硬脆性泥页岩地层井壁稳定性研究

Research on wellbore stability for hard brittle shale

北部湾盆地流沙港组属于典型硬脆性泥页岩地层,地层层理、微裂缝发育明显,非均质性强,钻井过程中易引发井下复杂情况和事故。基于页岩理化室内实验结果及力学参数,根据页岩水化分析结果和页岩破坏准则,建立了井壁稳定力化耦合模型。实例验证及影响因素分析结果表明:本文建立的井壁稳定力化耦合模型能更准确预测地层坍塌压力分布;层理面的存在会显著增加地层坍塌压力当量密度,不同层理产状使得坍塌压力分布更为复杂;当硬脆性泥页岩与流体作用时,岩石强度下降且层理面受水化影响更为明显,从而导致坍塌压力上升,尤其清水作用后地层坍塌压力上升幅度明显高于钻井液作用。本文研究结果可为泥页岩地层井壁稳定性研究提供参考。

Liushagang Formation in Beibuwan basin is of typical hard brittle shale, with abundant bedding planes and micro cracks development, as well as strong anisotropy, which may lead to high risks of collap-sing during drilling. Based on physical-chemical properties and mechanical parameters obtained from exper-ments , and under shale hydration analysis and failure criterion, wellbore stability model with chemo-mechanical coupling has been established. Results of application verification and affecting factors analysis in-dicate that this chemo-mechanical coupling model can predict collapse pressure distribution more accurate-y. The existence of bedding planes can dramatically increase collapse pressure equivalent density, and the occurrence of various bedding planes may make the distribution of collapse pressure more complicated. When shale contacts with fluid, shale strength would decline and bedding plane may show more obvious strength declining, thus leading to increment of collapse pressure. Especially after being impacted by water , collapse pressure will increase more significantly than drilling fluid. The study results can provide ref-eree for wellbore stability in shale formation.