页岩压裂微观裂缝起裂扩展及其增渗机理研究

The reserves of shale gas in China are large with deep dury. The permeability of shale formation is very low, so hydraulic fracturing technology is needed to stimulate shale reservoir to form complex fracture networks for effective development. The existing researches on fracture propagation mechanism in shale by fracturing mainly focused on macro-scale, while, micro-scale and nano-scale pores in shale matrix are the main effective reservoir spaces of shale gas. Therefore, it is urgent to carry out the research on crack propagation mechanism in shale by hydraulic fracturing treatment at micro scale. This project intends to combine experimental tests, theoretical analysis with numerical simulation to do this research. Uniaxial compression tests were combined with CT scanning tests for anisotropic shale cores to capture the initiation and propagation process of fractures at micro-scale, establish the damage constitutive equation, and analyze the evolution mechanism of cracks at micro-scale. In addition, the discrete element method (DEM) was employed to explore the influences of heterogeneity on the fracturing results. SEM and FIB-SEM were used to acquire and analyze the morphological parameters, propagation modes and connectivity mechanism of micro-cracks. Digital cores of micro-nano scale pore structure of shale before and after fracturing were established. Based on the Lattice-Boltzmann method (LBM), seepage simulations of shale before and after fracturing were realized. The permeability enhancement mechanism by the connection of micro crack was revealed. The research can provide theoretical basis for fracturing evaluation and design for shale gas reservoir.

我国页岩气储量大，埋藏深，渗透率极低，需要水力压裂技术对页岩储层进行改造形成复杂的裂缝网络实现有效开发。目前对页岩在压裂作用下的裂缝扩展机理研究基本局限于宏观尺度，而页岩基质内的微纳孔隙是页岩气主要的有效储集空间，因此亟需开展页岩在压裂作用下的微观尺度裂缝扩展机理方面的相关研究。本项目拟通过实验测试、理论分析和数值模拟相结合的方式，探索页岩压裂微观裂缝的起裂扩展及其增渗机理。利用微纳CT结合内置单轴压缩试验机进行各向异性页岩单轴压缩CT实时扫描试验，获取微观裂缝起裂扩展过程，建立损伤本构方程；采用离散元方法，基于损伤本构方程，探索页岩非均质性对压裂结果的影响；结合SEM及FIB-SEM获取分析微观裂缝的形态特征参数，扩展模式和连通机理，建立压裂前后页岩微纳孔隙结构数字岩心；基于格子玻尔兹曼方法，实现压裂前后页岩微纳尺度渗流模拟，揭示微纳裂缝连通增渗机制，为页岩储层压裂评价设计提供理论依据。

我国页岩气储量大，埋藏深，渗透率极低，需要水力压裂技术对页岩储层进行改造形成复杂的裂缝网络实现有效开发。目前对页岩在压裂作用下的裂缝扩展机理研究基本局限于宏观尺度，而页岩基质内的微纳孔隙是页岩气主要的有效储集空间，因此亟需开展页岩在压裂作用下的微观尺度裂缝扩展机理方面的相关研究。本项目拟通过实验测试、理论分析相结合的方式，探索页岩压裂微观裂缝的起裂扩展及其增渗机理。利用微纳CT结合内置单轴压缩试验机进行各向异性页岩单轴压缩CT实时扫描试验，获取微观裂缝起裂扩展过程；通过龙马溪海相页岩的水力压裂试验，研究了页岩的各向异性，围压，偏应力和注入速率等试验因素对压裂结果的影响，探讨了各向异性海相页岩水力裂缝的扩展机理和破裂机制；建立了水力裂缝宽度的计算公式, 研究了裂缝宽度在压裂过程中的演化过程；结合SEM及FIB-SEM获取了微观裂缝的二维及三维图像信息，分析微观裂缝的形态特征参数，总结了微观裂缝的四类扩展模式，揭示了微观裂缝与矿物、有机质等的连通机理，建立了压裂前后页岩微纳孔隙结构数字岩心；揭示了微纳裂缝连通增渗机制，为页岩储层压裂评价设计提供理论依据。