深部煤层水力压裂煤体裂隙演化及其对瓦斯渗流的控制机理

In the deep coal seam, the mechanisms of permeability increasing by hydraulic fracturing are inadequate and the effects have uncertainties. According to this problem, in the gas contained coal body around the hydraulic fracturing hole, the evolution behavior of fractures and changing behavior of permeability during the fracturing process were analyzed theoretically through the acoustic emission test and permeability test. The evolution model of coal body fracture was also constructed. Then, using the theory of rock mechanics, fluid mechanics in porous medium and solid-fluid coupling principle, the dynamic changing equation of permeability and the solid-fluid coupling numerical model of gas and coal during the gas extraction after hydraulic fracturing were constructed. The process of hydraulic fracturing and coal seam extraction were also simulated using the three dimensional physical simulation experiment device of simultaneous extracting of coal and methane. Combined with the numerical simulation results and field monitoring results of coal body fractures, coal seam stresses and gas flow under the process of gas extracting of hydraulic fracturing hole, the evolution behavior of coal body fracture, gas seepage behavior and the coupling relations between them were studied under different coal mechanical characteristics, gas pressure and ground stress. It will reveal the control mechanism of coal body fractures evolution on gas seepage under the process of hydraulic fracturing. This work would provide theoretical basis to the effects improvement of hydraulic fracturing for gas extraction and the safety production of coal mines.

针对深部煤层水力压裂增透机理不完善，增透效果不确定等焦点问题，以水力压裂孔周围含瓦斯煤体为研究对象，通过声发射试验和渗透性试验，对水力压裂过程中煤体裂隙演化规律和渗透性变化规律进行理论分析，构建水力压裂煤体裂隙演化模型。运用岩石力学、多孔介质渗流力学及固流耦合原理等理论建立水力压裂孔抽采瓦斯过程中含瓦斯煤渗透率动态变化方程和瓦斯固流耦合数学模型。应用煤与瓦斯安全共采三维物理模拟实验装置对水力压裂增透抽采煤层瓦斯进行物理模拟，并结合数值模拟结果和现场水力压裂增透抽采瓦斯过程中煤体裂隙、应力及瓦斯监测结果，研究不同煤体力学特性、不同瓦斯压力和不同地应力条件下水力压裂孔周围煤体裂隙演化规律、瓦斯渗流规律及其耦合关系，揭示深部煤层水力压裂煤体裂隙演化对瓦斯渗流的控制机理，为提高深部煤层水力压裂增透抽采瓦斯效果和保障煤矿安全生产提供理论依据。

瓦斯既是煤矿的“第一杀手”，又是一种不可再生的清洁能源，瓦斯高效抽采是瓦斯灾害治理和资源利用的主要途径。我国煤层受多期构造史的影响，煤层赋存地质条件复杂、瓦斯压力大、瓦斯含量高、渗透率低，瓦斯抽采非常困难，煤层瓦斯安全高效抽采一直是我国煤炭工业可持续发展面临的重大课题。水力压裂增透技术是增加煤层渗透性、提高瓦斯抽采效果的有效措施，本项目以水力压裂孔周围的含瓦斯煤体为研究对象，综合运用岩石力学、多孔介质渗流力学、损伤断裂力学和计算数学等理论，采用理论分析、实验室试验、数值模拟、物理模拟与现场工程试验相结合的研究方法，对深部煤层水力压裂增透抽采瓦斯实践中煤体裂隙演化及其对瓦斯渗流的控制机理进行了深入研究。.通过单轴抗压试验、劈拉试验和角模压剪试验，获得了煤的抗压强度、抗拉强度、内聚力和内摩擦角等力学性质；通过渗透性试验和声发射仪信号采集试验，获得了煤样在不同围压时破裂过程中的渗透性变化规律、声发射特征和在固定围压、轴压的情况下逐步减小孔隙水压力时的渗透性变化规律；基于煤样的三轴压缩渗透性试验结果，构建了煤体破裂过程中的渗透率-损伤演化方程、煤体裂隙闭合时的渗透率-应力方程和含瓦斯煤渗透率动态变化方程，运用岩石力学、多孔介质渗流力学及固流耦合原理等理论，建立以煤岩体变形场控制方程和瓦斯渗流场控制方程为基础的水力压裂孔抽采瓦斯固流耦合数学模型；利用多场耦合煤层气开采物理模拟实验装置开展了不同地应力条件和不同煤体抗压强度条件下水力压裂孔增透抽采瓦斯的全过程进行物理模拟试验；采用RFPA数值模拟软件研究了不同地应力和不同煤体抗压强度条件下煤层水力裂隙的演化规律，通过实验室试验结果、物理模拟试验结果和现场水力压裂增透措施实施前后观测到的瓦斯数据进行对比研究，获得了不同煤体力学特性和不同地应力下煤体裂隙演化规律、瓦斯渗流规律及其耦合关系，揭示了深部煤层水力压裂煤体裂隙演化对瓦斯渗流的控制机理，为水力压裂增透抽采瓦斯工程实践提供理论依据。