地层各向异性对硬脆性泥页岩井壁稳定性的影响

Effect of formation anisotropy on wellbore stability of hard brittle shale

硬脆性泥页岩地层内部裂缝、微裂缝等弱面结构发育,表现较强的非均质性和各向异性,常规坍塌压力的计算方法不能准确反映硬脆性泥页岩的破坏规律。基于弱面破坏准则,考虑弱面产状、各向异性、井眼轨迹的影响,建立硬脆性泥页岩坍塌压力的计算模型,分析不同地应力状态下的地层力学各向异性对坍塌压力的影响,确定钻井的安全及危险分布区域。结果表明:当水平地应力为均匀地应力时,地层各向异性对坍塌压力的影响较小,坍塌压力差异程度小于5.00%,基本可以忽略;其他应力条件下,需综合考虑地层力学各向异性和弱面对坍塌压力的影响,确定地层坍塌压力。井斜角与弱面角接近时(井斜角为45°~75°),沿任何方位钻进的定向井井壁失稳风险较高。该结果对硬脆性泥页岩地层钻井具有指导意义,为现场采取合理的工程对策解决井壁失稳问题提供依据。

Cracks and micro-cracks are developed in the hard brittle shale,which makes shale behaving strong heterogeneity and anisotropy.Conventional calculation method of collapse pressure cannot accurately reflect the failure law of hard brittle shale.Combining with weak plane occurrence,anisotropy and borehole trajectory,a calculation model of collapse pressure for hard brittle shale was established by weak failure criterion.The influence degree of anisotropic formation mechanics on collapse pressure under different in-situ stress was evaluated,and the safety and dangerous distribution area of drilling were determined.Results show that formation anisotropy exerts less effect on the collapse pressure when horizontal in-situ stress is uniform ground stress,and difference degree of collapse pressure is less than 5.00%,then formation anisotropy can be ignored;the other in-situ stress conditions should comprehensively consider the influence of anisotropic formation mechanics and weak face on collapse pressure,and determines the formation collapse pressure value.Borehole instability risk is higher when directional well is drilled along any direction when the well deviation is closed to weak face angle within the scope of the 45°-75°inclination.The results are of guiding significance for drilling in hard brittle shale formation,which provides basis for reasonable engineering measures to solve the problem of borehole instability.