南海某高温高压井剪切破裂上限压力时变规律研究

成像测井显示南海某高温高压井钻进过程中高密度钻井液不仅导致井壁拉伸缝，还诱发井壁高角度梯形剪切缝。为此，通过分析井周应力和裂缝形状，根据不同的应力组合模式，建立了一套以莫尔-库伦准则为基础的剪切破裂压力计算模型。在此基础上，定量分析了拉伸破裂模型和剪切破裂模型计算所得破裂压力的大小，给定了两个模型的适用范围，并进一步分析了破裂压力随钻井循环时间的变化规律。分析认为，地层强度较小时，剪切破裂压力决定钻井液密度上限；地层强度较大时，拉伸破裂压力决定钻井液密度上限；随井周温度降低，两种破裂压力均降低，但是拉伸破裂降低得更快。

Study on the Time-varying Law of the Upper Limit of Fracture Pressure in High Temperature and High Pressure Wells

During the drilling of high-temperature and high-pressure wells, not only tensile fracture fractures but also high-angle trapezoidal shear fractures exist on the well wall. To this end, a set of shear fracture pressure calculation models based on the Mohr-Coulomb criterion was established by analyzing the stress state of the formation around the well and the shape of the cracks in the well wall, and according to different stress combination modes. On this basis, the calculated fracture pressures of the tensile fracture model and the shear fracture model were quantitatively analyzed, the application scope of the two models was given, and the variation law of fracture pressure with drilling time was analyzed. The analysis shows that the upper limit of drilling fluid density is determined by shear fracture pressure when the formation strength is small. The upper limit of drilling fluid density is determined by tensile fracture pressure when the formation strength is large. The longer the drilling fluid circulation time, the greater the decrease of wall temperature, the two kinds of fracture pressure decreased, but the tensile fracture decreased faster.