页岩储层射孔簇内复杂三维多裂缝起裂与竞争扩展机理研究

Volume fracturing is one of the core technologies of shale gas exploitation. Some problems, such as high breakdown pressure, sand plugging, and un-propagated fractures in most perforation clusters, are often encountered during the volume fracturing treatment in shale gas reservoir, which seriously hampered the efficient large-scale development of shale gas. Due to the combined influences of perforation cluster parameters, treatment parameters and reservoir geological characteristics, the mechanisms of multiple three-dimensional (3D) hydraulic fractures initiation and propagation in naturally fractured shale reservoir are very complex, which need to be studied urgently for the shale gas exploitation of China. To this end, considering the coupling effects among stress, seepage, and fluid flow within hydraulic fractures, a 3D seepage-stress coupling model for naturally fractured shale is deduced and calculated based on the Abaqus software. A novel cohesive pore pressure model is developed based on the user subroutines of Abaqus, and an explicit temporal integration scheme is adopted to solve the motion and flow equations in the mechanical and hydraulic solvers, respectively. Then, the natural fractures in shale are meshed in the coupling model by an interface code, and fluid redistribution equations of the wellbore, perforation and hydraulic fractures are also introduced into the model. Finally, a complex hydraulic fracture network propagation model is established. Laboratory hydraulic fracturing experiments of 3D printed rock sample and shale outcrops are carried out for model calibration. Based on rock mechanics experiments of deep shale and the fractal description of natural fractures, the initiation and propagation characteristics of hydraulic fractures within a perforation cluster are investigated systematically. The exclusion, attraction, bifurcation, and intersecting mechanisms between the complex hydraulic fractures and the natural fractures are revealed. And then, the optimal cluster parameters, liquid type, treatment parameters and shale characteristic parameters, which are beneficial to reduce the near wellbore friction and form a dominant main hydraulic fracture quickly within a cluster, are obtained for the shale gas volume fracturing.

页岩分段多簇体积压裂时，常遇施工压力高、多数簇内裂缝无法扩展等难题，严重制约了我国页岩气的高效开发，因此亟待开展射孔簇内复杂裂缝起裂与竞争扩展的力学机理研究。本项目基于ABAQUS平台的二次开发功能，发展显式时间积分的三维裂缝粘弹塑性损伤模型，在页岩基质网格边界嵌入零厚度裂缝单元模拟水力裂缝遇天然裂缝的相交和分岔，建立页岩双重介质多场耦合的复杂三维裂缝交错扩展模型，并进行页岩露头和3D打印岩样的水力压裂室内大型物理模拟实验验证；基于页岩岩石力学实验及天然裂缝分形描述，分别开展页岩双重介质体积压裂射孔簇内复杂三维多裂缝的起裂和竞争扩展数值模拟研究，揭示射孔簇内多裂缝起裂以及多条水力主裂缝和天然裂缝交错扩展的排斥、吸引、分岔和相交机理，找出降低破裂压力、近井摩阻以及多裂缝串联形成优势主裂缝的工艺参数和储层条件。研究成果不仅促进了页岩气开发理论发展，同时对复杂裂缝扩展模拟技术进步提供了支撑。

我国页岩储层普遍埋藏较深，呈现地质构造复杂、页理发育、高应力和高强度等典型特征。分段多簇体积压裂作为我国页岩油气经济高效开发的主体技术，在施工时常遇压力高、多数簇内裂缝无法扩展等难题，严重制约了我国页岩油气的高效开发，因此亟待开展射孔簇内复杂裂缝起裂与竞争扩展的力学机理研究。本项目①首先通过研究我国页岩储层地质力学与损伤力学特性，发展了拉-剪混合的裂缝粘弹塑性起裂与损伤准则，并在页岩基质网格边界嵌入零厚度裂缝单元模拟水力裂缝遇天然裂缝的相交和分岔，建立基于DFN-FEM的页岩双重介质多场耦合的复杂裂缝交错扩展模型，编制了基于显式时间积分的裂缝性页岩水力压裂复杂多裂缝扩展数值程序，并通过对比解析解、公开室内物理模拟实验和开展水力压裂室内大型物理模拟实验等多种方式对模型及数值程序进行了验证；②基于页岩岩石力学实验及天然裂缝分形描述，形成了高精度天然裂缝DFN建模方法，探讨了不同地质及工程参数影响下的水力-天然裂缝互作用机理，并绘制了页岩水力-天然裂缝交错机制图版。在此基础上，分别开展页岩双重介质体积压裂射孔簇内复杂三维多裂缝的起裂和竞争扩展数值模拟研究，揭示射孔簇内多裂缝起裂以及多条水力主裂缝和天然裂缝交错扩展的排斥、吸引、分岔和相交机理；③开展了复杂裂缝扩展模型及机理对页岩储层水力压裂多种特殊复杂工艺适应性评价研究，揭示了包括页岩暂堵压裂、限流压裂、组合缝网压裂、立体井网压裂等新型复杂工艺条件下的复杂裂缝扩展机理，并针对性开展了包括暂堵时机与施工参数优化、限流压裂施工参数优化、组合缝网变粘度工艺参数优化、立体井网井距与施工参数优化等相应的优化措施。研究成果已在四川盆地涪陵、长宁页岩气和济阳坳陷页岩油等储层中进行了规模化推广应用，并已获省部级科技进步/技术发明一等奖5项，不仅促进了页岩油气开发理论发展，同时对包括干热岩、煤层气、致密气等脆性非常规储层压裂的复杂裂缝扩展模拟技术进步提供了支撑。