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16.HumnE o M su m n nc ny When surveyors refer to error models they are usually referring to the inaccuracies of instruments or measurement systems. Clearly these cannot compensate for bad practice and human error such as using grid North instead of True or GPS on the wrong datum. The instrument inaccuracies are often referred to as modellable errors whereas the practical mistakes are referred to as unmodellable errors. Great care has to be taken when using industry standard error models to ensure that best practice has been adopted before assuming that the calculated uncertainties are representative. The following section points out a few common mistakes.
16.1 Common Human errors
Common Pitfalls The following common well planning pitfalls are listed to raise awareness. Past experience has shown that these are likely errors that can be overlooked. Best policy is to review this list after producing a new well plan. Missing Data h mos f qu n sonfo collson sno poo su yngbu h h h obj c w llc m s “su p s ”. s essential that the well planner ensures that he has all the data needed to plan a safe well path. Always check a list of wells with the client to ensure that nothing was lost in migration. Using Gyro Error Models for Undrilled Sections m yw llb h gyosu y spl nn d nfo x mpl ,95/8c sng. u p o o hsgyosu y“conf mngo mo ccu lyd sc bng” h w llp hof h w ll, hss c onwllusu llyb 12¼ nchhol whchwllb d ll d with MWD. It is important during well planning that the anticipated survey program as each well section is drilled, is used. The updated or most accurate survey and error model should be entered and used for anti-collision purposes right before a new section is drilled. But during the initial planning the as drilled error models should be used to the total depth of the well to confirm the entire well can be drilled conforming to required separation factor rules.
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 fo d n d m n ng h unc n y of h s s’ su f c loc ons. Caging “ gng” s h mfo h cond onwh n ww llc nnolong b d ll ddu o h poo pos onngof 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.
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