各向异性煤岩水力压裂缝网动态扩展机理及数值仿真技术研究

The coal is dynamically fractured under high fluid pressure during the hydraulic fracturing progress. Unlike other unconventional gas reservoirs, cleats and natural fractures are well developed in a coal bed, and a complex network of fractures forms when hydraulic fracturing. The dynamic evolution mechanism of a hydraulic fracture network in an anisotropic CBM (coalbed methane) reservoir is an important theoretical basis for ensuring the rational and efficient development of CBM in China, and it is also an urgent problem to be solved in the study field of unconventional natural gas development. In this project, taking the Qinshui CBM field as an engineering background, anisotropic mechanical tests of coal under dynamic and static loading conditions are carried out firstly, and the dynamic constitutive model of coal is theoretically derived. Secondly, the extended finite element-cohesive element combination method is used to establish a hydro-mechanical-damage coupled numerical model for an anisotropic CBM reservoir, revealing the hydraulic fracture dynamic initiation and propagation mode transition mechanism in an anisotropic CBM reservoir, the influence of natural fractures and formation interfaces on the dynamic propagation of a hydraulic fracture and the shear-slip characteristics of natural fractures, and then revealing the dynamic evolution mechanism of the hydraulic fracture network in an anisotropic CBM reservoir. Finally, large-scale indoor model tests are carried out to study the initiation and propagation characteristics of a hydraulic fracture.

煤岩中水力压裂是煤岩在高压流体作用下发生动态破裂的过程。有别于其他非常规气储层，煤层中割理、天然裂隙十分发育，水力压裂时会形成十分复杂的裂缝网络。各向异性煤岩中水力裂缝网络动态扩展机理及仿真技术是现阶段我国煤层气合理、高效开发所需的重要理论和技术基础，也是非常规天然气领域亟待解决的关键科学问题。本项目拟以沁水盆地煤层气开采工程为背景，首先开展动、静荷载作用下煤岩各向异性力学试验，建立煤岩动态本构模型；其次，提出扩展有限元-粘结单元结合算法，建立各向异性煤岩中水-力-损伤耦合数值模型，揭示各向异性煤岩水力裂缝动态起裂和扩展模式演化机制、天然裂缝和地层界面对水力裂缝动态扩展的影响及天然裂缝自身的剪切滑移特征，进而揭示各向异性煤岩中水力裂缝网络的动态扩展机理；最后，结合大型水力压裂室内模型试验结果，对比研究水力裂缝的延伸特征和空间展布规律。

各向异性煤岩中水力裂缝网络动态扩展机理及仿真技术是现阶段我国煤层气合理、高效开发所需的重要理论和技术基础，也是非常规天然气领域亟待解决的关键科学问题。本项目开展了动、静荷载作用下煤岩各向异性力学室内试验，通过理论推导，建立了煤岩动态本构模型；通过扩展有限元-粘结单元结合数值算法，建立了水-力-损伤耦合数值模型，实现了对各向异性煤岩中水力裂缝动态扩展及与天然裂缝和地层界面交叉扩展全过程的直观模拟，揭示了各向异性煤岩水力裂缝动态起裂和扩展模式转变机制、天然裂缝对水力裂缝动态扩展的诱导作用机理及天然裂缝自身的剪切滑移机理；建立了三维裂缝网络系统模型，揭示了各向异性煤岩水力压裂过程中缝网的形成及动态扩展演化机制。以本项目为依托，以第一/通讯作者发表SCI论文8篇。研究成果为我国煤层气开发水力压裂的方案决策、施工设计等提供理论依据和技术支持。