考虑时效影响的深部煤层瓦斯运移特性

Characteristics of deep coalbed gas migration based on the time-dependent effect

为了考虑长期抽采过程中时间效应对煤体渗透率的影响,结合平均有效应力建立了时间效应和气体解吸效应耦合作用下的深部煤体孔隙率及渗透率演化模型。运用COMSOL Multiphysics对钻孔周围瓦斯运移过程进行了定量计算,结合现场数据对是否考虑时间效应的瓦斯渗流场变化规律进行了对比分析,并对长期抽采过程中深部煤层瓦斯运移规律进行了模拟分析。结果表明:煤层渗透率随瓦斯压力的下降呈指数型上升趋势;考虑时间效应的孔隙率、渗透率模拟结果明显小于未考虑时间效应模型的结果,且随着抽采时间的增长,蠕变本构中的黏弹性元件使得煤体更为致密,深部煤层的时间效应越发明显,考虑时间效应的孔隙率、渗透率模拟结果与未考虑时间效应的结果差值逐渐增大;考虑时间效应的模拟结果与现场数据匹配度较高,更符合深部煤层孔隙率和渗透率的实际演化特征。在同一抽采时刻,随着距钻孔中心距离的减小,渗透率呈现升高的趋势,压力呈现降低的趋势,当模拟抽采时间为1 d时,临近钻孔中心处渗透率较大、瓦斯压力较小;在不同抽采时刻,当抽采时间逐渐增长时,相同位置处的渗透率逐渐增大,瓦斯压力逐渐减小,当抽采时间由1d增至30d时,临近钻孔中心处的渗透率增长近1.4倍,瓦斯压力降低近3.8倍,且模型内渗透率与瓦斯压力的演化趋于平衡状态。

In order to study the permeability of deep coal seam with time-dependent effect during the long-term extraction process,the improved porosity and permeability model coupling with gas desorption effect and time-dependent effect was developed by incorporating mean effective stress into the gas migration equation.By using COMSOL Multiphysics,the process of gas migration around the borehole was calculated quantitatively.In combination with the field data,the variation law of gas seepage field with or without time-dependent effect was compared and analyzed,and the law of gas migration in deep coal seam during long-term extraction was simulated and analyzed.The results show that coal seam permeability increases exponentially with gas pressure decreasing.The porosity and permeability simulation results considering time-dependent effect are significantly smaller than those without considering the time-dependent effect model,and as the extraction time increases,the viscoelastic elements make the coal body denser in the creep constitutive.With the increase of extraction time,the time-dependent effect of deep coal seam becomes more obvious,and the difference increases gradually between the simulation results of porosity and permeability considering time-dependent effect or not.The time-dependent simulation results are in good agreement with the field data and are more in line with the actual evolutionary characteristics.At the same time,as the distance from the center of the borehole decreases,the permeability shows an increasing trend,while the pressure shows a decreasing trend.When the extraction time is 1 d,the permeability is large and the gas pressure is small near the center of the borehole.At different extraction times,when the extraction time gradually increases,the permeability gradually increases at the same position,and the gas pressure gradually decreases.When the extraction time increases from 1 d to 30 d,the permeability increases by nearly 1.4 times near the center of the borehole,the gas pressure decreases by nearly 3.8 times,and the evolution of permeability and gas pressure tends to balance in the model.