It is a common occurrence where drilling engineers would assign a tool code to an entire well despite the well having several drilling sections, particularly when measured using only MWD tools. Such practice leads to much higher wellbore positional uncertainty. Consequently, more resources are required for anti-collision procedures. We provided an example scenario to study this practice in detail.
Let us assume that we have standard well profile ISCWSA 1.
In this example, the well was drilled in 3 sections. The table below shows the details of the wellbore construction.
Two survey programs were analyzed: the first survey program uses MWD rev 4 tool code for the entire well, and the second one uses a separate MWD rev 4 for each section.
Both survey programs look essentially identical. However, based on the resulting uncertainty ellipsoids, we can clearly see there is a difference. The first survey program has uncertainty σh = 21.6 m and σl = 94.79 m while the second one has uncertainty σh = 15.51 m and σl = 76.11 m.
This difference is due to systematic errors, such as BHA sag, are supposed to be fully correlated for the first program. The BHA sag for each section were summed as follows:
However, for the second program, these errors were considered independent and the total uncertainty was calculated as the square root of the sum of error squared:
By assigning one tool code for the entire well, we would receive a more conservative positional uncertainty. The second approach is more realistic. This is because systematic errors between the borehole sections should not be correlated as different BHAs, drillstring, and MWD tools are used. Therefore, having the correct tool code assignment can provide better EOUs with no additional cost.
A detailed summary of the decorrelation conditions are listed below. Generally, if these conditions are met (or certain errors can be ignored), we can split a drilled interval into two independent survey legs, even within the same drilling section.
