Extended Reach

Except in some cases of high (reverse fault) stress and soft rock conditions, there is no reason why an ideal drilling direction for a horizontal wellbore cannot be found if the full 3D stress volume is known.


Parallel strike slip stress states afford a simple inclined to horizontal drilling direction at 45° to SHmax (SHD). That stress state is common in many fields at reservoir levels. As SH/SV increases relative to Sh/SV, ideally a wellbore needs to be directed closer to SHD but the same 3D seismic survey may provide evidence of hazards such as faults planes which may be sub-parallel with the optional well design and stress dictated trade-offs may be required; all best known in the planning stage from 3D seismic. It can then be determined if the stresses can be used to advantage or avoided.


If the stress state is nearer normal fault, the wellbore needs to be sub-parallel with Shmin (ShD). PSI accurately predicted SHD (hence ShD) at the eastern part of the Tullich field in the UK (SHD = 50° rotating to 095°) which was confirmed as 060° from later tensile fracture measurements. In the northwestern part of the field SHD rotated further to 135°. This 85° (60 to 135°) of SHD rotation of stress from seismic compares with the single pre-drill plate tectonic assertion of 135° for the entire area of the map and the post-drill 060° at the northeasterly well.


The low stress state of extensional strike slip faulting, almost normal faulting, required a horizontal wellbore direction sub-parallel with ShD or 025° (205°) in the central part of the field. This gave a low wellbore stress ratio of 1.03 for the SHM = extensional strike slip stress state. The planned 5 well programme was completed by 4, trouble free and showed the importance of both SHMagnitude (SHM) and SHD in well planning, especially in shallow soft rocks.