深部煤岩体三维微细观孔裂隙结构表征及其渗透机理研究

Based on the fundamental scientific problems such as theoretical modeling of damage and permeability evolution of surrounding rock and coal in front of goaf under the multi-field coupling conditions such as high in-situ stress and high osmotic pressure in the condition of the deep coal mining in China, this project aims at studying the micro- and meso-structure characterization and permeability mechanism of 3D pore-fracture networks of deep coal and rock. In this study, in order to realize the quantitative characterization of pore-fracture networks under micro- and meso-scale in deep coal and rock, the evolution of elastic parameters, porosity, fractal dimension and permeability of coal with deformation under different confining pressures are investigated using CT scanning, acoustic emission, mercury intrusion and seepage experiments, combined with fractal self-similarity law. The damage variables of elastic-plastic brittle materials are introduced to establish the relationship model between damage and permeability of mining-induced coal and rock, and the microscopic physical and mechanical mechanism of coal and rock permeability is revealed from the perspective of fracture damage. What’s more, with the help of CT digital three-dimensional reconstruction technology, a numerical model of microscopic structure of coal and rock is established to simulate the mechanical experiment and seepage experiment of deep coal and rock combined with the relationship model between damage and permeability. The numerical simulation reveals the failure mechanism, damage evolution law of coal and rock and the distribution law of seepage field (pressure field, velocity field and gas flow field) under the condition of deep mining, which desires to provide a certain theoretical basis for the analysis of multi-field and multi-phase coupling, and for prevention and control of disasters such as deep gas outburst and water inrush.

本研究课题以我国深部煤炭开采为工程背景，针对其中面临的高地应力、高渗透压等条件下采空区围岩及其前方煤体损伤与渗透率演化的理论建模等基础科学问题，开展深部煤岩体三维微细观孔裂隙结构定量表征及渗透机理研究。本课题拟通过CT扫描、声发射、压汞和渗流实验等手段，结合分形自相似尺度定律，分析深部应力条件下煤岩体弹性参数、孔隙度、分形维数和渗透率随应变的演化规律，实现微细观尺度下深部煤岩体孔隙-裂隙网络的定量表征；引入弹塑脆性材料的损伤变量建立采动煤岩体损伤和渗透率的关系模型，从裂隙损伤角度揭示煤岩体渗透的微细观物理力学机理。进一步，基于CT三维重构建立煤岩体微细观结构的数值模型，嵌入损伤与渗透率关系模型进行渗流数值模拟研究，揭示深部开采条件下煤岩体损伤演化规律以及渗流场（压力场、速度场和瓦斯流场）分布规律，为深部煤炭资源开发中多场多相耦合分析以及深部瓦斯突出、突水等灾害防治等提供一定的理论基础。

本项目充分运用压汞技术、CT扫描技术、核磁共振（NMR）成像技术、声发射（AE）技术和分形几何理论知识，通过大量的精密测试、系统分析及数值模拟，较为详细地研究了深部煤岩体（煤体和顶板砂岩）中不同测量精度下的孔裂隙结构特征以及常规单轴、三轴压缩和分级循环加卸载（深部煤炭开采过程中煤岩体所处的应力状态）作用下的孔裂隙结构（损伤）演化规律及演化机理；进一步，基于分数阶导数的低渗岩石瞬态法渗透率计算方法得到了分级循环加卸载作用下渗透率的演化规律，并分别基于渗透率损失率(PLR)和不可恢复的渗透率损失率(IPLR)对渗透率演化机理进行了研究。结果表明：（1）深部煤体中吸附孔隙的体积分数超过50%，但是其与渗透孔间连通性较差；渗透孔相对不发育，但是与裂隙之间具有一定的连通性；顶板砂岩内部孔隙分布较均匀，只存在渗透孔和微裂隙；大孔、微裂缝贡献了煤岩体几乎所有的渗透率。（2）单轴及三轴压缩过程中，煤岩体中吸附孔的分形维数DA随轴向偏应力的增大表现为先小幅度下降后稳定增长的规律；渗透孔、大孔及裂隙的分形维数DSF随轴向偏应力的增加呈先小幅度下降再稳定增长，最后再小幅度下降的规律。（3）分级循环加卸载渗流实验过程中煤岩体初始局部裂隙损伤是由于加载的偏应力导致的，但是裂隙的贯通过程主要是由于煤岩体内部孔隙水压力的作用引起的。（4）AE时间-空间分布的关联维数的变化情况可以反映煤岩体损伤演化的过程，当关联维数不变或者增大时，煤岩体处于安全的稳定状态；当关联维数开始下降时，煤岩体将要破坏，形成局部裂隙；当关联维数持续下降时，煤岩体局部裂隙将贯通成宏观裂隙。（5）煤岩体在不同偏应力下和静水压力下的渗透特性可以分别通过偏应力下的PLR和塑性应变产生的IPLR很好地表征。PLR在峰前阶段随偏应力呈先增大后减小的二次曲线趋势，峰后阶段呈现线性负相关；IPLR与塑性应变之间具有较好的线性关系。本研究对于煤与瓦斯突出、围岩突水防治以及煤层气开采具有重要意义。