A Method to Estimate Borehole Collapse Pressure Based on Shale Damage Law

Shale failure has three models:splitting,single shear and double shear,but Mohr-Coulomb criterion that commonly used in predicting borehole collapse pressure for shale formations at present is only suitable for the shale failure of single shear mode.In view of this issue,based on the principle of minimum energy and damage mechanics,the shale damage mechanics model including shale damage-evolution equation under confining pressure,damage-threshold strain formula and ultimate damage variable formula was established first;then conventional triaxial compression test was carried out with a typical shale core from Tarim basin,the comparison of stress-strain curve from testing and theoretical curve shows sufficient coincidence;at last,the relationship among strain energy density,strain and damage threshold strain was determined,and the minimum drilling fluid density to keep wellbore stable was established,by which the collapse pressure of shale can be calculated.