钻井液作用下页岩破裂失稳行为试验

为揭示富有机质脆性页岩与钻井液之间的相互作用对井壁失稳潜在影响,以层理发育的脆性页岩为研究对象,利用蒸馏水、油基钻井液、硅酸盐钻井液及其滤液,开展钻井液高温浸泡试验,从宏观和微观方面描述页岩破裂失稳过程与机制。结果表明:钻井液侵入带来的水化膨胀与碱液侵蚀是页岩化学损伤的主要形式,也是诱发脆性页岩破裂失稳的直接原因;钻井液作用下,页岩主要沿层理面破裂,破裂根本原因是层理微缝的扩展、延伸乃至贯通形成宏观裂缝,诱发页岩失稳。室内试验与现场应用均证实,提高钻井液封堵性、降低滤失量,减弱水化与碱液侵蚀对页岩破裂失稳影响,有助于提高井壁稳定性,也是页岩气井防塌水基钻井液体系设计的主要技术思路。

Laboratory studies of shale fracturing behaviors with rock-drilling fluid interactions

Drilling in shale formations has been a challenge due to wellbore instability problems. Loss of drilling fluid into the formation can cause complex interactions of rock-fluid and change the cracking behavior of shale during chemical erosion with reactive clays. In this study, high temperature immersion testing was conducted to investigate the interactions of shale rock samples with various drilling fluids in order to reveal the mechanisms of rock failure, in which the fracture development and microstructure changes of shale rocks during drilling fluid immersion were analyzed, and the fracturing behaviors and its controlling factors were investigated. The results show that strong chemical interactions between the shale samples and drilling fluids in terms of fracture development have been observed. These interactions can enhance the propagation of fracture, and the fracturing behavior is time and chemical dependent. The fracturing of shale can be significantly reduced by increasing the plugging capability and adjusting the pH value of drilling fluids. The shale fracturing induced by chemical erosion of drilling fluids can be attributed to hydration reactions and alkali effects. Optimized oil-based fluid and well designed water-based fluid are essential for wellbore stability. This study can provide valuable insights into the nature of the wellbore instability in shale drilling.