Well Seeker Pro Change to UD LR Logic at low inclination
With each new survey added to Well Seeker, the program automatically calculates the centre to centre (C-C), up/down (U/D) and left/right (L/R) distances against the principal plan. When the well has a high inclination these values are generally stable (survey dependent) and provide the user with values that they expect to see.
When drilling a vertical portion of the well, these U/D L/R values could change significantly from survey to survey which was a source of confusion for many users. From v2.1.0.35 Innova has implemented a change which reduces the fluctuating nature of these values at low inclination.
This document will help provide the user with a better understanding of the initial problem and the implemented solution.
When calculating U/D and L/R values, Well Seeker finds the closest point on the plan to each survey station and as long as the planned point has an inclination greater than zero, Well Seeker uses the azimuth of the plan at this point as the high-side (HS) reference. The planned azimuth is stable in most instances and will not exhibit large swings over a short measured interval. As a result, the U/D L/R values are stable and predictable. No change has been made to this logic in the new version.
In any situation where the well plan is vertical, there is no azimuth for the program to use as a reference. Historically, Well Seeker would use the azimuth of each individual survey station as the HS reference. At low inclinations the survey azimuth can change significantly between survey stations, which in turn would cause the U/D and L/R values to also show a significant change.
To illustrate how this works, we will look at an example.
In the below example, we have a principal plan that is vertical to 700 Usft MD, at which point it kicks off.
We also have the associated survey which contains 8 survey stations in the vertical portion of the well.
For each of these stations the closest point on the principal plan is vertical with zero inclination.
In this example, we will focus on the last 2 survey stations at 530 and 630 Usft MD. In the above screen shot you can see there is a large change in the azimuth (320 to 110°) between these 2 surveys and despite there only being a change of 0.26 Usft in distance (C-C) from the plan, there is a noticeable change in the U/D (8.21 to -6.83 Usft) and L/R (-0.56 to 5.03 Usft) values.
The centre to centre (C-C) distance is calculated using the 3D closest approach scan method and as such, this is stable at all inclinations and not something that requires further explanation.
Below is a plan view plot showing the position of the 2 survey stations, relative to the plan. Looking at this alone, it is not immediately clear why we are seeing such a big change in the UD L/R values.
The 3D plot provides a clearer visual representation of the 2 surveys, which includes the HS line. Both plots below are positioned with Grid North at the top of the screen. Although the survey at 630 Usft MD (below right) has only deviated 0.26 Usft further from the plan than the previous survey at 530 Usft MD (below left), the HS is very different. Since the HS of the survey is the reference used when calculating our U/D L/R values in this situation, this is the reason for the significant change we are seeing at this low angle.
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While the U/D L/R values are an important output when drilling a well, as the above example demonstrates, large azimuth changes at low inclination can have a marked effect on the U/D and L/R values. The tendency of low inclination surveys to drastically switch high-side is the reason why these values should be avoided in a vertical portion of the well, as the U/D L/R values are not as meaningful as they are at higher inclination.
To address this issue, Innova have implemented the following change. If the closest point to the survey on the principal plan is vertical, Well Seeker will now no longer use the HS azimuth of the survey as the reference, but rather it will switch to using the North reference. This is a more relevant reference at low inclinations and will not fluctuate in the same way as the HS reference.
The below plots are identical to the ones above, with the exception that north reference is selected instead of HS. Here you can see the U/D L/R values are replaced by North/South and East/West, which does not fluctuate in the same way and is more relevant to the low inclination surveys.
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Below left are the U/D L/R values calculated using the old logic while below right shows the output using the new logic. Both of these are correct based on the references used; however, it is clear to see that the new logic provides a more stable output.
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With the implementation of the new logic, there are some things that the user should be aware of.
Firstly, unless the well is planned as purely vertical, the user can expect there to be a switch in the U/D L/R logic once the plan starts to deviate from vertical. Depending on the position of the survey relative to the plan, the user may see a significant change in the values between the new survey and the previous one.
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In the above example, there is a switch in logic between the survey at 630 and 730 Usft MD, as the closest point on the plan changes from being vertical to having an inclination. You can see that there is a noticeable change in the U/D value. This change is no worse than what the user would experience with the old logic. The main benefit is that this will only happen once, compared to every survey.
When kicking off at a shallower depth than the plan, the user needs to remember that the U/D & L/R values will continue to be referenced to North until such time as the closest point on the plan deviates from vertical. As a result, when the switch in logic occurs, the user may experience a larger change in the U/D L/R values, since the surveys will be further away from the plan at that point.
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In the above example, the well starts to KO around 430 Usft MD but the plan does not KO till 700 Usft MD. You can see when the switch happens at 730 Usft MD, the change in U/D is almost double that of the previous example.
When a plan has a change in azimuth, the reference direction changes and as a result the U/D L/R values do as well. When close to the plan, this change is negligible and will rarely be noticed. However, this is amplified the further away from the plan that the survey is. This can cause significant shifts in the U/D L/R values, when there is a change in the plan azimuth and the user is a long way from the plan, even when the survey an d plan inclinations are well above vertical.