基于相场法的深层页岩压裂流-固-热耦合裂缝扩展数值模拟研究

Shale gas exploitation in China is marching towards the deep formation, hydraulic fracturing is the key technology for the deep shale gas exploitation. With the characteristics of high temperature, rich in natural fractures and complex in-situ stress, deep shale fracturing involves the mechanism of heat transfer between low temperature fracturing fluid and high temperature formation, the influence mechanism of temperature change on ground stress and fluid properties, and the damage mechanism of complex tensile and shear stress on natural fractures. At present, the understanding of these mechanisms is not clear, leading to a lack of pertinence in the optimization of fracturing parameters. Thus results in the problems of easy screening-out and unclear fracture geometry. Numerical simulation is the major method to study the fracture propagation of hydraulic fracturing. The commonly used simulation methods such as extended finite element method have problems such as difficult in tracking fracture path, thus cannot effectively simulate complex fracture propagation. The phase field method can automatically compute the optimal propagation direction, which has inherent advantages for simulating complex fracture geometry. Therefore, according to the aforementioned mechanism of deep shale hydraulic fracturing, this project establishes the rock damage model considering shear failure with phase field method, the stress distribution model and fluid pressure model considering temperature influence. Then couples those models to establish the hydraulic fracture propagation model with phase field method. Studies the fracture propagation mechanism and complex fracture propagation pattern at the influence of thermally effect and shear failure, and provides theoretical guidance for improving the deep shale hydraulic fracturing.

我国页岩气开采正加速向深层进军。深层页岩具有储层温度高、天然裂缝发育和地应力复杂的特征，在水力压裂过程中涉及低温压裂液与高温地层的传热机理、温度变化对地应力和流体性质的影响机理、张剪复杂应力对天然裂缝的损伤机理。当前对这些机理认识不清，导致压裂施工参数优选缺乏针对性，施工中出现了易砂堵和裂缝形态认识不清等问题。本项目基于深层页岩地质特征，通过实验获取压裂液流变性和页岩力学参数随温度变化规律；根据实验结果，建立考虑热效应的地应力和流体压力模型；推导裂缝扩展的弹性能演化泛函、耗散势泛函和外力功泛函，利用变分原理得到考虑剪切破坏的岩石损伤演化相场法模型；基于损伤、应力和流体压力模型，建立深层页岩水力压裂流-固-热耦合裂缝扩展相场法模型。揭示热效应和剪切破坏因素影响下的天然裂缝损伤机理和复杂裂缝扩展规律，形成促进深层页岩形成复杂缝网的压裂参数优化技术，为提高深层页岩水力压裂改造效果提供理论指导。

我国深层页岩储层温度高、天然裂缝发育且地应力复杂，高温页岩岩体在低温热冲击作用下的裂缝扩展机理极为复杂。准确模拟深层页岩水力压裂裂缝扩展过程对提高水力压裂设计效果有重要意义。本项目紧密围绕深层页岩水力压裂裂缝扩展机理进行研究，开展了页岩低温热冲击实验研究，发现低温冲击对岩石结构产生损伤，导致微裂纹产生，显著提升岩石渗透率；同时对岩石强度和刚度造成损伤，且损伤效果随着接触温差的增加而加强。通过建立温度-压力耦合井筒流动换热模型、地下双重介质动态温度场控制方程、考虑热应力影响的地下岩体动态应力场控制方程，形成了质流-固-热多物理场耦合流动模型。基于变分和泛函，建立了考虑张剪破坏的相场法岩体损伤模型，多个模型耦合形成了流-固-热耦合的相场法深层页岩裂缝扩展模型。通过C++语言编程形成裂缝扩展模拟程序。基于建立的流-固-热耦合水力压裂裂缝扩展模型，揭示了低温冲击作用下的均质页岩和含天然裂缝页岩的岩体温度、应力分布和变化机理；分析了低温冲击对裂缝性页岩中裂缝扩展规律，并对多簇裂缝扩展下的裂缝扩展影响因素进行了分析，优化了施工参数。利用研究成果在威远深层页岩气开展了20余口井的压裂方案设计，平均压后测试产量前期提高25.7%，增产效果显著。研究成果发表标注本基金的论文5篇；授权中国和美国发明专利5项；授权软件著作权1件；参加国际学术会议2次；培养博士研究生2名，硕士研究生4名。