free space such as coal fracture；and it shows no nitrogen discharge out in the phase，meanwhile a large amount of methane is desorbed and discharged.In the period within 14-200 min of injection experiment，the dominant position of the displacement effect will be gradually lost，and begin to enter the transition phase which the displacement effect is weakened and the replacement effect is enhanced.At the last stage after 400 min，it is the period that the displacement and the replacement are combined to balance each other.But the total efficiency of displacement and replacement is at a low level.According to the experimental results that the discharge methane efficiency is low after 400 min (6.67 hours).Therefore，the study suggests that the period of gas injection should be controlled within eight hours timeframe.