基于围岩本体Mogi-Coulomb强度准则的层理性岩层斜井井壁稳定模型

 层理性地层中进行大斜度井施工的井壁失稳问题较为突出，在传统井壁稳定模型基础上，以弱面对岩石强度的弱化作用实验为依据，引入欧拉变换充分考虑三维地应力方向的任意性，利用Mogi-Coulomb准则强化中主应力对围岩本体强度的影响，结合单弱面强度准则建立分析层理性地层斜井井壁稳定问题的模型，并进一步给出计算斜井坍塌压力与破裂压力的方法与公式。实验与计算结果表明：当加载方向与弱面夹角为30°时岩石强度最低；Mogi-Coulomb准则因考虑了中主应力的影响而对围岩本体强度的估计更为有效；考虑层理弱面影响的井壁坍塌破坏区域明显增大，破坏位置也发生改变；取得最小坍塌压力值的钻井方向在空间中与层理面并非简单的垂直关系；空间中关于地应力主平面对称的井孔破裂压力相同。利用所建模型进行安全泥浆压力窗口的计算可为安全钻井以及斜井轨迹设计提供理论依据。

A model for inclined borehole stability in bedding rocks based on Mogi-Coulomb criterion of rock matrix

The problem of instability is more prominent when drilling inclined wells in bedding layers. A new wellbore stability model is established considering the weakening action of bedding plane on rocks on the basis of the traditional borehole stability model. The model takes into account the arbitrary nature of 3D in-situ stress with three Euler angles，and applies Mogi-Coulomb criterion to strengthen the effect of intermediate principal stress，as well as combines the single weak-plane strength theory to fulfill the goal of guided drilling. The method and formula for calculating the collapse and fracture pressure using this model are given. The experimental and calculated results show that the rock gains a minimum strength when the angle between the loading direction and bedding plane is 30°.. Mogi-Coulomb criterion is more effective on the estimation of rock matrix strength because of the consideration of the influence of intermediate principal stress. The collapse area is markedly enlarged under the effect of bedding plane，and the collapse position is changed. The drilling direction for obtaining the minimum collapse pressure is not simply perpendicular to the bedding plane in space. And the boreholes in the symmetry in terms of principle in-situ stress plane underground have the same fracture pressure. The safe mud density window calculated with the model above can provide theoretical basis for the safe drilling and trajectory design of inclined boreholes.