水力压裂诱导含瓦斯煤体动态损伤与瓦斯运移机理研究

Hydraulic fracturing underground to improve the efficiency of gas drainage is an important method for preventing gas disaster of coal seam. Mechanism on hydraulic fracturing during the water injection process was focused on nowadays, and the water keeping and dewatering process were not inferred in literature. However, the water keeping and dewatering process have an important impact on distribution of damage and the gas concentration characteristic, thus affect efficiency of gas drainage. Therefore, this application proposed that the water keeping and dewatering process are of the same importance as the water injection process, and the mechanism of dynamic damage and gas migration induced by hydraulic fracturing should be studied with the method of acoustic emission monitoring and digital microscopic. The characteristic of gas concentration and migration will be analyzed and control mechanism of hydraulic fracturing on coal seam gas field will be revealed in this research. The flow model of gas in damaged coal will be obtained. This research is of great importance for hydraulic fracturing to preventing gas disaster of coal seam.

水力压裂卸压增透是煤层瓦斯治理的重要手段，对水力压裂机理的研究目前多侧重于高压水楔入，针对“保水”和“排水卸荷”研究不够深入，保水和排水卸荷直接影响到煤体损伤分布及瓦斯富集特征，进而影响到卸压增透效果。本项目以煤体动态损伤和瓦斯运移规律为研究主线，针对水力压裂的“高压水楔入”、“保水”和“排水卸荷”三个阶段，采用实验室实验、数值模拟和理论分析相结合的方法，借助于声发射、数字显微等测试分析技术，研究揭示水力压裂三个阶段含瓦斯煤体的动态损伤规律及机理，得到水力压裂过程瓦斯富集及压裂后裂缝分布特征，建立压裂损伤煤体的瓦斯渗流控制方程，揭示水力压裂过程瓦斯运移规律及水力压裂对煤层瓦斯场的控制机理。本研究对于水力压裂技术进行深部瓦斯治理具有重要的科学意义和应用价值。

井下水力压裂是很有潜力的煤层卸压增透技术，具有压裂范围大、卸压增透效果明显的特点，而且能大幅降低抽采钻孔施工量、降低成本。水力压裂将水压入煤层会导致含瓦斯状态下的煤体发生损伤，同时驱赶瓦斯导致瓦斯在煤层内重新分布，但目前对这两个方面的研究还不够深入。鉴于此，本项目采用多种研究手段相结合的方法，针对水力压裂过程含瓦斯煤体的动态损伤特征及瓦斯的运移机理开展了系统研究。构建了含瓦斯煤岩水力压裂试验系统，实现了对不同地应力、不同瓦斯压力条件下煤岩体的水力压裂，同时能够对水力压裂过程声发射、孔壁应变及视电阻率的实时监测。提出了通过监测压裂孔孔壁应变研究煤岩体动态损伤的方法，并开展了含瓦斯煤岩水力压裂试验，发现了压裂孔承压破坏过程的孔壁应变规律，得到了注水流量与压力孔破坏的相关性；基于孔壁应变特征，提出了压裂孔起裂三阶段模型，即水气作用诱导微损伤形成阶段、局部损伤带形成阶段及试件失稳破坏阶段。分析了瓦斯在水力压裂过程中的作用，发现瓦斯压力增大了水压致裂阻力，提高了起裂压力，同时高压水驱气作用诱导水流进入、激活了原生裂隙。开展了工程现场的水力压裂增透试验，通过取样测试的方法发现了煤层压裂后的水分与瓦斯呈现明显的分区特征，水分与瓦斯的富集特征规律明显相反，含水量大的区域内瓦斯少，而水分低的区域内瓦斯含量高。结合瓦斯与水分的云图分析，揭示了高压水楔入、保水及排水卸荷阶段的瓦斯运移机制。