基于井眼坍塌角度和坍塌深度预测模型的泥岩水平段井壁稳定性评价方法

Wellbore stability evaluation of horizontal wellbore in mudstone: A method based on wellbore collapse angle and depth model

徐声驰 刘锐 孟鑫 刘博文 孙志高 付基友 翟晓鹏 张军

XU Shengchi LIU Rui MENG Xin LIU Bowen SUN Zhigao FU Jiyou ZHAI Xiaopeng ZHANG Jun

中国石油西部钻探录井工程分公司 中国石油新疆油田分公司 油气钻采工程湖北省重点实验室(长江大学)

CNPC XDEC Mud Logging Branch Company, Karamay 834000, Xinjiang, China PetroChina Xinjiang Oilfield Company, Karamay 834000, Xinjiang, China Hubei Key Laboratory of Drilling and Production Engineering for Oil and Gas, Yangtze University, Wuhan 430100, Hubei, China

在水平井钻进过程中，泥岩水化会导致岩石强度逐渐降低。目前依据临界坍塌压力确定的钻井液密度只能维持初始阶段井眼平衡，难以保持整个钻进过程中水平井井壁稳定。为此，根据线弹性岩石力学理论，结合Mohr-Coulomb准则，建立了泥岩水化后的井眼坍塌角度和坍塌深度模型，分析不同地应力作用下井眼的破坏程度和扩径率。结果表明，井眼坍塌深度随着岩石强度衰减而增大，总体坍塌面积增大；随着岩石浸泡时间增加，岩石强度和弹性模量逐渐减小，岩石泊松比和内摩擦角逐渐增大，适时调整钻井液密度才能减少岩石强度衰减造成的井眼坍塌。在一口水平井案例应用中，利用建立的岩石强度衰减井眼扩径率模型，设计了井眼扩径率与合理钻井液密度匹配图版，根据该图版适时定量调整钻井液密度，可有效改善井眼坍塌风险。该方法可为水平井地层强度衰减的井壁稳定评价及合理钻井液密度设计提供理论依据。

Mudstone hydration leads to a reducing rock strength during horizontal well drilling. The drilling fluid density determined depending upon the critical collapse pressure can maintain the wellbore equilibrium only at the initial stage, but fails to sustain the wellbore stability of horizontal wellbore through the drilling process. Based on the linear elastic rock mechanics and the Mohr-Coulomb rule, a model was developed to predict the wellbore collapse angle and depth after mudstone hydration and subsequently, analyze the wellbore damage degree and enlargement rate under different in-situ stress conditions. The results show that the wellbore collapse depth increases with the reducing rock strength, leading to an expanding overall collapse area. With the soaking time, the rock strength and elastic modulus decline gradually, while the Poisson's ratio and internal friction angle rise. Properly adjusting the drilling fluid density in a timely fashion can reduce wellbore collapse caused by reducing rock strength. For an example horizontal well, the correlation between wellbore enlargement rate and rational drilling fluid density was charted using the presented wellbore collapse angle and depth model. On this basis, the drilling fluid density was adjusted properly and quantitatively to effectively migitate the risk of wellbore collapse. This method provides a theoretical basis for wellbore stability evaluation of horizontal wellbore in rocks with reducing strength and design optimization of drilling fluid density.

10.13639/j.odpt.2023.02.002