弱面结构对页岩地层井壁稳定性影响研究

页岩地层大量发育弱面结构,造成井壁垮塌现象频发,严重制约了页岩气的高效开发。因此,本文在Mohr-Coulomb准则的基础上,建立多弱面条件下的强度破坏准则。应用该准则,对不同弱面产状、弱面数量、钻井时间下的岩石强度进行分析,从而建立页岩地层坍塌压力预测模型,计算分析表明:围压增大后,页岩强度增加,不易沿弱面破坏;弱面产状和数量变化,使得岩石强度产生变化,从而造成坍塌压力分布复杂。尤其弱面数量增多,岩石强度下降明显,逐渐受弱面强度控制,坍塌压力出现明显增加;实例分析表明多弱面条件下的坍塌压力预测方法能更为准确地预测页岩地层坍塌压力,对页岩地层钻井工程有一定指导意义。

Influence of Weak Plane on Wellbore Stability in Shale Formation

Shale formation contains rich weak planes, causing wellbore collapse and restricting efficient development of shale gas. Therefore, based on the Mohr-Coulomb criterion, failure criterion with multiple weak planes has been developed. By using this new criterion, shale strength in different occurrence of weak plane, weak plane number and drilling time have been investigated. In addition, wellbore stability model with multiple weak planes have been established. The results indicate that with increasing confining stress, shale strength increases and tends to failure across rock matrix. Besides, the occurrence and number of weak planes lead to variation of shale strength and complex distribution of collapse pressure. In particular, with increasing number of weak plane, shale strength declines and is gradually controlled by weak plane strength, leading to growth of collapse pressure. Finally, this model has been applied to oilfield application, showing better prediction of collapse pressure. So it can be a viable tool and reference for drilling operation in shale formation.