Up/Down discrepancies with Target Line Changes in 3D Laterals

Covers differences in Up/Down results when using Well Seeker Pro compared to manual calculation, in the context of target line changes in 3D laterals.


When calculating above / below distances of a survey against new geosteering plan lines, manual calculations will not work unless the vertical section of the well matches the azimuth of the lateral due to limitations with this type of calculation. The values calculated by Well Seeker are not bound by these limitations and it is recommended that manual calculations are avoided as these can lead to confusion.


While drilling a well, the directional driller is constantly checking the position (U/D & L/R) of the wellbore compared to the plan, to ensure the drilled well is where it needs to be. When drilling ahead in the lateral, it is common to receive geosteering updates which are used to create a new target line which supersedes the original plan. These updates are often provided referenced to zero vertical section.
Well Seeker has the ability to create new plans using this information, which the surveys can then reference when providing U/D & L/R values. In many cases directional drillers also utilise excel spreadsheets to give them their distance above and below the line. Sometimes the Well Seeker output and the excel output do not tie in leading to questions regarding the accuracy of Well Seeker.


Vertical section defines the vertical plane or planes used to measure the well displacement. The plane requires an origin and a direction. The excel documents that are commonly used to calculate the distance of the survey above or below the target line, achieve this by calculating the TVD of the target line at a given Vertical Section and then working out the difference compared to the survey TVD at the same vertical section.
In this document we will look at a geosteering example and will compare the manual calculation to the Well Seeker output. The focus will be on the U/D values.
Original Well Plan
The below well plan has a Vertical Section of 152.11°. Note that the azimuth for the final ~5000’ of the plan is 133.54°
Latest Survey
The below survey was the most recent survey point when the geosteering update was received.
Geosteering Update
Geosteering update received @ 15627’ MD: New target line at 10378' TVD at 0 VS with 87.5° INC
Manual Calculation
Basic calculations can be used to determine the TVD at any given vertical section, which can then be used to work out if the survey is above or below the line. The azimuth of this target line is taken to be the same as the plan which at this depth is 133.54° to TD.
In this example, the Vertical Section is taken from the last survey (5121.63’) and the inclination is provided in the geosteering update (87.5°)
This is a simple method for calculating distance above and below the line, but it has its limitations.
Manual Calculation Limitations
If the VS of the plan and survey (Red line below @ 152.11°) is not the same as the azimuth of the plan (Green line below @ 133.54°) at the current survey depth of 15627’, then this calculation breaks down.
The blue dashed line in the above diagram, represents the vertical section at 5121.63’ as per the last survey point @ 15627’ MD. It has been drawn perpendicular to the red line, which represents the vertical section which has been assigned to the plan and survey. Note the point at which it crosses the green line, which represents the azimuth that the new target line is on. Because of the difference in the VS azimuth and the target line azimuth, the point the blue line crosses the green line is further away from the origin than the point it crosses the red line.
The purple line crosses the green line (not to scale) at the same distance from the origin as the blue line crosses the red line. Based on the above, the manual calculation is not looking at the correct point on the target line. It is actually referencing a point which is closer to the origin.
Well Seeker
Well Seeker has a “Create Offset Plan” function, which allows the user to enter the geosteering update to create a new plan that will effectively become the desired target line. The user enters the measured depth of the last survey, which will be the point on the original plan that the new plan ties on.
The TVD @ zero vertical section and the dip are entered and the new TVD is calculated.
Below left is the original plan and below right is the new plan created. The part of the plan in the red box represents the new target line
Manual Calculation vs Well Seeker Differences
When calculating distances against offset plans and surveys, Well Seeker uses the selected anticollision scanning method (most often 3D closest approach) to determine the closest point on the offset. It then provides an output for Distance (closest C-C distance), U/D and L/R for each survey station. Vertical section is not factored into this and does not therefore cause any limitations to the calculation.
Below is a continuation of our example, where we now have additional surveys. Below left is from Well Seeker and below right is from an excel spreadsheet. Note that at our tie on of 15627’ MD, the values for U/D are very similar, but by the last survey point @ 19039’ MD, they differ by ~7.6’
The manual calculation, using the VS from the final survey (8364’) puts the target line @ 10743.2’ TVD at the same point, meaning the survey is 18.45’ above the line. This is NOT correct. Due to the setup of this example, the manual calculation is not looking at the correct point on the target line.
In Well Seeker, a simple way to confirm the U/D values is by interpolating the plan at the same vertical section as the survey (below). We can then subtract the survey TVD (10724.73’) from the interpolated TVD (10750.98’). In this case it gives us a value of +26.25’, which ties in with the values reported on the survey interface.