毛管力作用下的欠平衡钻井水侵规律研究

Research of water invasion of underbalanced drilling under capillary force

欠平衡钻井过程中,毛管力作用下侵入地层的微量钻井滤液常被忽略,降低了泥页岩地层井壁稳定性预测的准确性。基于油水两相渗流理论,建立了毛管力作用下水基欠平衡钻井滤液侵入地层的数学模型,采用有限差分和交错网格系统方法对所建立的数学模型进行数值求解,得到表征水侵程度的含水饱和度与相对渗透率的时空分布。模拟结果表明,毛管力作用下,钻井时间越长、欠压值与原始含水饱和度越小,含水饱和度变化范围越大。由模型计算的水侵量与现有实验结果进行对比,两者基本吻合。水侵规律研究能为欠平衡钻井的泥页岩水化及其井壁稳定提供理论依据,以及更合理地设计欠压值。

In the process of underbalanced drilling, the micro mud filtrate under capillary force is often neglected, which decreases the accuracy of predicting borehole＇s stability for the shale formation. Based on oil-water two phase flow theory, the mathematical model of water-based underbalanced drilling is established considering the capillary force. The finite difference method and the staggered grid system are used to present the numerical solution of the model, mapping the space and time distribution curve of the water saturation and relative permeability. The simulated results show that, under the capillary force, the longer the drilling time and the smaller the underbalance value and the original water saturation is, the greater the change of water saturation will be. The calculated water influx is basically consistent with the experiment results. This research provides theoretical basis for the shale hydration of underbalanced drilling and its stable borehole. Therefore, more reasonable underbalance value can be designed.