南海深水海域高温高压地层破裂压力预测模型

Fracture pressure prediction model of high temperature and high pressure formation in deep water area of the South China Sea

在南海海域深水高温高压地层环境下,钻井液安全密度窗口窄,井壁容易失稳。南海北部WZ12-X区块的岩石抗拉强度实验发现,岩石强度受到抗拉强度的影响,而抗拉强度又受到温度的影响。目前,传统的地层破裂压力预测模型均未考虑岩石抗拉强度的影响。为此,文中利用测井资料计算了南海琼东南盆地深部地层岩石抗拉强度,得出岩石抗拉强度随地层深度变化的拟合方程,从而建立了一种新的适合深水海域钻井的考虑岩石抗拉强度的地层破裂压力预测模型。研究结果表明:在25~200℃时,砂泥岩抗拉强度随温度的升高,先升高、后降低,最后趋于稳定;岩石破裂压力与抗拉强度呈线性正相关关系;深水海域地层的岩石抗拉强度与地层深度呈对数关系。在琼东南盆地L-X区块运用了新模型,与现场地漏实验结果对比,地层破裂压力当量密度预测误差介于0.54%~0.98%,均值仅为0.7%。新模型为深水海域钻井过程中地层破裂压力的窄密度窗口计算提供了理论支持。

Under the environment of high temperature and high pressure in the deep water area of the South China Sea, the safe density window of drilling fluid is narrow, and the wellbore is easy to lose stability. From the rock tensile strength experiment in WZ12-X block in the north of the South China Sea, it is found that the rock strength is affected by the tensile strength, while tensile strength is affected by the temperature. At present, the traditional fracture pressure prediction model does not consider the influence of tensile strength. Therefore, the rock tensile strength of deep strata in Qiongdongnan Basin in the South China Sea is calculated by using acoustic logging data, and the fitting equation of rock tensile strength changing with depth is established, so as to establish a new prediction model of formation fracture pressure considering tensile strength for deep water drilling. The research results show that the tensile strength of sand and mudstone under the temperature of 25-200 ℃ increases first, then decreases with the temperature, and finally tends to be stable. The fracture pressure of the rock has a linear positive correlation with the tensile strength. The rock tensile strength of the deep water formation has a logarithmic relationship with the depth of the formation. The new model is applied in the L-X block of Qiongdongnan Basin. Compared with the ground leakage test results, the error of equivalent density is between 0.54% to 0.98%, and the average error is 0.70%. The new model provides theoretical support for the narrow density window calculation of formation fracture pressure during deep water drilling.