水力压裂诱发断层活化机理分析

Analysis of fault activation mechanism induced by hydraulic fracture

我国油气储层中含有大量断层,水力压裂开采油气资源的同时易引起断层活化,进而引发地震。为研究水力压裂诱发断层活化机理,在裂缝尖端和附近储层应力状态研究基础上,结合滤失引起的孔隙压力变化规律,得出断层受拉活化和受剪活化条件。通过对断层活化条件进行解析研究,得出以下结论:断层与水力压裂裂缝尖端相遇时可被水力压裂裂缝尖端激活,首先裂缝尖端应力集中可使断层小范围内受拉或受剪破坏,然后压裂液进入断层使断层受拉张开,两阶段条件均满足时断层被激活,水力压裂裂缝转向断层扩展;水力压裂裂缝穿过断层后,随着压裂时间的累积,压裂液滤失使储层孔隙压力增大、有效应力减小,可引起断层剪切滑动;随着地应力差增大,断层倾角、粘聚力以及内摩擦角减小,断层剪切滑动所需的孔隙压力降低。

There are a lot of faults in oil and gas reservoirs in China. The faults can be activated by hydraulic frac- ture and then lead to earthquake. In order to study the fault activation mechanism induced by hydraulic fracture, on the basis of the study of stress state at fracture tip and around fracture, tensile and shear activation conditions of fault are obtained combining with the pore variation law caused by leak-off. It draws some conclusions by analytical research on the fault activation conditions. The interference of fracture tip makes fault tension activation when the fracture tip just touch fault and it is divided into two stages. The stress concentration induced fracture tip may make fault broken within a small range in the first stage. The fracturing fluid flows into the small range of fault generated in the first stage, which makes the fault tension broken in the second stage. The fracture turns towards fault when both the first stage and the second stage are satisfied. After the fault is crossed by hydraulic fracture, the leak off of fracturing fluid can make the reservoir pore pressure increase and effective stress decrease with the hydraulic fracturing time accumulation which can make fault shear slip. The minimum pore pressure made fault shearing slip goes down with the in-situ stress difference increased and the fault dip angle, cohesion and internal friction angle decreased.