长73泥页岩地层井壁失稳物化机理及力学特征研究

Study on Physicochemical Mechanism and Mechanical Characteristics of Wellbore Instability in Chang 73 Shale Formation

摘要: 弄清鄂尔多斯盆地长73层位井壁频繁失稳物化机理及岩石力学特征，对该地区钻井至关重要。通过扫描电镜、X射线衍射、滚动回收率、膨胀性和Zeta电位试验，分析长73层位井壁失稳物化机理。结果表明：岩石层理、微裂缝和微裂隙发育；矿物组分中石英含量最高(平均28.33%)，其次为粘土矿物(平均18.5%)，其中粘土矿物以伊/蒙间层(平均62.67%)和伊利石(30.33%)为主，属于典型的硬脆性泥页岩地层；回收率为97.32%~98.62%，CST毛细管测试为13.65 s~20.67 s，Zeta电位绝对值介于8~22 mV，岩石水化分散性较弱。物化分析表明，长73层位井壁失稳主要是由于钻井液渗入地层，引起泥页岩结构力变化导致剥落掉块。钻井液浸泡岩石结果显示：钻井液滤液进入微裂缝后，伊利石和蒙脱石在碱性条件下与溶液发生离子反应，改变了层理面矿物成分，降低了微裂缝面的摩擦性能，从而影响岩石强度，其中水基钻井液浸泡后岩石力学参数变化幅度最大，岩石强度降低32.40%；随着浸泡时间增加，岩石力学参数呈递减趋势，岩石强度降低幅度可达11.11%~36.81%，且浸泡时间增加，微裂缝尖应力强度因子增加，临界断裂韧性降低，裂缝更容易扩展。

Abstract: It is of marked importance for drilling to understand the physicochemical mechanism and mechanical characteristics of frequent instability occurrence of borehole at Chang 73 formation in Ordos Basin. The physicochemical mechanism of borehole instability at Chang 73 formation was analysed by the scanning electron microscope (SEM), X-ray diffraction, rolling recovery and zeta potential test. The results showed that rock bedding, microcracks and microfractures were developed and the content of quartz in mineral components was the highest (average 28.33%), followed by clay minerals (average 18.5%), in which clay minerals were mainly Iraq/Mongolia interlayer (average 62.67%) and illite (30.33%), belonging to typical hard brittle shale formation. The recovery was 97.32%~98.62%, the CST capillary test was 13.65 s~20.67 s, and the absolute value of zeta potential is between 8~22 MV, indicating its weak hydration dispersion. Physicochemical analysis showed that the wellbore instability of Chang 73 formation was mainly due to the change of shale structural force caused by the penetration of drilling fluid into the formation, leading to peeling and block falling. The results of rock immersion by the different drilling fluids showed that after the filtration entered the microcracks illite and Montmorillonite reacted with the solution under alkaline conditions, which changed the mineral compositions of bedding surface and reduced the friction performance of microfracture surface, thus affecting the rock strength. Among them, the variation range of rock mechanical parameters was the largest after soaking in water-based drilling fluids, reduced by 32.40%. The mechanical parameters of rock tended to decrease with soaking time, and the reduction ranged from 11.11% to 36.81%. With the increase of soaking time, the stress intensity factor at the tip of microcrack increased, the critical fracture toughness decreased, and the crack was inclined to expand.