基于蒙特卡洛法的井壁稳定可靠度随机变量敏感性分析

Sensitivity analysis on the random variables of borehole stability reliability based on Monte-Carlo method

利用蒙特卡洛模拟对井壁稳定性模型中随机变量与结果的相互关系进行分析,获取了不同应力场和破坏准则下影响井壁稳定性最为敏感的因素。通过分析可知,基于Mohr-Coulomb准则的坍塌压力,无论处于哪种地应力场,最为敏感的因素都是最大水平地应力;基于Drucker-Prager准则的坍塌压力,当垂应力为最大主应力时,最为敏感的因素是孔隙压力;当垂向应力为中间主应力或最小主应力时,最为敏感的因素均为内摩擦角;地层破裂压力无论处于哪种地应力场,最为敏感的因素均为最小水平地应力。通过研究确定了影响井壁稳定可靠度最为敏感的因素,通过相应的措施提高敏感因素的准确程度,从而减少坍塌及破裂压力预测结果的不确定性,最终提高井壁稳定的可靠度。

The relationships between random variables and results of borehole stability model were analyzed by means of Monte-Carlo simulation to figure out the most sensitive factors affecting borehole stability under different stresses and failure criteria.It is indicated that based on the collapse pressure of Mohr-Coulomb criteria,the most sensitive factor in any ground stress field is always the maximum horizontal ground stress.Based on the collapse pressure of Drucker-Prager criteria,the most sensitive factor is pore pressure when the vertical stress is the maximum major stress,and it is the internal friction angle when the vertical stress is the medium or the minimum major stress.The most sensitive factor of formation fracture pressure in any ground stress field is always the minimum horizontal stress.After the most sensitive factors affecting the reliability of borehole stability were defined,the accuracy degree of sensitive factors was improved by taking the measures correspondingly,so that the uncertainty of predicted collapse and fracture pressures is decreased and the reliability of borehole stability is ultimately improved.