Abstract
Abstract
Whilst drilling development wells in the Norwegian North Sea over the last several years, significant drilling dysfunction in the form of bottom hole assembly (BHA) whirl has presented while drilling both the Grid and Skade sandstone units which resulted in trip for failures and non-productive time. These sandstone units form part of the overburden section and are laterally extensive across many fields in the Norwegian continental shelf. The historical consensus with regards to the causative mechanism was that the cleaner intervals of the sandstone units was being hydraulically enlarged by the flow exiting the bit and subsequently providing the bottom hole assembly room to move within the wellbore and develop damaging vibration mechanisms.
During a recent campaign of both exploration and development wells, the BHA whirl vibration mechanism once again presented itself where the occurrence was documented across a range of hole sizes, wellbore inclinations and drive types from rotary assemblies to rotary steerable tools. While the whirling dysfunction had caused non-productive time on historical drilling campaigns, there was not a documented root cause in place to clearly tie the dysfunction to wellbore enlargement.
After the initial instances of BHA whirl during the recent drilling campaign, a multi-disciplined investigation was initiated to evaluate the root cause of the drilling dysfunction in the clean Grid and Skade sandstone units where the corrective action was to re-evaluate the drill bit design criteria and hydraulic configuration. The data and interpretation presented in this paper will demonstrate that the source of the drilling dysfunction originates at the drill bit / rock interface and is not initiated by a mass-imbalance mechanism due to wellbore hydraulic enlargement. To conclude on the data interpretation, the stability benefits of a different bit type will be presented to demonstrate the effectiveness in eliminating bit induced whirl and the criticality of interpretating drillstring dynamics data inn both the time and depth domains.