井底压差是影响钻井机械钻速的重要因素之一。实践证明欠平衡钻井与常规钻井相比机械钻速有显著提高,不同钻井条件下的井底应力场需要深入地研究。在分析井底岩石所受载荷的基础上,建立了轴对称条件下井底岩石流固耦合模型,并利用有限元方法进行求解,通过对不同井底压差下的井底岩石应力场进行数值模拟计算,针对井壁岩石的理论解和数值模拟结果进行对比验证模型的合理性,并对井底待破碎岩石进行了力学分析。结果表明,数值模拟结果与理论结果相符,验证了模型的正确性;井底待破碎岩石按照应力状态分为3个区域,分别为3向拉伸区、2向压缩区和3向压缩区。井底压差对井底应力场的影响研究为快速高效破岩提供理论基础。
Bottom-hole differential pressure is one of the important factors which affect the drilling penetration rate;and it is proved that underbalanced drilling is usually more efficient than conventional drilling.The purpose of the article is to study the effect of bottom-hole differential pressure on the vertical bottom-hole stress field.Based on the of mechanical analysis of the bottom-hole rock,the fluid-solid coupling model with the bottom-hole differential pressure is established under axisymmetric condition and the numerical simulation calculation of the bottom-hole stress field under different differential pressures is calculated with the finite element method.The comparison between the numerical and theoretical solutions and analysis of the stress state of bottom-hole rock to break are carried out.The results show that the numerical solution is consistent with the theoretical solution and the fluid-solid coupling model is reasonable.The bottom-hole rock is divided into three regions: triaxial tension region,biaxial compression region and triaxial compression region.Study of the bottom hole rock stress field under different drilling methods is the theoretical basis for faster and more efficient drilling.