Vertical Section is the horizontal distance of wellbore that moves in the direction of the target per each station or in total. For instance, in the figure below, vertical section is the distance from survey to survey point and it’s measured in the same direction of the vertical section direction.
The two factors that affect vertical section are as follows:
1. The Incremental horizontal displacement (? HD)
2. Vertical section direction (VSD) is the azimuth that is used to reference to the vertical section. Normally, VSD is the azimuth of the last target.
The simple mathematics as Average Angle Method calculation demonstrates the relationship of the VS as below:
VS = cos (VSD – Az avg) X ?HD
VS: Vertical Section
VSD: Vertical Section Direction
Az avg: Average Azimuth between 2 points (Az1 + Az2) ÷2
?HD: Delta Horizontal Displacement
In order to get the Positive Vertical Section or Zero Vertical Section, a well path must have difference of angle between VSD and Az avg, (VSD – Az avg), within a range of +90 to -90 degree. On the other hands, the negative Vertical Section can occur because the difference of angle between VSD and A zavg, (VSD – Az avg), is out of range of +90 to -90 degree AZI.
Ref book: Formulas and Calculations for Drilling, Production and Workover, Second Edition