Consideration of the stress field around an arbitrarily oriented borehole shows that in an extensional stress regime (sv > sH > sh), wellbores parallel to the direction of minimum horizontal principal stress are the least prone to compressive shear failure (breakout). The most stable deviation angle (from the vertical) depends on the ratio of the horizontal principal stresses to the vertical stresses, and the higher the ratio sH/sv, the higher the deviation angle for minimizing breakout. In a strike-slip stress regime (sH > sv > sh), horizontal wells are the least prone to breakout, and the higher the ratio sH/sv, the closer the drilling direction should be to the azimuth of sH.
A new compressive shear failure criterion, which is a combination of the effective strength concept and the Drucker-Prager criterion, is proposed for quantifying the stresses at which borehole breakout occurs. The lowest mud weight, at and below which breakout will occur, can be predicted by combining this criterion with the stress field around an arbitrarily oriented borehole. The highest mud weight at and above which a tensional or hydraulic fracture is induced can be predicted by combining the tensile strength of the rocks of the wellbore wall with the stress field around an arbitrarily oriented borehole. For the in-situ stress environments considered, the optimally oriented inclined wellbore is less prone to breakout (i.e., allows a lower mud weight) and tensional or hydraulic fracture (i.e., supports a higher mud weight) than a vertical well.
It has been widely recognized that highly deviated, extended-reach and horizontal wells can offer economic benefits through lower field development costs, faster production rates, and higher recovery factors. However, inclined and horizontal wells may be prone to mechanical instability problems associated with the in-situ stress field. Hence, an understanding and analytical design capability to manage wellbore stability in high in-situ stress fields should help realize the full benefits offered by current and emerging inclined well drilling technology.