Drilling-induced lateral shifts along pre-existing fractures: A common cause of drilling problems

Two gas well collapses showed a peculiar failure mechanism. Excessive fluid levels and temperature variations during the ultimate drilling phase and later workovers induced microannuli and cementation damage. Abnormal fluid pressures from deeper gas- or water-bearing layers channeled through damaged cementation and increased the pore pressure in uphole faults or bedding joints. Thus the effective normal stress on fault's was released, inducing shear displacements and casing deformation. Weakened casings were further collapsed by later fluid-level variations. It was then suspected that this mechanism also occurred in open holes during drilling. This was shown to be so with borehole televiewer tool (BHTV) images showing borehole lateral shifts. Although never mentioned in the past in the domain of borehole stability, this mechanism explains some poorly understood drilling incidents, such as tight-holes, problems to run in hole (RIH) or pull out of hole (POOH), and abnormal torques, in spite of no indications of instability, such as cavings. This mechanism can be mitigated by taking the necessary precautions during drilling and by the adequate selection of mud characteristics.