页岩气储层气压致裂中的流固耦合力学机理研究

The shale gas has occupied a significant share in the world energy structure as an important clean energy. However, the shale gas reservoirs in China are characterized by low porosity, low permeability, great depth and poor physical property which lead to the great dependence of the effective fracturing measures for the exploitation efficiency of shale gas. Regarded as the conventional and artificial technology for permeability enhancement, the hydraulic fracturing influences the permeability of reservoirs and requires a large amount of water due to the impact of water block, Jamin effect, Anti discharge of fracturing fluid, and water-swell clay. At present, to use the gas instead of water as the fracturing material has been successfully applied for the stimulation operations in several gas fields of North America, while the fracturing mechanism of gas still needs further study. In order to research the pivotal mechanical problems for gas fracturing of shale gas reservoirs which consist of the anisotropic to shale deformation and failure, the permeation and diffusion law in the shale for different gases, the damage rupture law and fractures extension law of shale under different stress conditions, the multi-field coupling mathematical model for gas fracturing in the shale gas reservoir has been constructed, and the numerical simulation tools for quantitative forecast and analysis of the gas fracturing effect in the shale has been developed in the project through experimental and theoretical research methods. As a consequence, the project research results will provide an important theoretical support for the development of shale gas reservoir transformation technology and the improvement of shale gas recovery efficiency.

页岩气作为重要的清洁能源已经在世界能源结构中占有重要的比重。中国页岩气储层具有典型的低孔、低渗特征，且埋深大，物性较差，导致其采收率极大地依赖于有效的压裂措施。水力压裂是人工增渗的常用技术，但存在产生水锁和贾敏效应、压裂液反排、粘土吸水膨胀等问题，影响渗透性提高，且需要大量的水资源。目前国际上开始采用气体代替水作为压裂介质，并在北美多个气田成功实施了增产作业，但其致裂机理尚需进行深入的研究。本项目在多年地下岩体多物理场耦合研究的基础上针对我国页岩气储层气体压裂中的关键力学问题：页岩变形破坏的各向异性特征；不同气体在页岩中的渗流扩散规律；不同应力条件下页岩的损伤破裂规律和裂隙扩展规律等进行深入研究，建立页岩气储层中气压致裂的多场耦合作用数学模型，并开发能够定量预测和分析页岩中气压致裂效果的数值模拟工具。研究成果将为页岩气储层改造技术发展和提高页岩气采收率提供重要的理论支持。

本项目采用实验与理论相结合的研究手段针对我国页岩气储层气体压裂中的关键力学问题开展了深入研究。项目开展的主要研究包括：（1）开展了页岩各向异性力学特性实验，研究了层理对页岩物理力学特性的影响，同时考虑液氮的冷却作用，研究了不同储层温度下页岩试样液氮作用后的拉伸、压缩特性和渗透率演化规律。（2）开展了高温高压三轴条件下页岩注气压裂实验，研究了页岩中气体的渗透扩散规律，给出了不同应力条件、不同压裂气体、不同储层温度条件下页岩破裂压力的变化规律。同时开展了液氮预注后的氮气压裂实验，并研究了液氮预注时间、岩样温度和围压等对破裂压力的影响。（3）利用三维数字扫描系统和电镜扫描等手段，研究了页岩气压致裂后裂隙的宏细观特征，分析了液氮预注后再接氮气压裂的机理和裂隙扩展特征。（4）研究了页岩储层气压致裂中应力场、渗流场及温度场的全耦合作用，并考虑损伤对岩石弹模、热传导系数和强度的弱化作用及孔隙率和渗透率的增强作用，建立了考虑围岩损伤演化作用的钻孔围岩热-流-固耦合数学模型。项目研究取得的主要成果包括：（1）通过开展高温三轴条件下页岩试样气体压裂实验，发现了液氮注入能大幅降低氮气压裂的破裂压力，同时得到了液氮注入时间、储层温度、围压和层理对破裂压力的影响规律。（2）通过研究岩样破裂面宏细观形貌特征，发现了液氮注入能提高压裂岩样破裂表面粗糙度，得到了岩样破裂表面粗糙度和细观断裂模式的联系，揭示了液氮注入后页岩气储层压裂的破坏机理。（3）提出了一种将液氮注入页岩气储层后再进行氮气压裂的无水压裂技术，液氮注入使得压裂效果明显改善，从而提高页岩气产量，并有效解决了水基压裂液造成的水锁效应和环境污染问题。（4）建立了考虑围岩损伤演化的热-流-固耦合模型，并实现了液氮预注后氮气压裂的数值求解，得到了液氮注入对不同储层温度、不同应力状态、不同层理倾角下页岩气储层氮气压裂损伤演化规律的影响。