16.2 Misapplication of Uncertainty in Top Hole

There are two common problems in applying uncertainty in the top-hole section.

Top-hole uncertainty must include the radius dimension of the well. This is unlike deep sections where the measurement uncertainty dimension is so great that the radius dimension component of the uncertainty is a very small percentage. In the top hole when measurement uncertainty is just beginning to accumulate, the well radius dimension is a significant component in anti-collision.

If the area being scanned includes multiple sites (platforms or surface locations from which wells have been drilled) then the uncertainty as to the actual coordinates of the surface site must be determined and included in the calculations. Refer to Section 3.2.1 for aid in determining the uncertainty of the sites’ surface locations.

Caging
“Caging” is the term for the condition where a new well can no longer be drilled due to the poor positioning of previously drilled wells. This usually happens later in the development cycle of pad or platform drilling. It is useful to include all future planned wells in the collision scanning at the planning stage and during drilling operations so that, whilst not safety critical, sensible avoiding action can be taken so that the potential for future caging is minimized

Overly Conservative Targets
Small targets cost a large amount of money to drill as the directional driller in the field will not be afforded much flexibility from the planned trajectory and will have to spend more rig time steering to the “line”.

Curved Section Close Approaches
Interpolating at long intervals when scanning can completely miss a close approach to approach another well.
It is recommended to always perform a 3D visualization run down the planned well to visually sense the effects all close approaches.
Initial numerical scan reports should always be run at 100 ft or 30 m intervals. Then refine localized zones with searches using 15 ft or 5 m intervals to better characterize the near close approaches.

Long Parallels
It is almost impossible to keep a well straight, so the situation of having two wells with long parallel vertical sections before very deep kick offs should be avoided. (Even if it were possible to maintain a straight well, the survey uncertainty will be growing and with it the risk of collision). Where this happens the planner should include a ‘nudge’ to raise the inclination to around 5 degrees and return to vertical once enough separation had been achieved to compensate for the uncertainty at the deep kick off point.