Shale wellbore instability is a major cause of high cost and long time drilling and completion for shale gas .The effect of working fluid soaking on shale crack propagation and me‐chanical properties was analyzed to optimize drilling technology and protect wellbore stability . The interaction between shale and fluid during soaking experiments was observed with scanning electron microscopy (SEM ) .Shale fracture propagation mechanism and influencing factors under working fluid soaking were discussed through microstructural changes .It is found that after soa‐king the shale ,the rock strength decreases ,the mechanical parameters change significantly ,that is the elastic modulus decreases ,the Poisson ratio increases ,and longer soaking time results in grea‐ter decrease in the shale rock strength .The extension of weak structure plane along the bedding surface and micro cracks is common cause of shale wellbore instability .%采用页岩浸泡实验直观地观测了页岩与流体之间的反应和裂缝延展情况，结合电镜扫描实验中页岩微观结构变化，分析了工作液浸泡下页岩裂缝扩展的机理。研究发现，由于工作液碱度较高和页岩中黏土矿物的存在，页岩在工作液中浸泡后，岩石强度降低，力学参数发生了明显变化：弹性模量减小，泊松比增加；浸泡时间越长，强度值降低越大。由此得出，浸泡下页岩微裂缝沿层理面和弱结构面不断扩展延伸是引起页岩井壁失稳的主要原因。
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