页岩水力压裂过程的态型近场动力学模拟研究

水力压裂技术广泛使用于页岩气开采工程中。为了分析压裂过程中多裂缝扩展形成复杂裂缝网的机制,尝试将态型近场动力学理论引入页岩水平井水力压裂过程的力学建模与数值仿真,在物质点间相互作用力模型中加入等效水压力项以实现在新生裂缝面上跟踪施加水压力,建立了水力压裂过程的近场动力学分析模型。通过模拟页岩储层的水力压裂过程,可得到复杂的裂缝扩展路径、裂缝网络的形成过程以及裂缝扩展受射孔间距及页岩天然裂缝和层理的影响。研究结果表明:射孔间距过小会造成起裂干扰,使中心射孔的裂缝扩展受到抑制;在压裂压力一定的情况下适当增大射孔间距,可以显著增强页岩压裂形成裂缝网的能力;压裂过程中水平层理面可能张开形成水平裂缝,且天然裂缝会诱导形成更复杂的垂直裂缝。模型和方法可为页岩水力压裂过程和机制研究及工程实践提供参考。

State-based peridynamic study on the hydraulic fracture of shale

Hydraulic fracturing technology is widely applied in shale gas extraction engineering. In this study, the state-based peridynamic theory was introduced into the modeling and analysis of hydraulic fracture in horizontal shale wells. The mechanism of the multi-fracture propagation and the formation of complex fracture networks were analyzed as well. An equivalent hydraulic pressure term was embedded in the constitutive function, which describes the interactive forces between material points, to track the applied hydraulic pressure on the new crack surfaces. Thus, a simplified state-based peridynamic model was established to describe the hydraulic fracturing. Through the proposed model, the hydraulic fracturing process of shale reservoirs was simulated to investigate the complex crack propagation paths, the formation process of crack networks, as well as the effects of natural stratification, cracks, and perforation spacing on the hydraulic crack propagation. Numerical results indicate that when the perforation spacing is sufficiently small, the cracking process is disturbed and the crack propagation along the central perforation is inhibited. Under certain fracturing pressure, a proper increase of perforation distance can significantly accelerate the hydraulic fracture process. During the hydraulic fracturing process, horizontal cracks may be formed in the horizontal stratification, and complex vertical cracks may be formed by the induction of natural cracks. The proposed model and numerical methods provide alternative and promising ways for the profound investigation into shale hydraulic fracturing and engineering applications.