煤体脉动水力压裂离散裂缝网络演化及碎裂规律研究

In-situ treatment of coal towards pumpable flowing mediums by fracturing is one of the critical technologies associated with fluidized mining of underground solid coal resources. To address the challenge of transforming in-situ coal mass into pumpable flowing mediums, this study proposes to fracture the coal by using the pulse hydraulic fracturing (PHF) method. This study delves deep into PHF-induced fracturing, with a special focus on coal fragmentation in control. Using the typical coal mass containing naturally occurring fractures as the research object, this study intends to construct a large-size true axial PHF model. The model will be employed to carry out a series of studies. First, make clear the microscopic role of the pulsating pressure wave in fracture propagation and fracturing the coal mass. Second, explore the mechanism of interaction between the hydraulic fractures and naturally occurring fractures with the pulsating pressure at play. Third, reveal the dynamic evolution of the discrete fracture network in the coal mass. Fourth, analyze the multi-factor synergistic effect of pulsating load on coal fragmentation, and clarify the relationship between fatigue fragmentation characteristics and main controlling factors. Finally, investigate the fracturing behavior of the coal mass subject to pulsating pressure. It is expected that this study can be of great scientific value for coal mass controlled fracturing and in-situ treatment. In addition, the research provide reference for permeability enhancement of coalbed methane reservoir, hard roof pre-fracturing, and top coal caving.

通过物理破碎实现煤体原位改造转化是煤炭资源流态化开采的关键技术之一。针对如何使煤体原位形成符合管路传送流态物质这一难题，基于脉动水力压裂方法，提出对煤体进行体积改造的思路。本课题聚焦如何实现煤体充分碎裂方面的基础研究，拟选取典型含天然裂隙的煤体为研究对象，构建大尺寸真三轴脉动水力压裂模型；研究脉动产生的压力波对水力裂缝扩展及煤体破坏的细观作用机制；分析脉动载荷下水力裂缝和天然裂隙的互馈贯通特征，阐明煤体离散裂缝网络动态演化规律；探讨脉动载荷多因素协同作用对煤体碎裂的影响，明确疲劳碎裂特征与主控因素之间的关系，揭示脉动水力压裂作用下煤体充分碎裂的规律。预计研究成果对煤体受控破裂、实现煤体原位改造转化具有重要的科学和工程意义，同时在煤层气抽采增透、坚硬顶板预裂以及放顶煤破碎等相关领域也有一定的参考价值。

通过物理破碎实现煤体原位改造转化是煤炭资源流态化开采的关键技术之一。针对如何使煤体原位破碎这一难题，提出了基于脉动水力压裂对煤体进行体积改造的思路。建立了天然裂隙储层流固耦合离散元模型，对水力裂缝与天然裂隙产生的互馈模式进行了分类，揭示了3种因素对离散裂缝网络形成和渗流形态的影响规律。发明了大尺寸真三轴煤岩体多场多相变频压裂试验装置，利用该装置可以对煤岩样施加真三轴应力、注入不同压裂介质、控制多物理场变化的压裂试验。提出了一种岩石数字图像脉动水力裂隙精准识别的目标检测与评价方法。实现了裂隙目标检测与像素点的单独分割，去除了兴趣区域池化层中简单粗糙的量化过程，构建了基于Mask R-CNN算法的岩石数字图像裂隙识别模型，提出了基于裂隙数量准确性的模型评价指标。通过子程序在模型中提出了评价地震活动的算法且模型参数的正确性和可靠性得到了验证。之后利用所提出的模型研究了不同工况下水力裂缝与天然裂隙相互作用产生的微震的特征。最后对应力差，侵入角及天然裂隙的摩擦系数对地震活动的影响进行了敏感性分析。提出了通过LN2循环冻融的方式来提高煤层的破碎程度。物理实验采取了AE参数方法(KDE)、分形理论、核磁共振研究了煤样破碎特征和孔隙结构演化。主要讨论的因素包括LN2冷冻时间、循环冻融次数、煤样的含水率。这些因素的变化改变了冻融煤样的物理和力学特征、孔隙结构。研究成果对煤体受控破裂、实现煤体原位改造转化具有重要的科学和工程意义，同时在煤层气抽采增透、坚硬顶板预裂以及放顶煤破碎等相关领域也有一定的参考价值。