重复采动煤体裂隙结构定量表征及其对应力敏感性影响特性研究

In the repeated pressure relief mining of coal seam group, the coal seam of mining in the later period is affected several times by the early coal seam mining. The internal fractures of later mining coal seam are changed and the pressure relief gas drainage efficiency is influenced. The preliminary experiments from the applicant show that the stress sensitivity of coal seepage is influenced by the fracture structure. Therefore, the actual stress path of coal under the repeated mining can be obtained with the field measurement and physical simulation analysis in this project, the loading and unloading plan of the experiment can be designed accordingly. The actual fracture structure distribution of coal can be obtained with experiment simulating the repeated mining of coal seam group. The fracture structure distribution characteristics of coal obtained under the repeated mining experiment are quantitatively analyzed by CT scanning technology. The coal of corresponding fracture structure can be characterized and three-dimensionally reconstructed. At the same time, the seepage stress sensitivity study of different fractured coal is carried out in both axial and radius directions by the self-designed loading coal seepage and strain testing experimental system. The influence of fracture structure on the coal stress sensitivity can be analyzed. The stress-fracture-seepage computational model can be obtained according to the quantitative structure characteristics and seepage experiment testing results. Finally, the computational model can be modified and validated based on the fracture generation, stress and permeability variation testing data in the practical mining process. The seepage stress sensitivity characteristics of coal influenced by fracture structure under the repeated mining can be obtained, which provide the theoretical basis for the mastering the evolution characteristics of gas seepage in coal seam group mining.

煤层群卸压开采导致后期开采的煤层受到前期煤层开采的多次采动作用，其裂隙结构发生变化，影响卸压瓦斯抽采效果。申请人初步实验表明，裂隙结构对煤体渗流的应力敏感性存在影响。据此，本项目借助现场实测及物理模拟获得重复采动作用下煤体的实际应力路径以设计实验室加卸载方案。通过模拟煤层群重复采动实验得到煤体的实际裂隙分布结构。运用CT扫描定量分析实验室所得重复采动影响煤体的裂隙结构分布特征，并对不同裂隙结构煤体进行结构表征及三维重构。采用自行设计的受载煤体渗流和应变测试实验系统进行不同裂隙结构煤体轴向及径向应力渗流实验，以分析裂隙结构对煤体应力敏感性的影响。根据煤体裂隙结构量化特征及渗流实验结果得出应力-裂隙-渗流的计算模型。最后，利用现场开采过程中实测的裂隙发育、应力以及渗透率变化数据对计算模型进行修正及验证，得出重复采动煤体裂隙结构对应力敏感性影响特性，为掌握煤层群开采瓦斯渗流演化特征提供理论基础。

煤层群卸压开采中，前期煤层开采将对未采煤层造成多次采动作用，使煤层裂隙结构发生变化，影响卸压煤层瓦斯抽采效果。在已有的保护层开采应力路径和保护层煤体应力分布规律基础上，提出煤层群多保护层开采情况下，不同采动影响强度的重复加卸载应力路径。在此基础上，通过现场钻样取芯制备烟煤试样，在自主研发受载煤体注气驱替瓦斯测试仪上开展重复采动物理模拟实验。通过理论分析等方法，研究分析高应力集中重复采动应力作用下裂隙结构的定量表征方法，探究重复采动过程中重复采动次数、采动煤层瓦斯压力、重复采动强度与煤体渗流应力敏感性的关系，以及不同应力指标对应的渗流敏感性；并对煤体基本裂隙结构，和重复采动发育裂隙结构的渗流应力敏感性进行讨论。主要取得了以下研究成果：.（1）分析和归纳现场保护层开采实测采动应力分布规律，总结提炼得到近距离煤层群采动应力路径；通过改变气体压力、重复采动次数、重复采动强度，得到针对不同重复采动参数下的实验研究方案。.（2）重复采动循环加卸载实验前后煤体裂隙分布差异较大，裂隙的阈值分割采用平均灰度值法和深度学习分割的组合方法能够取得非常好的分割结果。并成功获得了煤体裂隙结构的三维重构模型。煤体受重复采动应力影响破裂前后的内部裂隙网络均具有良好的自相似性。破裂前后的裂隙网络的分形数增加。.（3）随着轴向偏应力升高，煤体渗流轴向偏应力敏感性先降低后升高。煤体渗流有效应力敏感性存在临界值，有效应力低于临界值时煤体有效应力敏感性较强，高于临界值时较弱。重复采动应力路径不会明显改变孔隙压力敏感性，但轴压及围压升高会使渗流孔隙压力敏感性整体降低。.（4）在重复采动应力路径下，倾斜裂隙煤样表现出的应力敏感性最大，水平裂隙煤样次之，竖直裂隙煤样最小。对于采动作用下煤体发育裂隙结构而言，剧烈采动影响后的煤体渗流有效应力敏感性更高；多采动作用次数的煤体渗流有效应力敏感更低。