薄煤层水力压裂裂缝网络传播规律及其控制

Thin coal seams at shallow depths account for an important share of the total coal resources in China. To address the issue regarding the difficult extraction of the thin coal seams, we propose the integrated in-situ manless fluidized mining, which enables operations, such as mining, transporting, sorting, refining, and backfilling, to be completed in the underground. Used in conjunction with other methods, hydraulic fracturing is a viable approach for transforming coal to pumpable particles on site. The proposed study focuses mainly on how the coal seam can be fractured uniformly and thoroughly with hydraulic fracturing. First, we attempt to develop a hydraulic fracturing model to investigate the critical factors controlling the propagation of the hydraulic fractures in coal seam, floor strata, and roof strata. Second, we intend to study the mechanism of fracture initiation and propagation, understand how pre-existing fractures couple or coalesce with fracturing-induced fractures, and make clear the effect of coal-rock interface and seam thickness on hydraulic fracture network (HFN) propagation. Third, we strive to explore the evolution of the stress field in the rock mass over the entire course of hydraulic fracturing thereby revealing the fracturing-induced stress perturbation. Finally, we intend to optimize the parameters used in coal seam hydraulic fracturing, establish an evaluation system for hydraulic fracturing in coal seam, and reveal the mechanism for the control of HFN propagation in the thin coal seam. The proposed study is part of efforts to realize controlled fracturing and integrated in-situ manless fluidized mining. Also, the results of this study hold good prospects for the application of hydraulic fracturing in top coal breaking, seam permeability enhancement, and gas desorption.

针对我国浅部薄煤层资源丰富和开采困难的问题，提出通过水力压裂结合其他辅助手段将煤体原位体积改造为可管路输送的颗粒，在井下完成煤炭的采、运、选、充等工作，实现薄煤层少/无人化的绿色原位混液流态化开采的思路。本项目聚焦如何实现煤体充分、均匀破裂方面的基础研究，拟通过构建“顶板-煤层-底板”控制条件下煤体水力压裂模型，分析压裂裂缝传播的主控因素；研究压裂裂缝的萌生、扩展及其与天然裂缝的耦合贯通过程，以及煤岩交界面和煤厚变化对裂缝网络扩展的影响。探讨水力压裂过程中围岩应力场的演化规律，揭示煤体压裂的应力扰动机制；对煤体压裂工艺参数进行优化，建立压裂效果评价指标和体系，揭示薄煤层水力压裂裂缝网络的传播规律并提出其控制方法。预计研究成果可为煤体受控破裂、实现混液流态化开采奠定基础，在煤层压裂增透解吸瓦斯、顶煤破碎等相关领域也具有良好的应用前景。

本项目主要聚焦于通过以水力压裂技术为主实现薄煤层煤岩体充分、均匀致裂方面的基础研究。考虑薄煤层的“顶板-煤层-底板”的工程控制条件，分析了煤体天然裂缝、煤岩体力学性质差异、煤岩分界面、顶底板围岩等开采技术因素对压裂效果的影响。针对含天然裂隙岩石受载后裂隙扩展的发育规律和影响因素开展研究，对充分认识煤岩体水力压裂裂缝扩展和影响因素提供了借鉴。基于水力压裂穿越煤岩的客观条件，结合典型煤岩体致裂工程案例，针对此条件下水力压裂主要工艺参数对裂缝扩展、贯通过程及压裂范围、应力振动的影响，研究并实践了水力压裂工艺参数设计破裂煤岩的工程实践，取得了良好的技术效果，为探讨煤岩分界对水力压裂的影响提供了依据。以典型工程地质条件下的“顶板-煤层-底板”岩性组合模式为基础，通过SCB试验方法，建立了煤系岩层I/II混合型断裂试验研究模型，分析了煤系顶底板岩层不同岩性的裂纹扩展敏感性，为层状组合岩体水力裂缝起裂准则的理论建模和煤系层状组合岩体水力裂缝扩展规律的数值计算研究提供了基础依据。开展了人工切缝对改造煤岩体压裂效果的实践研究，取得了良好的效果。同时依托本课题，发明了一种基于示踪技术的水-氨气复合压裂岩石的试验方法，改进升级了水力压裂地应力原位测试方法及配套设备。这些研究成果为煤岩体受控破裂、实现混液流态化开采奠定了基础，还进一步丰富与完善了煤层瓦斯增透、顶煤破碎等领域的理论与工艺体系。