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gyro drift are time dependent and therefore continue to get worse, the longer the route and others are random like the effects of inadequate survey intervals but we can still come up with a rough estimate that will allow us to assess the realism of any quoted error models and the likelihood of hitting a target. In this example a target is to be drilled with MWD with the following typical accuracies: measured depth is good to 2m/1000m, inclination to +/- 0.3° and azimuth to +/- 1°. ’s y owokou h ppoxm llps ofunc ny by the time we reach the target.
Figure 75: Plotting the uncertainty.
Fslyl ’sd mn h ffc of h m su ddph o of 2 m/1000. In this simple example, this can be determined without knowing the planned trajectory. The accumulated error at the target will be the same no matter what path we take to get there. This is not necessarily obvious. Using the fact that survey error propagates with distance from origin we can draw the uncertainty axis created by depth error as a line of 10 m (2m/1000m x 5000m distance) towards and away from the origin. By way of example, if the well was drilled as a horizontal well with 4000 m vertical drilling and 3000 m horizontal drilling after a sharp build to horizontal, the total MD would be 7000 m. ’ssyh h depth was overshooting by 2 m/1000 m. In this case, the depth error would be 2m/1000m x 7000m = +/-14m. If this had been a straight vertical well our depth could actually be anywhere between 6986 m and 7014 m.
Figure 76a: Calculating positional error.
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