复杂载荷下深层脆性页岩力学性能及破坏规律实验研究

四川自贡、泸州等区块深层页岩硬度高、脆性强、层理缝发育,水平钻井过程中井下阻卡、溢流等事故频发。针对该问题,通过开展深层脆性页岩钻井液封堵性实验及不同卸载围压速度下力学性能实验,研究了钻井液的封堵性及起钻速度对脆性页岩力学性能的影响及页岩破坏规律。实验结果表明:钻井液封堵性对深层脆性页岩强度和弹性模量具有明显影响,封堵性好时,页岩抗压强度可达350 MPa,弹性模量为16 000 MPa;封堵性差时,页岩抗压强度仅为40 MPa,弹性模量为6 000 MPa;围压卸载速度越快,脆性页岩越容易发生破坏,即起钻速度过快会导致井底有效液柱压力降低,引发井壁岩石崩落;井底压力环境下页岩多为沿层理裂缝发生破裂。该研究解释了复杂工况下深层脆性页岩力学性能及破坏规律,为现场制订防塌工艺措施提供了科学依据。

Test and Analysis of Mechanical Properties and Failure Patterns for the Deep Brittle Shale Under Complex Load

The deep shale in Zigong, Luzhou and other regions of Sichuan Province are characterized by high hardness, strong brittleness and well-developed bedding fractures, which results in frequent downhole stuck, overflow and other accidents during horizontal well drilling. The drilling fluid plugging performance and mechanical property tests under different confining pressure unloading rates were conducted for the deep brittle shale to clarify the effects of drilling fluid plugging performance and tripping out speed on the mechanical properties of brittle shale and the failure patterns. The test indicates that the drilling fluid plugging performance shows a significant effect on the strength and elastic modulus. The compressive strength and elastic modulus can reach 350 MPa and 16 000 MPa respectively with favorable drilling fluid plugging performance. The compressive strength and elastic modulus are only 40 MPa and 6 000 MPa respectively in case of poor drilling fluid plugging performance. The brittle shale is more likely to be failure under high confining pressure unloading rate. High tripping out speed will result in a sharp decrease of downhole effective liquid column pressure, which will lead to sidewall rock collapse. The shale failure always along the bedding fracture under bottom-hole pressure. This research reveals the mechanical properties and failure patterns of deep brittle shale under complex conditions, which could provide a scientific reference for formulating anti-collapse countermeasures.