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脉冲水射流对煤体破裂特征影响分析

Analysis of influence of pulsed water jet on fracture characteristics of coal body

  • 摘要: 高瓦斯低渗透煤层透气性差,瓦斯抽采难度大,严重影响和制约了矿井的采掘接替和安全高效生产。常规抽采方法难以解决消突难题,而脉冲水射流技术能够显著提高煤体渗透性,进而提升钻孔瓦斯抽采能力。因此,基于水射流冲击致裂实验平台,通过单因素物理实验方法,研究了脉冲水射流压力P和冲击靶距S对冲击煤体破裂特征的影响,分析了脉冲水射流破岩致裂机制,并通过工程实验验证了脉冲水射流技术对煤体瓦斯浓度、压力、增透和抽采的影响。结果指出:当喷嘴直径D=2.0 mm,冲击靶距S=30 mm,随着冲击水压P从5 MPa增大至40 MPa,试样发生应变和应变突变的时间均逐渐减少,应变量累计量逐渐减小,冲击破裂深度h从75 mm逐渐降低至37 mm,冲击坑直径d先增大后减小;当脉冲水射流的冲击水压在5~15 MPa时,初期响应和稳定破坏阶段煤体裂纹扩展迅速;当脉冲水射流的冲击水压在15~40 MPa时,初期响应阶段煤体裂纹扩展较为缓慢,试样破裂时间减短,断裂突变后煤体裂纹加速扩展;脉冲水射流不同压力破裂煤岩体时,煤岩体以纵向主裂纹为主,次级裂纹较发育,随着冲击水压的升高,主裂纹由水平(0°)和竖直(90°)方向转变为倾斜(45°)方向。当喷嘴直径D=2.0 mm,冲击水压P=20 MPa,随着脉冲水射流靶距S从30 mm增大至120 mm,试样初期响应阶段的应变量降低,稳定破坏阶段时间增长,应变变化率降低。水射流冲击破岩过程分为4个阶段:I表面冲击阶段,II冲击坑萌生阶段,III冲击坑延伸阶段和IV冲击缓滞阶段。通过现场实验验证,脉冲水射流切槽破岩钻孔相比常规钻孔周围煤体瓦斯涌出浓度显著升高,瓦斯压力降低了约0.17、0.19 MPa,煤体透气性系数平均增大9~13倍,钻孔瓦斯抽采瞬时混合量升高了2~21倍。脉冲水射流能够有效破碎钻孔煤体,研究结果可为低渗透性煤层的瓦斯治理提供一定的依据。

     

    Abstract: The poor permeability of high gas and low permeability coal seams and the difficulty with gas drainage seriously affect and restrict the mining replacement and safe and efficient production of the mine. It is difficult to solve the problem of outburst elimination through the conventional drainage method, while the pulse water jet technology can significantly improve the permeability of the coal body and improve the gas drainage capacity of borehole. Therefore, based on the experimental platform for water jet impact cracking, the influence of pulsed water jet pressure P and impact target distance S on the fracture characteristics of impacted coal is studied via the single-factor physical test method, the rock-breaking mechanism of pulsed water jet is analyzed, and the effects of pulsed water jet technology on coal gas concentration, pressure, permeability enhancement and drainage are verified through engineering experiments. According to the results, when the nozzle diameter D=2.0 mm and the impact target distance S=30 mm, with the impact water pressure P increasing from 5 MPa to 40 MPa, the time for strain and strain mutation of the sample gradually decreases, the cumulative amount of strain gradually decreases, the impact fracture depth h gradually decreases from 75 mm to 37 mm, and the impact pit diameter d first increases and then decreases; when the impact water pressure of the pulse water jet is 5−15 MPa, the cracks of the coal body propagate rapidly in the initial response and stable failure stages; when the impact water pressure of the pulse water jet is 15−40 MPa, the crack propagation of the coal body is relatively slow in the initial response stage, the fracture time of the sample is shortened, and the crack propagation of the coal body is accelerated after the fracture mutation; when the coal and rock mass is fractured by pulsed water jet under different pressures, the main longitudinal cracks play a dominant role, and the secondary cracks are relatively developed. With the increase of impact water pressure, the main cracks change from horizontal (0°) and vertical (90°) directions to the inclined (45°) direction. When the nozzle diameter D=2.0 mm and the impact water pressure P=20 MPa, with the target distance S of pulsed water jet increasing from 30 mm to 120 mm, the strain in the initial response stage of the sample decreases, the time in the stable failure stage increases, and the variation rate of strain decreases. The process of rock breaking by water jet impact can be divided into four stages: I surface impact stage, II impact pit initiation stage, III impact pit extension stage and IV impact retardation stage. According to the verification results through on-site tests, the concentration of gas emission from the coal body around the slotting and rock-breaking boreholes by pulsed water jet is significantly higher than that around the conventional boreholes. The gas pressure decreases by approximately 0.17 and 0.19 MPa, respectively. The average permeability coefficient of the coal body increases by 9 to 13 times, and the instantaneous mixing amount of gas extraction from the boreholes increases by 2 to 21 times. Pulsed water jet can effectively break the coal body in the boreholes. The research results can provide a certain basis for the gas control of low-permeability coal seams.

     

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