页岩气井脆性页岩井壁裂缝扩展机理

页岩钻井中井壁失稳是制约页岩气大规模开发的关键技术难题。阐述了页岩井壁稳定技术现状和存在的问题,借助固体力学、断裂力学和界面化学理论,建立了介质润湿特性控制的裂缝扩展模型,提出了基于润湿理论的页岩井壁稳定评价方法。研究结果表明,页岩地层钻井时水基钻井液应减小钻井液界面张力和增大钻井液与岩石的润湿角;油基钻井液应减小钻井液界面张力和润湿角,从而强化井壁围岩强度、防止页岩井壁发生垮塌。页岩地层井壁稳定性尺度效应明显,不同尺度条件下井壁围岩裂缝扩展机制各异。探索不同尺度条件下页岩井壁围岩失稳机制,对页岩气井钻井液设计具有重要的意义。 

Brittle Shale Wellbore Fracture Propagation Mechanism

Borehole instability is the critical technological issue restricting the large-scale development of shale gas.Based on the demand of borehole stability during shale well gas drilling,this paper presented the status of and challenges to borehole stabilization technology,established a fracture propagation model dominated by wetting behavior of rock medium in accordance with related theories of solid mechanics,fracturing mechanics and interface chemistry,and proposed a shale well-stability evaluating method based on wetting theory.The result showed that the WBM should be available for reducing the interfacial tension and enlarging wetting angle between drilling fluid and rock medium;while the OBM should be available for reducing both interfacial tension and slight wetting angle to reinforce surrounding rock and avoid well collapse.Meanwhile,the shale well borehole stability had obvious scale effect and the fracture propagation mechanism of surrounding rock with different scale varies.The studies on borehole instability of the surrounding rock of the shale formation with different scale had important guiding significance for fluid design of shale drilling.