波动注入水力压裂诱发微地震的力学机制及其对压裂效果的影响

水力压裂岩石裂缝的扩展诱发天然微地震。传统水力压裂区微地震引起的应力重新分布范围有限，小震级地震波的作用也有限，这些微地震不可能触发较远储层的地震，一定程度上限制了水力压裂改造效果。为此，提出了波动注入水力压裂诱发较大范围“微地震”的想法。根据流体力学理论建立了波动注入水力压裂井筒不稳定流动的压力振动模型，分析了人为诱发“微地震”的井底压力振动源和岩石损伤断裂机制；研究了波动注入水力压裂人为诱发的“微地震”震源特性和力学机制，分析了微地震对水力压裂效果的影响机制。分析结果表明，波动注入水力压裂人为诱发的微地震震源破裂信号频率随裂缝长度的增加而降低；对于断层和天然裂缝发育较多的储层，波动注入水力压裂导致岩石破裂诱发微地震的力学机制主要是储层中断层或天然裂缝的剪切滑移；对于非裂缝性储层，波动注入水力压裂导致岩石破裂诱发微地震的力学机制是人为地震效应和损伤—断裂效应。研究结果表明，波动注入水力压裂能够人为诱发“微地震”，增加储层的“微裂缝”，扩大自然微地震的波及面积，增强压裂作业的增产效果。

Mechanical principles of the fluctuation injection based hydraulic fracturing to induce microseismic and its influence on the fracturing effect

The propagation of rock fractures created by hydraulic fracturing can induce natural microseismic. The stress’s redistribution range caused by the microseismic of the traditional hydraulic fracturing area is limited, and the seismic wave of low magnitude has limited effects, so they cannot excite the seismic in the farther reservoirs, which, to some extent, restricts the stimulation effect of hydraulic fracturing. In this regard, the idea of making use of the fluctuation injection based hydraulic fracturing to induce “microseismic” in a larger area was proposed in this paper. Firstly, the pressure vibration model for the wellbore unsteady flow in the process of the fluctuation injection based hydraulic fracturing was established according to the theory of fluid mechanics, and the bottom hole pressure’s vibration sources and the rocks’ damage and fracturing mechanisms of artificially induced “microseismic” were analyzed. Then, seismic source characteristics and mechanical principles of the “microseismic” induced artificially by the fluctuation injection based hydraulic fracturing were investigated, and the influential mechanisms of microseismic on the hydraulic fracturing effect were analyzed. It is indicated that the frequency of the source fracturing signal of the microseismic artificially induced by the fluctuation injection based hydraulic fracturing decreases with the increase of fracture length. As for the fractured reservoirs, the mechanical principle that the fluctuation injection based hydraulic fracturing induces microseismic by leading to rock fracturing is the shear glide of faults or natural fractures in reservoirs. As for the non-fractured reservoirs, however, it is the artificial seismic effect and damage-fracturing effect. In conclusion, the fluctuation injection based hydraulic fracturing can artificially induce “microseismic”, which can increase the “microfractures” in reservoirs and enlarge the sweep area of natural microseismic, so as to enhance the stimulation effect of fracturing.