The Impact of Drilling Unloading on Wellbore Stability of Shale Formations
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摘要: 钻进泥页岩地层时井壁易失稳，且井周岩石极易因钻井卸载产生诱导缝，导致岩石强度降低，从而加剧井壁失稳。因此，为确保泥页岩地层井壁的稳定，采用三轴力学试验仪模拟钻井卸载过程中的岩石应力变化过程，分析了卸载对泥页岩力学特性的影响，采用回归方法回归了泥页岩内聚力、内摩擦角与卸载幅度的关系，并将该关系引入到常规井壁稳定性模型中，建立了考虑卸载作用的泥页岩井壁稳定性模型。结果表明：卸载会使泥页岩的强度降低，随着卸载幅度变大，泥页岩强度的降低幅度增大；考虑卸载作用后，泥页岩地层的坍塌压力增大，尤其在高地应力和各向异性较强的泥页岩地层，由卸载造成的坍塌压力增量更为明显；井筒与最小水平主应力的夹角较小时，可以降低卸载对泥页岩井壁稳定性的影响。研究结果表明，卸载对泥页岩地层坍塌压力的影响不可忽视，进行钻井液设计时应考虑卸载对坍塌压力的影响。Abstract: It is easy for wellbores to lose stability during drilling through shale formations where induced fractures and cracks might be generated due to drilling unloading in rocks around the wellbore and the consequent reduces rock strength which thereby can cause wellbore instability.To ensure consistent wellbore stability while drilling through shale,the author simulated stress change process during drilling unloading with a triaxial testing system to analyze the impact of unloading on shale mechanical properties.Further,the relationship between shale cohesion,internal friction angle and the unloading amplitude was set up with the regression method,and a wellbore stability model was established that took into consideration the unloading effect by introducing the newly defined relationship into normal wellbore stability model.Testing results demonstrated that unloading will reduce shale strength and accordingly shale strength reduction will increase with the increase of unloading.Considering the unloading effect,wellbore collapse pressure will increase for the shale(especially for shale under high in-situ stress and strong anisotropy).When the angle between the borehole and the minimum horizontal stress is comparatively small,the effect of unloading on the shale wellbore stability is limited.The research results indicated that the effect of unloading on wellbore collapse pressure of shale should not be overlooked,and it should be taken into account in designing the drilling fluid.
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Keywords:
- unloading /
- shale /
- wellbore stability /
- wellbore collapse pressure
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