煤岩塑性软化及扩容特性对钻孔密封性的影响

钻孔密封性是钻孔瓦斯抽采的关键基础之一,而煤岩力学特性对钻孔密封参数的选择及密封效果具有重要影响作用。因此,为准确认识钻孔围岩密封环境并深入分析煤岩塑性软化及扩容特性对钻孔密封性的控制作用,进而为钻孔密封参数的合理选择提供可靠的依据。基于煤岩全应力应变曲线及煤层力学参数,通过引入煤层软化系数k及扩容系数η1和η2,在构建煤岩线弹塑性力学软化“三折线”模型的基础上,依据线性Mohr-Coulomb屈服准则,对钻孔围岩弹性区、塑性软化区及塑性流动区半径、钻孔围岩径向位移以及钻孔围岩理论密封半径等参数进行理论数值解析计算,并对相应解析结果进行对比分析。同时结合钻孔成像技术及钻孔瓦斯抽采浓度数据监测等手段,综合分析塑性软化及扩容特性对煤层钻孔密封性的影响作用。研究结果表明,与理想弹塑性模型(Kastner)解相比,受塑性软化及扩容特性共同作用下的煤层钻孔围岩径向位移及理论密封半径分别增加12.93倍和2.60倍,且在构造煤层区域内钻孔缩径现象明显。在钻孔密封参数及密封材料相同的条件下,缩径现象明显的钻孔内初始平均抽采瓦斯体积分数较相邻完整钻孔降低59%,在29d考察期内平均抽采体积分数同比降低64.92%,瓦斯体积分数衰减率同比增加3倍。因此,塑性软化及扩容效应是造成钻孔初始密封性降低及后期抽采瓦斯衰减的主要原因之一,而采用Kastner方程求解获得的煤层钻孔围岩理论渗透半径及注浆压力等参数并不合理;同时,钻孔密封参数需应根据钻孔围岩特性进行区别选择。

Effect of plastic-softening and dilatancy of coal on sealing property of borehole

Sealability of borehole is one of the key foundations of efficient gas drainage.The mechanical properties of coal play important roles in the selection of sealing parameters and the results of borehole sealing.Therefore,this paper aims at understanding the environment of surrounding coal and rock of borehole and analyzing the impacts of coal plastic softening and expansion characteristics on borehole sealing and providing a reliable foundation for the selection of borehole sealing parameters.A linear elastic-plastic stress softening model of “three-fold line” was proposed based on the full stress-strain curve and the mechanical parameters of coal.According to the Linear Mohr-Coulomb Yield Criterion,the radius and displacement of elastic zone,plastic softening zone and plastic flow zone and displacement of the model were calculated.Borehole imaging and data monitoring of gas drainage were conducted.The results show that radial displacement and theoretical sealing radius of surrounding of boreholes was increased by 12.93 and 2.60 times compared with the calculated results of Kastner solution.Compared with the adjacent borehole which is unbroken and the sealing parameters are the same,the initial average extraction gas concentration of the borehole decreased by 59%,the average extraction gas concentration decreased by 64.92% over the same period of 29 d and gas concentration decay rate increased by 3 times.Based on the results,the influence of plastic-softening and dilatancy is one of the main reasons inducing the decrease of initial sealing performance and the decline of gas extraction in late drainage stage.The theoretical permeability radius and grouting pressure for sealing boreholes obtained from Kastner equation was not reasonable.The sealing parameters of boreholes should be selected according to the characteristics of borehole surrounding rock as well.