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Clearly, this is very inaccurate when the field strength is weak and the degradation is very small from one signal to the other. This occurs when the target is a long way off and thus the uncertainty is very large. A first ranging shot is not worth taking unless you are within about 100ft of the target well.
The uncertainty on a ranging shot is on both distance (Range) and Bearing (Toolface Angle) and the normal distribution of likely positions of the target well looks like a wedge shaped mountain as shown. The surveyed target well trajectory (shown here in red) is then adjusted to the centre of the normal distribution after each shot and the plan to target is updated based on the latest estimate of the relative position. Clearly this becomes much more accurate as the proximity improves.
Figure 116: How the uncertainty forms on the ranging shot .
Some Practical Considerations
1. A relief well can only be drilled from a safe location. This usually means that a relief well plan will estimate the p lngwndd c on nd‘smok ng ’ nd hsd mn s h loc onof h l fw ll g. hswllb a considerable distance from the target well so accurate surveying including surface positioning, is required in both for a successful relief well. 2. The kill point in a relief well cannot be any shallower than the point at which the hydrostatic head of kill mud exceeds the pressure in the target well. At this point the uncertainty may well be more than 100ft. When planning the relief well, it is often qu d o d s gn o ‘loc on’ o ‘hom ng n’ po n w ll sh llow of h k ll point. This point has to be shallow enough that the combined uncertainty of the relief well and the target well does not exceed 100ft for active ranging and 50ft for passive ranging. 3. It is never allowed to approach the target well with a separation factor less than one before ranging. Once ranging is underway, the relative positioning uncertainty takes over but prior to the first ranging shot, there must be no danger of colliding early with the target well or you may have two blowouts for the price of one.
4. When selecting the homing in point, it is important to remember that the ranging tool will only see the effects of one magnetic field no matter how many wells have contributed to it. It is vital then that at this point there is no danger of ranging to the wrong well. This then forces discipline in the planning stage especially on crowded platforms and pads where there has to be enough space for the relief well to reach the target without significant collision risk and for the target well response to be unambiguous. As a rough guide, all other wells should be at least three times the distance away that the target well is when taking the first ranging shot. 5. After each ranging shot, the survey on the target
Figure 117: Ranging on crowded platforms.
well is shifted to pass through the observed position. As you approach closer, this shift should reduce in size. If it increases in size, there is a danger that the ranging is seeing more than one well and the procedure should build in more ranging checks to establish an unambiguous target. Another reason though may simply be a
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