Sidetrack tie-on: where the kick-off station belongs

A sidetrack leaves its parent wellbore at the kick-off point, which almost never falls on one of the parent's survey stations. So the directional driller interpolates a station at the kick-off depth from the parent stations above and below it. We call it the kick-off station.

The short version

1. Tie on to a measured station. The tie-in station is the last measured station in the parent's definitive survey above the kick-off point.
2. Put the kick-off station in the sidetrack's survey, directly after the tie-in station, flagged as interpolated.
3. Leave the parent's survey as it was measured. Do not insert the kick-off station into it.
4. Calculate the kick-off station by minimum curvature, not by averaging the inclinations and azimuths.
5. Calculate it again whenever the parent's survey changes.
6. Between the sidetrack and its parent, start the relative uncertainty at the kick-off point. Down to that point, they are one hole.

If the kick-off station is calculated correctly, both layouts in Figure 2 give the same wellpaths. The difference shows when the station is wrong, or when the parent's survey changes later.

Why the parent's survey stays as measured

A definitive survey is a record of measurements. The kick-off station measured nothing. In the sidetrack's survey, it is simply where a new record starts. Inserted into the parent, it edits the finished record of another well:

• The parent changes with every sidetrack. A parent with three sidetracks gains three stations that were never measured.
• A mistake spreads. A wrong kick-off station in the parent moves the parent below it, and every deeper sidetrack with it.
• The parent's uncertainty changes. Some error terms depend on the distance between stations, so an extra station changes the parent's ellipse. The change is small, typically a fraction of a percent, but it should be zero.

Whichever layout you use, keep the kick-off station. Without it, the sidetrack is calculated as leaving the parent at the tie-in station instead of the kick-off point. That typically moves the sidetrack by 0.1 to 0.4 m.

If the parent was surveyed more than once, for example by a gyro run to set a whipstock, agree which survey is definitive before you tie on. Some software ties on directly at the kick-off depth and calculates the station from the parent. That also keeps it out of the parent's survey.

Where the shared hole ends

Down to the kick-off point, the parent and the sidetrack are one hole. They share the same position and the same uncertainty, so the relative uncertainty between them is zero. It grows only below the kick-off point (Figure 3).

Two things follow:

• Start the relative uncertainty at the kick-off point, not at the tie-in station. Some anti-collision software starts it at the deepest station that both surveys share, which with the recommended layout is the tie-in station. That typically makes the sidetrack's relative uncertainty 25 to 75% larger at its first survey. That is on the safe side, but the sidetrack then fails the clearance check against its parent for longer. If your software lets you set where the sidetrack's errors start, set it to the kick-off depth.
• Give the kick-off station the parent's error model. In the sidetrack's survey program, run the parent's survey down to the kick-off depth, and start the sidetrack's own survey below it. Otherwise the kick-off station takes the sidetrack's tool, and the two wells disagree about a point they share. That can change the sidetrack's ellipse at TD by up to about 1.5%.

This applies only between the sidetrack and its own parent. Against every other well, the sidetrack's uncertainty starts at surface, as usual. Between two sidetracks from the same parent, the shared hole ends at the shallower kick-off point.

Right at the kick-off point, the sidetrack fails any clearance rule against its parent, because both wells start in the same place. Agree in advance how far the rig may drill while that check fails, for example the first 30 m or until the wells are 3 m apart, and what happens if it still fails at that limit. An open or producing parent is a bigger risk than a plugged and abandoned one.

The kick-off depth is an estimate

The calculation can be exact while the depth it uses is not (Figure 1).

• Cased hole. Use the top of the whipstock, the tip of its ramp. The new hole leaves the parent's centreline there, so it is the deepest point the two holes certainly share, and it is the depth the crew tallies and tags. Use the top of the whipstock itself, not the anchor or setting depth. The top of the window is less fixed: multi-mill assemblies usually start cutting the casing a little above the top of the whipstock, by up to about 1.5 m. Do not use a depth lower in the window, where the new hole has already turned. If a regulator asks for the top of the window as the sidetrack depth, record both.
• Open hole. The bit leaves the old hole gradually while time drilling. Some take the kick-off point at the top of the trough, where time drilling started. Others take the depth where the cuttings show mostly new formation.

Record which depth you used and how certain it is. A 5 m error in the kick-off depth typically moves the sidetrack's TD by about 0.1 m. If the actual exit turns out to be somewhere else, calculate the kick-off station again at that depth.

Two quick checks:

• Look at the dogleg from the kick-off station to the sidetrack's first survey. If the whipstock or the assembly could not have made it, check the kick-off depth and the tie-in.
• If another rig drills the sidetrack, convert its depths to the parent's depth reference first. The sidetrack's measured depths run from the parent's reference, not from the kick-off point.

References

• Sawaryn, S.J. and Thorogood, J.L. (2005). A Compendium of Directional Calculations Based on the Minimum Curvature Method. SPE Drilling & Completion 20(1), 24–36, SPE 84246. Interpolation along the minimum curvature arc.