Abstract:

Complex problems，such as HTHP，wellbore collapse，and lost circulation were encountered during drilling Hutubi Anticline. The fabric，mechanical properties，and in-situ stress state of the troublesome medium-deep shale formation were analyzed to determine the countermeasures necessary to stabilize the borehole and reduce risk. A multi-field calculation model for collapse pressure was established linking mechanical factors，drilling fluid chemistry，and fluid seepage. The mechanism of wellbore instability when using water-based drilling fluid in the middle-deep shale formation of Hutubi anticline was identified. The results indicate that the montmorillonite content of the illite-smectite mixed layers in the mudstone in Paleogene Anjihaihe-Ziniquanzi Formation （which collapses readily） exceeds 35%. The formation exhibits moderate expansion and high dispersion，with water expansion rate and recovery rate both being less than 10%. The formation strength also shows anisotropic properties. The mudstone in Cretaceous Hutubihe-Qingshuihe Formation shows reduced swelling and increased hardness and brittleness. Hutubi Anticline is subject to significant tectonic stress，with the maximum horizontal principal stress being equivalent to nearly 2.50 g/cm³，which is higher than the overburden pressure. Applying the multi-field coupling model indicates that the main causes of wellbore instability are seepage of drilling fluid along cracks，hydration of mudstone after contact with drilling fluid，and insufficient effective mechanical support of drilling fluid for the wellbore. The collapse pressure calculated using the multi-field coupling model is 0.05-0.25 g/cm³ higher than when only considering mechanical factors. It was established that the wellbore can be kept stable by maintaining the sealing ability of the drilling fluid to the fractured formation，preventing contact with easily hydrated mudstone during drilling，and increasing the density of the drilling fluid to slightly higher than the collapse pressure.

Key words:

drilling fluid, Hutubi anticline, wellbore stability, multi-field coupling, collapse mechanism

摘要：

呼图壁背斜钻井过程中面临高温高压、井壁坍塌、钻井液漏失等复杂问题。为了制定稳定井壁的技术对策，降低事故风险，通过室内实验与理论分析对中深部复杂泥岩地层的组构特征、力学特性和地应力状态进行了研究，建立了“力学-化学-渗流”多场耦合的井壁坍塌压力计算模型，明确了呼图壁背斜构造中深部泥岩地层在水基钻井液作用下的井壁失稳机理。研究结果表明，古近系安集海河组至紫泥泉子组易坍塌泥岩伊蒙混层中蒙脱石含量占35%以上，中等膨胀、高分散（清水膨胀率和回收率均低于10%），强度具有各向异性；白垩系呼图壁河组至清水河组泥岩膨胀性减弱，硬脆性增强。呼图壁背斜受强构造应力作用，水平最大主应力当量密度接近2.50 g/cm³，高于上覆岩层压力。利用多场耦合方法分析可知，钻井液沿裂隙的渗流作用、泥岩地层与钻井液接触后的水化作用以及钻井液对井壁的有效力学支撑不足是造成井壁失稳的主要原因。考虑多场耦合模型计算得出的坍塌压力比仅考虑力学因素高0.05~0.25 g/cm³。在钻井过程中需保持钻井液对裂缝性地层的封堵性能和对易水化泥岩地层的抑制性能，在保证有效封堵的基础上提高钻井液密度，使其略高于多场耦合模型计算得出的坍塌压力，以保持井壁稳定。

关键词:

钻井液, 呼图壁背斜, 井壁稳定, 多场耦合, 坍塌机理