压裂液作用下页岩储层的摩擦特性及其对缝网形成的影响

Shale gas reservoir with low porosity and permeability only can be developed economically by fracturing treatment to form a large-scale space fracture network. Shear slip is an important factor in the formation of shale network and is controlled by the friction strength of fracture surface. Previous studies have focused on the formation mechanism of the network under the interaction between the natural fractures and hydraulic fractures. However, less study has focused on the influence of shale friction during hydraulic stimulating; especially the influence fracturing fluid on shale friction properties rare has been mentioned. This study will make effort on the shale friction properties and its effects on the formation of fracture network under the effect of fracturing fluids.Combined effect of high net pressure in fractures and friction strength decreasing by the fluids, could be the main mechanisms behind shale large-scale hydraulic network fracturing. Several research methods will be used in this research, such as lab frictional experiments, micro-scale analysis, theoretical analysis of rock friction and mechanical analysis of fracture network formation and propagation. The research goals can be reached by gradually solving the three following contents: ①the friction characteristics of shale reservoirs; ②the effects of fracturing fluid on shale friction properties; ③the effects of shale shear slip on the formation of fracturing network under the condition of fracturing fluid wetting. These results are significant in revealing the mechanisms of fracturing network of shale reservoir under the synthetic effects of stress, fracturing fluid and fractures. Theoretical foundations will be provided for improving the fracture network forming efficiency and optimizing the fracturing fluid types.

储层压裂改造是页岩油气开发最核心的技术。低孔低渗的页岩气储层必须经过压裂改造、形成大规模空间缝网才能形成产能。剪切滑移是页岩压裂缝网形成的重要控制因素，而剪切滑移是由裂缝表面的摩擦强度控制的。以往缝网压裂研究主要集中在岩石脆性、天然裂缝和人工裂缝相互作用等方面，而对缝网形成中的缝面摩擦效应，尤其是压裂液作用下页岩储层的摩擦特性研究较少。很可能，大排量带来的高缝内净压力和滑溜水带来的缝面超低摩擦系数的共同作用，是页岩储层水力压裂缝网得以形成的主要机理。本研究欲通过室内摩擦实验、岩石摩擦理论分析和缝网形成的力学分析，围绕压裂液作用下页岩储层摩擦特性及其对压裂缝网形成的影响，研究：①页岩储层摩擦特性；②压裂液对页岩摩擦特性的影响；③页岩缝面剪切滑移对缝网形成的影响。揭示页岩储层压裂改造中的“应力-流体-裂缝”联合作用机理，为提高缝网改造效率和压裂液类型优选提供理论依据。

压裂液作用下页岩内部微裂缝的剪切滑移和“页岩-流体化学作用”的后效作用对于页岩储层复杂缝网的形成具有重要影响，高缝内净压力、滑溜水带来的缝面低摩擦系数降低以及缝尖的亚临界裂纹扩展的共同作用，是页岩储层水力压裂复杂缝网形成的主要机理。本研究通过室内摩擦实验、岩石摩擦理论分析、亚临界裂纹扩展实验和缝网形成有限元建模分析，围绕压裂液作用下页岩储层复杂缝网形成的影响开展了系统研究，得出了以下结论与认识：①压裂液作用下页岩储层的摩擦特性与页岩的矿物组份和表面润湿情况关系密切，对于高含粘土的富有机质页岩，吸水后缝面可由硬颗粒支撑转变为软颗粒支撑，大幅降低了缝面摩擦系数，有利于页岩剪切缝网的形成；②流体类型对缝面摩阻降低也有显著影响，滑溜水的降缝面摩阻能力优于KCl溶液；③建立了“应力-流体-裂缝”耦合作用下的缝网扩展有限元模型，发现在同等天然裂缝分布的条件下，低缝面摩擦系数（0.4）更有利于形成复杂形态裂缝，高缝面摩阻系数（0.8）水力裂缝趋于形成一维直缝；④压裂液作用下，页岩裂缝尖端的亚临界裂纹扩展行为对于复杂裂纹的网络的形成也有积极意义，滑溜水可大幅降低页岩I型裂纹扩展的门槛值，有助有间歇压裂期间页岩缝尖的继续延伸扩展。项目研究结果对于优化非常规致密储层压裂改造工艺、提高缝网改造效果及压后产能具有重要理论指导意义。