卸压区不同钻孔长度抽采条件下瓦斯运移特性试验

研究抽采过程中瓦斯运移特性有助于了解抽采气体来源、不同位置对抽采效果的贡献及抽采降压规律，为合理确定抽采时间、设计抽采位置和钻孔长度等提供依据。利用自主研发的多场耦合煤层气开采物理模拟试验系统，开展了卸压区不同钻孔长度条件下瓦斯抽采的物理模拟试验，分析了抽采过程中煤层瓦斯运移相对速度和方向特征。研究结果表明：抽采前期和钻孔周围区域分别是抽采量主要贡献时期和区域，瓦斯压力梯度大，流动快。卸压区瓦斯流动相对速度最快，应力集中区使得瓦斯相对流动速度衰减加速，且对原始区的瓦斯流动形成一道屏障,使其相对流动速度趋于0。随着抽采时间的增加，瓦斯相对流动速度逐渐衰减，对于瓦斯运移方向而言，抽采一旦开始便在煤层中形成了较为固定的运移通道，但在抽采后期和钻孔深部区域，由于瓦斯压力梯度小，流动缓慢，运移方向的不稳定性增强。而随着钻孔长度增加，卸压区内瓦斯相对流动速度表现出增大的趋势，因此，适当增大卸压区钻孔长度将有利于现场瓦斯开采。

Test on gas migration characteristics during coal bed methane exploitation under different lengths of drilling hole in distressed zone

The research on gas migration law during coal bed methane (CBM) exploitation is beneficial to understand the source of gas, impact of coal seam position on gas production and the law of gas pressure decay, thus providing foundation for design of exploitation time and the position and length of drilling hole. Using the multi-field coupling testing system for CBM exploitation developed by the authors, physical simulation experiments of CBM exploitation under different lengths of drilling hole in distressed zone were carried out, and relative gas flow speed and direction were analyzed. The results show that the initial stage of exploitation is the main contribution period to gas production and the surrounding area of drilling hole is the main contribution area to gas production, because gas pressure gradient is high and thus gas flows fast. Gas flow has the highest speed at the distressed zone. Existence of concentrated stress zone makes the relative flow speed attenuate quickly, and also creates a barrier to original stress zone, leading relative gas flow speed to be zero. Relative gas flow speed decreases with time while the migration direction does not change much because relatively stable seepage channels are formed at the beginning of exploitation. Due to gas pressure drop that leads gas pressure gradient and relative gas flow speed to decrease, the uncertainty of migration direction rises. With the length of drilling hole growing, the relative gas flow speed increases in the distressed zone, so it is meaningful to moderately lengthen the drilling hole.