焦石坝页岩气储层水平井井壁失稳机理

Mechanisms of sidewall stability loss in horizontal wells drilled for shale gas development in Jiaoshiba Block

焦石坝区块页岩气储层岩石层理发育,导致页岩强度各向异性特征明显,钻井过程中易发生井壁失稳问题。为此,采用龙马溪组井下岩心开展直接剪切和三轴实验,结合单组弱面强度理论,建立了页岩强度各向异性表征方法;综合页岩强度各向异性表征方法、井壁应力分布模型,建立了页岩井壁坍塌压力计算方法;最后,以目标井JY1HF井为例进行了实例计算与分析。结果表明:龙马溪组页岩发育了大量软弱层理,沿软弱层理剪切滑移是井壁失稳的主要力学机制;钻井液浸泡后页岩强度出现劣化,但强度劣化效应并不十分显著;常规模型计算的坍塌压力随着井斜角增加而减小,文中模型计算的坍塌压力随着井斜角先增加后减小,且计算结果与实际情况更加吻合;页岩薄弱层理的剪切滑移是井斜井段井壁垮塌的关键,而强度的劣化并不是主控因素。研究结果可为礁石坝区块和其他地区页岩气井钻井设计、轨迹优化等提供参考。

With beddings well developed, shale gas reservoirs in Jiaoshiba Block are characterized by high strength anisotrophy, so loss of sidewall stability is frequent in these reservoirs. Therefore, direct shearing and triaxial tests were performed on core samples taken from the Longmaxi Formation to develop a new way to characterize shale strength anisotropy based on single-group weak-plane strength theory. By combining the shale strength anisotropy characterization method and model of stress distribution on sidewall, a calculation method of sidewall collapsing pressure in shale formations has been worked out. Finally, calculation and analysis have been done with Well JY1 HF as an example. The calculation results show that, with large amounts of weak bedding, shale in the Longmaxi Formation shows apparent anisotropy, and shearing slippage along weak bedding is the main mechanism of sidewall stability loss. Strength of the shale soaked in drilling fluid turns weaker, but such strength reduction is not significant; collapsing pressure calcualted by using conventional models decreases with the increase of deviation angle, whereas collapsing pressure calculated by using the model proposed in this paper increases first and decreases later with the increase of deviation angle, and the calculation results match well with field observation results; shearing slippage of weak bedding in shale is the key of sidewall collapsing in building-up intervals, while reduction in rock strength is not the controlling factor. The research results may provide references and guidelines for drilling design, trajectory optimization of shale gas wells in Jiaoshiba Block and other regions.