Numerical and experimental study of the stability of non-circular boreholes in high-permeability formations

• Non-circular well is an innovative drilling concept currently under development.
• Borehole stability of non-circular well was studied numerically with ABAQUS.
• An unusual failure mode was discovered by finite-element analysis.
• Failure was due to tensile stresses generated in protrusions of the cross section.
• The results of numerical simulations were verified through a laboratory experiment.

One innovative drilling concept explored in recent years is based on the use of a non-circular well cross section, in particular a well with grooves running along the well axis. The grooves are expected to improve wellbore hydraulics and hole cleaning. In the present study, borehole stability analysis has been carried out for a horizontal rifle well in a high-permeability formation. It has been found that tensile failure is likely to develop at several locations around the non-circular cross section, a failure mode that is not normally found in conventional (circular) wells. The numerical results obtained with a finite-element code have been qualitatively confirmed in a dedicated laboratory experiment performed on a block of concrete with a section of non-circular well. In the experiment, the unusual failure mode was clearly observed. Traditional breakouts were also observed in the same test, but had smaller size. Numerical simulations have been carried out for different drilling conditions, both onshore and offshore, and depths from 1000 m to 2000 m, normal or abnormal pore pressure. The borehole was always found to fail in tension unless a considerable overbalance was applied by the drilling fluid. Similar results were obtained with formations that had hydrostatic pore pressure, elevated pore pressure (overpressure) or reduced pore pressure (underpressure, depleted reservoirs). The results are of importance for practical application of non-circular wells.