抽采长度对煤层气开采效果的影响分析

Analysis of the effect of the borehole length on the efficiency of coal-bed methane exploitation

运用自主研发的多场耦合煤层气开采物理模拟试验系统,开展三维应力状态下煤层气开采物理模拟试验,分析不同抽采长度条件下煤储层内气体压力参数的动态演化规律。研究表明:不同抽采长度条件下,气体压力都是从钻孔中心周围开始下降的;抽采前期气体压力下降显著,随着抽采时间的延伸,各测点气体压力值变化速率减小;随着抽采长度的增加,气体压力梯度较大的区域明显扩大,同时会提高瓦斯的解吸速率,抽采效率提高。抽采断面上的等压线则以钻孔为中心,呈圆环状分布,气体主要是汇流至抽采钻孔,层面流场中等压线则以钻孔为对称轴呈漏斗形分布。抽采长度对突出危险消除区域出现时间影响不大,但随抽采长度增加,消除突出危险性所需时间减少;由于应力集中区应力大、解吸速率慢,消除突出危险性抽采所需时间较其他区长。研究成果可为现场合理布置抽采钻孔,提高抽采效率以及确定合理的抽采时间提供一定的理论依据。

The physical simulation about the coal-bed methane（CBM） exploitation under 3D stress state was conducted with a multi-field coupling test system developed in-house. The development of gas pressure under the different borehole lengths was analyzed. The results show that the gas pressure decreases rapidly near the center of the borehole under the different borehole lengths. The rate of pressure drop is high at the beginning and then decreases gradually. With the increase of the borehole length, the area with high pressure gradient increases significantly, which leads to the rise of gas desorption rate and productivity. On the cross Sectional plane of borehole, the pressure contours are ring shaped with the center located at the borehole, indicating that the gas flows from the coal to the borehole. On the horizontal plane, the pressure contours present the funnel shaped distribution around the symmetry axis of the borehole. Although the length of the borehole has limited influence on the occurrence time of the danger elimination area of gas outburst, the time needed to prevent the gas outburst isgreatly reduced with the increase of the borehole length. Because of the high stress and the low rate of gas desorption in the coal seam area of stress concentration, more time is needed for the gas production to eliminate the outburst danger than in the other areas.