Christian Zurdo

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 faults 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.

Barite Sag: Measurement, Modelling and Management

A joint industry project was established to study barite sag mechanisms and to develop field guidelines to manage the consequences. A simple empirical model was developed to compare sag potential for a wide range of fluid types. In the study, physical properties of the mud, wellbore conditions, and characteristics of the weighting material were shown to have a large influence on sag behaviour. The study also included direct measurements of the properties of settled weight-material beds. These results provide new insight into the mechanisms of barite sag and how best to manage problems in the field. Data from the tests clearly demonstrate that the parameters affecting sag are interrelated and seldom act in isolation. For all muds tested, the highest sag occurred at low annular velocities over angles from 60-75°. Drill-pipe rotation was particularly beneficial in minimising barite settlement. Rotation also assisted in re-distributing barite deposits formed on the low side of the hole. The improved understanding of the mechanisms of barite sag enabled development of practical field guidelines. Case history studies presented in the paper demonstrate how the results of the work together with better field monitoring have been successfully applied to manage the effects of barite sag in HP/HT and extended-reach drilling operations.