Drillstring Vibrations and Wellbore Quality: Where Drillstring Design Meets Geomechanics

Abstract

Wellbore quality might not always be the top priority from a drilling perspective due to the misconception that quality comes at the expense of drilling efficiency. This is indeed a misconception since a compromised wellbore quality, which can be defined as non-uniform wellbore shape due to the presence of enlargements and tight spots, is a sign of energy waste. The result of this is actually drilling inefficiency. It is proposed that drilling quality and efficiency come hand in hand provided that they are planned to be connected by two factors: drillstring design and geomechanics modelling. Drillstrings are designed to prevent mechanical failure and produce the highest possible ROP. Another aspect of the design is preventing vibrations and hole patterns. This work employs surface drilling parameters and geomechanics principles to correlate the drilled wellbore quality to the drillstring design. Surface drilling parameters and bit wear grading reports are used as diagnostic tools to check for dissipated energy and string vibrations, where the dissipated energy can be either mechanical, hydraulic, or both. This is then cross-referenced with multi-arm caliper measurements to confirm the influence on wellbore quality. A geomechanics model is incorporated to filter out in-situ stresses induced breakouts from breakouts and hole patterns caused by interactions with the drillstring. The role played by the drillstring design on this whole process is explored. Finally, drillstring modifications are proposed based on geomechanics considerations. The illustrated case shows a strong correlation between the shape of the wellbore, the spacing of the different bottomhole assembly components, and the formation mechanical properties. Another illustrated case, which utilized a modified drillstring design showed significant improvement in the wellbore quality. The results of the different cases confirm that incorporating geomechanical analysis in the process of drillstring design will help reconcile both of drilling quality and of efficiency.