考虑热流固耦合的深层页岩气藏水力裂缝扩展实验与数值模拟研究

China's deep shale gas reservoirs have great potential for exploitation, but hydraulic fracturing faces the problem of “three highs” (high temperature, high pressure and high stress). The hydraulic fracturing process in deep formations involves fluid seepage, rock deformation and heat transfer. Besides, the rock may exhibit extensive plastic deformation. However, the current developed models do not consider the comprehensive effects of thermo-hydro-elasto-plastic coupling on the propagation of fractures. Its basic scientific issue is the mechanism of hydraulic fracture propagation accounting for thermo-hydro-elasto-plastic coupling effects under the condition of “three highs”. This project intends to combine the experimental and numerical simulations. The main innovative contents and methods are as follows: (1) Carry out mechanics experiment and true tri-axial hydraulic fracturing experiment of deep shale under the condition of high temperature and high confining pressure, and clarify the mechanism of fracture propagation under that condition; (2) Establish a thermo-hydro-elasto-plastic coupling model of hydraulic fracture propagation, and develop a new efficient numerical algorithm based on extended finite element method and embedded discrete fracture model. The correctness of mathematical model and algorithm is verified by comparing the numerical results with those of fracturing experiments; (3) Apply the established model to the multi-cluster fracturing of horizontal wells in deep shale gas reservoirs. Establish a multi-field coupled model to simulate simultaneous propagation of multi-cluster fractures, and clarify the mechanism of competitive propagation between multi-cluster fractures and the mechanism of fracture network formation under the condition of multi-field coupling. Finally the project is to provide theoretical basis for hydraulic fracturing treatment in deep shale gas reservoirs.

我国深层页岩气藏开采潜力大，但压裂施工面临“三高”（高温高压高应力）难题。深层水力压裂过程涉及流体渗流、岩石变形以及热量传输，且岩石可能出现大范围的塑性应变，但目前研究模型尚未综合考虑热流弹塑性耦合对裂缝扩展的影响。其基础科学问题是“三高”条件下考虑热流弹塑性耦合效应的水力裂缝扩展机理。本项目有机结合室内实验和数值模拟开展研究，主要创新内容和方法为：（1）开展高温高围压条件下深层页岩力学特性与真三轴水力压裂实验，阐明高温高围压条件下裂缝扩展机理；（2）建立考虑热流弹塑性耦合效应的水力裂缝扩展数学模型，基于扩展有限元与嵌入式离散裂缝模型发展新的高效数值算法，通过与室内压裂实验结果对比，验证模型及算法的正确性；（3）将所建立的模型应用于深层页岩气藏水平井分段多簇压裂中，开展多场耦合多簇裂缝同步扩展数值模拟，厘清多场耦合作用下多簇裂缝竞争扩展与缝网形成机制；为深层页岩气藏压裂施工提供理论基础。

与浅层页岩气藏相比，深层页岩气藏埋深大，处于高温高压高应力的“三高”地质环境，岩石塑性特征强，导致水力压裂改造存在裂缝延伸困难、缝网复杂性低等问题，热流固耦合效应对水力裂缝扩展影响更加显著。围绕“深层页岩气藏三高条件下水力裂缝扩展机理”基础科学问题，本项目开展了室内实验和数值模拟研究，主要的研究内容和研究结果包括：（1）开展了页岩岩石力学三轴实验，分析了不同围压、不同温度条件下岩石力学变形特征；进行了页岩水力压裂实验，采用CT扫描技术观测分析了不同应力条件下压裂裂缝扩展形态；（2）建立了考虑热流-弹塑性耦合效应的深层页岩水力裂缝扩展数学模型，基于D-P塑性屈服准则和相关联流动法则，推导了考虑热流固多场耦合效应的深部岩石变形本构关系，采用嵌入式离散裂缝模型建立了渗流场和温度场控制方程；（3）提出了扩展有限元和嵌入式离散裂缝模型相结合的高效数值求解方法，采用扩展有限元求解岩体应力场，且使用牛顿迭代和返回映射方法计算塑性应变，结合固定应力分裂算法、顺序迭代法、Picard迭代法构建了多物理场耦合迭代求解算法；（4）考虑井筒内压裂液流动和不同射孔流量动态分配，建立了深层页岩气藏水平井分段多簇裂缝扩展模型，并构建了迭代求解算法；（5）阐述了岩石塑性变形和温度场变化对水力裂缝扩展的影响规律，岩石塑性应变将导致水力裂缝延伸压力升高，水力裂缝长度减小，多簇裂缝同步扩展缝间应力干扰作用增强，压裂缝网改造面积减小；低温压裂液注入储层引起基质收缩，将导致水力裂缝延伸压力降低，岩石累积塑性应变影响减弱；（6）开展了页岩气藏水平井多簇压裂优化设计研究，优化后采用非均等簇间距和射孔参数有利于多缝均衡扩展；研究成果丰富了深部岩体多场耦合理论体系，可为页岩气藏压裂开发提供理论基础和技术支撑。