高压氮气冲击致裂煤岩体裂隙发育规律研究

发布时间：2018-05-09 10:38

本文选题：高压氮气 + 气射流　；参考：《中国矿业大学》2017年硕士论文

【摘要】：氮气作为惰性气体,在煤中不容易被吸附存储;当高压氮气在煤层中发生冲击致裂后,不会留存大量的膨胀能,不具有突出危险性。随着我国开采深度的增加,矿井的瓦斯含量随之上升而渗透率同时下降,通过在煤层中实施高压氮气致裂可以提高瓦斯抽采率、降低瓦斯含量。因此,高压氮气冲击致裂的研究对于矿井的安全生产具有重要意义。本文以对高压氮气气射流冲击过程以及高压氮气的准静态膨胀作用的研究为基础,分析了致裂过程中能量的变化,并以之作为理论指导开展高压氮气冲击致裂实验与数值模拟研究,获得了不同工况下高压氮气冲击致裂的相关规律。实验系统以高压氮气冲击致裂装置为核心,主要由高压氮气冲击致裂系统和实时监控系统两部分组成实验系统。其中高压氮气致裂系统包括高压氮气压力源、高压氮气冲击致裂实验装置、气动高压切断球阀、试块以及连接装置等部分。实验中采用毫秒级压力传送器以及声发射监测系统,对致裂过程中压力的升降和试块内部的能量等物理量的变化进行监测。通过强度测定等实验确定了不同强度相似材料的配比并确定28天的养护周期;同时通过应力加载的方式来模拟煤层在实际情况中应力场的情况。实验中采用1L的容器作为气体的初始体积,分别先对三种强度的试块进行初始压力为5MPa、7.5MPa以及10MPa三种工况条件下的冲击致裂实验;再改变三种强度试块的应力环境进行初始压力为5MPa的实验。从结果分析得出随着气体初始压力的升高,裂纹从最初只有一条纵向裂纹到随后以释放孔为中心呈星状发散的裂纹,从垂直于最小主应力方向到均匀分布。同时裂纹的分布于试块周边的应力差也有一定关系,应力差小则裂纹均匀分布,应力差大则垂直于最小主应力方向。再此基础上对于最小致裂压力进行了初步研究,结果圈定在2MPa-3MPa之间。根据对压力监测结果的分析,整个高压氮气致裂过程根据压力上升的快慢可以分为气体射流冲击阶段、起裂致裂阶段和动态止裂阶段。从声发射监测结果中得出,随着气体压力的上升,试块致裂时内部绝对能量也呈现上升趋势,尤其是在7.5MPa升至10MPa时能量上升明显。以理论研究为基础结合现场实际,通过ANSYS/LS-DYNA模拟软件建立模型进行精算并通过LS_PREPOST后处理,对5MPa、7.5MPa和10MPa三种工况下的高压氮气冲击致裂进行模拟。从模拟的运行过程中可以看出,裂隙的发育情况随着气体初始压力的升高而得到明显的提升,同时裂隙呈现出先沿轴向再沿径向扩展的变化规律,这一规律在应力云图中也得到了验证。通过对应力波传递结果的分析得出,致裂过程中的应力波呈现震动上升,当应力值大于煤层失效应力时,致裂发生;小于时则保持高应力状态直至发生致裂。
[Abstract]:Nitrogen, as an inert gas, is not easily adsorbed and stored in coal. With the increase of mining depth in China, the gas content in coal mine increases and the permeability decreases simultaneously. The gas extraction rate can be increased and the gas content can be reduced by the application of high pressure nitrogen in coal seam. Therefore, the research of high pressure nitrogen impact cracking is of great significance to mine safety production. Based on the study of the impinging process of high pressure nitrogen jet and the quasi-static expansion of high pressure nitrogen, the energy changes during the process of cracking are analyzed. The experiment and numerical simulation of high pressure nitrogen impact cracking were carried out under the guidance of the theory, and the related laws of high pressure nitrogen impact cracking under different working conditions were obtained. The experimental system is composed of two parts: high pressure nitrogen impact cracking system and real time monitoring system. The high pressure nitrogen fracturing system includes high pressure nitrogen pressure source, high pressure nitrogen impact cracking experimental device, pneumatic high pressure cut off ball valve, test block and connecting device and so on. In the experiment, millisecond pressure transmitter and acoustic emission monitoring system are used to monitor the change of physical quantities such as pressure rise and fall and energy inside the specimen during the process of cracking. The proportioning of similar materials with different strength and the curing period of 28 days were determined by strength measurement, and the stress field of coal seam was simulated by the way of stress loading. In the experiment, 1L vessel was used as the initial volume of gas, and the initial pressure of three kinds of strength samples was tested under the initial pressure of 5 MPA and 7.5 MPA, respectively, and the impact cracking test was carried out under three conditions of 10MPa. Then the stress environment of the three strength specimens was changed and the initial pressure was 5MPa. The results show that with the increase of the initial gas pressure, the crack changes from one longitudinal crack to a star-shaped crack centered on the release hole, from the direction perpendicular to the minimum principal stress to the uniform distribution. At the same time, the distribution of the crack is also related to the stress difference around the specimen. The crack distributes evenly when the stress difference is small, and the stress difference is perpendicular to the direction of the minimum principal stress. On this basis, a preliminary study of the minimum cracking pressure is carried out, and the results are delineated between 2MPa-3MPa. According to the analysis of pressure monitoring results, the whole process of high pressure nitrogen cracking can be divided into three stages: gas jet impingement stage, crack initiation stage and dynamic crack arrest stage according to the speed of pressure rise. From the results of acoustic emission monitoring, it can be concluded that with the increase of gas pressure, the internal absolute energy of the specimen also shows an upward trend, especially when 7.5MPa rises to 10MPa. On the basis of theoretical research and field practice, the model was established by ANSYS/LS-DYNA simulation software and LS_PREPOST post-treatment was used to simulate the impact cracking of high pressure nitrogen under three conditions: 5MPA 7.5MPa and 10MPa. It can be seen from the operation of the simulation that the development of fractures increases obviously with the increase of the initial gas pressure, and the fracture shows a variation law of first axial direction and then radial expansion. This rule is also verified in the stress cloud diagram. Through the analysis of the results of stress wave transmission, it is concluded that the stress wave in the process of crack appears vibration rising, when the stress value is greater than the failure stress of coal seam, the crack occurs, and when the stress value is larger than the failure stress of coal seam, the stress wave will remain in a high stress state until the crack occurs.
【学位授予单位】：中国矿业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TD712.6

【参考文献】