裂缝性地层坍塌压力分析

Collapse Pressure Study of Fractured Formation

钻井过程中钻遇裂缝性地层易发生井壁掉块、憋扭矩、卡钻等复杂现象,严重阻碍钻井进程,裂缝性地层井壁坍塌压力的确定尤其必要。根据裂缝性岩石的特点,将其分解为基质和裂缝面两种介质,岩石内部随机分布的裂缝面可视为具有不同倾斜角和倾向角的弱面,将井周基质与裂缝面的应力分布与Mohr-Coulomb强度准则进行比较,运用Jaeger弱面理论确定裂缝性岩石的坍塌压力。结果表明岩石可能发生基质剪切或者裂缝面剪切滑移两种形式的破坏,坍塌压力主要由岩石中裂缝面的产状决定。对顺北区块一口井钻遇的辉绿岩侵入体地层坍塌压力进行了计算,表明裂缝性地层坍塌压力分析模型对钻井工程具有指导价值。

The wellbore instability problems like breaking down,holding torque and drilling pipe sticking are easily to take place when fractured formation is encountered,which will impede drilling process seriously.Therefore,it is particully necessary to determine the wellbore collapse pressure.Fractured rock could be divided into two parts：matrix and fracture plane,fracture planes are regarded as weak planes with different dip angles and inclination angles.Compare stresses of rock matrix and fracture planes with Mohr-Coulomb failure criterion and collapse pressure could be determined using Jaeger＇s weak plane theory.The results show that fractured rock could break along matrix shear plane or fracture plane,the collapse pressure is mainly controlled by occurrence of fracture planes.A well drilled in Shunbei block is taken as an example,collapse pressure of diabase intrusion is analyzed,it shows that the model of fractured formation collapse pressure has guiding value for drilling engineering.