含瓦斯构造煤多重剪切损伤演化机制及对渗透率的控制作用

Tectonically deformed coal is widely distributed in the major mine areas of China, which is characterized by low strength, poor permeability, huge difficulty of gas drainage and serious coal and gas outburst dangers. Mining-induced unloading damage expansion is the technical direction to enhance permeability of tectonically deformed coal, and whose multi-shear damage evolution mechanism controls the permeability evolution, however, the targeted research is insufficient. To implement the project, theoretical analysis, laboratory experiments, numerical simulation and field verification will be used. The mesoscopic spatial characteristics of multi-shear fractures will be studied to analysis the cutting action on coal matrix blocks, and to illuminate the multi-shear damage evolution characteristics and its mechanical mechanism. Based on this, deduce the multi-shear damage constitutive model of tectonically deformed coal. Then the permeability evolution law along with multi-shear damage will be studied to reveal the control effect of multi-shear damage on permeability. Deduce the permeability model which is with multi-shear damage as the main control parameter and the coupled model of stress field, damage field and seepage field, and analysis the dynamic characteristic and seepage laws of the geostress relieved methane migration field. Finally, verify the results by the engineering practice. The research will be able to provide a theoretical basis for unloading damage enhanced permeability and efficient gas drainage of tectonically deformed coal.

构造煤在我国主要矿区普遍存在，具有强度低、渗透性差、瓦斯抽采困难和突出灾害严重等特点，卸荷损伤扩容是构造煤增透的技术方向，其多重剪切损伤机制控制着渗透率的演化特征，但相关理论研究尚不完善。项目将采用理论分析、实验室试验、数值模拟及工程验证相结合的方法，研究构造煤多重剪切裂隙的空间细观发育形态，分析其对煤基质的切割作用，阐明多重剪切损伤演化特征与力学机制，构建多重剪切损伤本构模型；研究构造煤渗透率演化对多重剪切损伤演化的响应特征，揭示多重剪切损伤演化对渗透率的控制作用，构建以多重剪切损伤变量为主控参数的渗透率演化模型及卸压瓦斯抽采“应力场-损伤场-渗流场”多场耦合模型，定量分析构造煤卸压瓦斯流场的动态特征与渗流规律，并结合工程实践进行验证。研究工作将为构造煤卸压损伤增透及高效瓦斯抽采提供理论基础。

构造煤是指煤层中分布的软弱分层，在我国主要矿区普遍存在，其渗透率一般仅为10-4~10-3 mD，同时具有低强度与高吸附能力的特点，瓦斯抽采十分困难，煤与瓦斯突出灾害严重。对于低渗构造煤，通过卸压增透强化瓦斯抽采是消除煤与瓦斯突出危险性的可靠手段。项目采用理论分析、实验室试验、数值模拟及工程验证相结合的方法，首先基于μCT研究了不同煤阶构造煤基质尺度演化特性，提出基质和裂隙图像分割优化方法，即在最大信息熵阈值分割的基础上，采用倒角距离变换模块和极值合并模块将二值图像重新生成为灰度图像，基于这些灰度切片图像计算分水岭分割线，结合用于渗透率试验煤样的几何特征、质量及视密度，提出以 100nm 作为孔裂隙结构的分界点。根据煤样体积应变以及裂隙体积应变的分析可知，煤体损伤扩容渐进破坏的过程就是煤体内部裂隙逐渐压密、尖端应力集中引起原生裂隙扩展、新生裂隙生成、进而发展、连接、贯通并形成宏观破坏面的过程。当煤体所处的应力状态达到损伤扩容屈服条件后，煤体开始塑性变形，渗透率会急剧上升，呈近似线性增长，构建煤岩全应力应变过程中渗透性演化模型，修正双重孔隙煤体瓦斯运移过程中的气固耦合模型。进一步提出一种水力造穴卸荷增透效果定量评价方法，等效渗透率表征水利冲孔后煤层渗透率分布与真实渗透率煤层进行瓦斯抽采，抽采相同时间的瓦斯流量、累计瓦斯抽采量和瓦斯压力都基本相同，即等效渗透率和真实渗透率表达的煤层具有相同的瓦斯抽采效果。最终，运用COMSOL软件，嵌入建立的气固耦合模型，分析了煤层透气性系数测定影响因素，为未来径向流量法测定煤层透气性系数优化奠定基础。