A Method to Estimate Borehole Collapse Pressure Based on Shale Damage Law
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摘要: 页岩的破坏有劈裂式、单剪切式、双剪切式等3种模式，而目前常用于预测井壁坍塌压力的Mohr-Coulomb准则仅适用于发生单剪切式破坏的井壁。针对该问题，首先基于最小耗散能原理和损伤力学，建立了包括页岩损伤演化方程、损伤阈值应变计算式、极限损伤变量计算式的页岩损伤力学模型；然后，以具有代表性的塔里木盆地群库恰克地区页岩地层标准岩心为研究对象，进行了常三轴抗压室内试验，并将试验得到的应力-应变曲线与理论曲线进行了对比分析，发现吻合较好；最后，将理论模型与室内试验结果相结合，研究了应变能密度与岩石破坏时应变、损伤阈值应变之间的关系曲线，并确定出维持井壁稳定的最低钻井液密度。分析认为，利用该种维持井壁稳定最低钻井液密度的确定方法，可以确定页岩地层的井壁坍塌压力。Abstract: 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.
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Keywords:
- rock mechanics /
- shale sloughing /
- mathematical model /
- stress /
- strain /
- hole stabilization
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