A STUDY OF THE DESIGN OF INCLINED WELLBORES WITH REGARD TO BOTH MECHANICAL STABILITY AND FRACTURE INTERSECTION, AND ITS APPLICATION TO THE AUSTRALIAN NORTH-WEST SHELF

Knowledge of the in-situ stress field can be applied both in planning most stable drilling trajectories and also in maximizing intersection of the wellbore with open, natural, and hydraulically-induced fracture s in the reservoir. Inclined wells drilled with the optimum drilling d irection (azimuth) and deviation (from the vertical), at which the she ar stress anisotropy around the wellbore wall is minimized, may be mor e mechanically stable than vertical wells in various stress regimes. S uch mechanically stable trajectories may also maximize open fracture i ntersection. Combining the objectives of maximum mechanical stability and maximum open fracture intersection wells in an extensional stress regime should be highly deviated from vertical (at about 55-70 degrees ) and drilled along the azimuth of the least horizontal principal stre ss; wells in a strike-slip stress regime should be horizontal and dril led at 55-70 degrees with respect to the major horizontal principal st ress; wells in a compressional stress regime should be slightly deviat ed from vertical (at about 20-35 degrees) and drilled along the azimut h of the major horizontal principal stress. Application of this study to the Wanaea/Cossack field of the Australian North West Shelf, where the in-situ stress field has been constrained, suggests that the most stable drilling direction and that which maximizes potential intersect ion with any open fractures in the reservoir is horizontal, in the azi muth of the least horizontal principal stress, 005-010 degrees N.