页岩气开发必须依靠水平井多段大规模压裂才能有效进行,具有液量大、返排率低、返排周期长等特点,大量的水长期滞留储层中与页岩作用,通过页岩孔隙、天然微裂缝和节理等进入页岩。国内外研究表明水进入页岩会诱发产生微裂缝,但其产生微裂缝的力学机理研究较少。针对该问题,首先开展了自吸诱发微裂缝的实验,分析了页岩吸水诱发微裂缝的现象；然后从页岩吸水受力分析出发,将页岩中吸水介质简化为椭圆形的孔,构建了椭圆孔边应力模型；再结合裂缝起裂准则,推导出诱导裂缝起裂力学模型；最后结合页岩储层的参数计算分析了椭圆孔边应力场和诱导裂缝起裂压力的影响因素。研究结果表明:椭圆孔起裂均在长轴方向发生拉张破坏;起裂方位在最大主应力方向;随长宽比增加,起裂压力大幅降低;起裂压力与抗张强度线性正相关。研究结果与前人实验研究所观察到的现象相吻合,对利用自吸诱发微裂缝机理增大页岩改造体积具有参考意义。

 Shale gas development can only be effectively carried out by multi-stage large-scale fracturing of horizontal wells, with high fluid volume, low flow back rate, and long flow-back cycle. A large amount of water stays in the reservoir for a long time to interact with the shale, and flows into the shale through the pores, natural micro-fractures and joints. Domestic and foreign research shows that water entering the shale can produce micro-fracture, but its mechanical mechanism is less studied. To address this issue, this paper first carried out experiments on self-absorption induced micro-fracture, analyzed the phenomenon of micro-fracture induced by shale water absorption. Then from the analysis of shale water absorption stress, the water absorption medium in the shale was simplified to an elliptical hole, establishing an elliptical hole edge stress model. Afterwards, the induced fracture initiation mechanical model was deduced on the basis of fracture initiation criteria. Finally, the influencing factors of the stress field at the edge of the elliptical hole and the induced fracture initiation pressure are analyzed. The results show that the tensile failure occurs in the direction of the long axis. The orientation of initiation is in the direction of the maximum principal stress. With the increase of length-width ratio, the initiation pressure decreases greatly. There is a positive linear correlation between initiation pressure and tensile strength. This result is consistent with the phenomenon observed by the previous experimental research, and has certain guiding significance for improving the volume of shale transformation by using self-absorption induced micro-fracturing mechanism.