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.