Changing Datum Elevation

A guide to changing datum elevation in an existing plan or survey.

1.0 - Introduction

The following document will help to guide the user through the process of changing the depth reference (elevation) in Well Seeker for an existing plan or survey, which has an elevation already assigned.

2.0 – Depth Reference

In Well Seeker, the depth reference is entered at Well Level via the Well Properties dialog. This dialog is accessed via the database tree, by right clicking on the Well level and selecting Properties.
The user can then enter their depth reference data into the Depth Reference section, highlighted in red below. Multiple depth references can be entered at Well Level.
Once the user has entered their depth reference information at Well Level, they can then select the relevant depth reference for any plan or survey under that well. This is done by right clicking on the Actual Well or Plan level and selecting Properties. This will open the Actual Wellbore/Plan Properties Dialog. The user can then select the desired depth reference from the dropdown menu in the Depth Reference section, highlighted in red below.
If the Plan/Actual Well already has a datum referenced, then changing the depth reference in the Actual Wellbore/Plan Properties Dialog will prompt the following warning:
The user has three options:
  1. 1.
    Change TVD’s: Picking this option will change the MD and TVD values of the survey/plan points to reflect the new depth reference
    1. 1.
      Note: The TVD Subsea values will remain unchanged if the GL to MSL value is the same for both datums.
    2. 2.
      This option is usually selected when you have an existing well that was drilled with one rig and you are now planning to perform additional work with a different rig. In this situation the user wants to see the survey or plan depths adjusted to reflect the new datum.
  2. 2.
    Preserve TVD’s: Picking this option will preserve the MD and TVD values of the survey/plan
    1. 1.
      Note: The TVD Subsea values in this case will change.
    2. 2.
      This option is usually selected when a plan or survey has mistakenly been referenced to the wrong datum, and the user wants to assign a new datum, but not adjust the MD and TVD values.
  3. 3.
    Cancel: The change of depth reference is cancelled, and no changes are made to the survey/plan points.
Note: The user will also get this warning if they edit the value of a depth reference that is currently being referenced by a survey or plan.

3.0 – Example

In this example, we have a simple set of surveys, referenced to the below left RTE (RTE Plan) @ 100Usft AMSL.
We are then given a new RTE (below left RTE Actual) to reference @ 101Usft AMSL. When we select this new depth reference, the below right warning appears.
If we choose to Change TVD’s the surveys will change to the below:
  • The MD and TVD values have increased by 1Usft
  • The TVDss values have remained the same, except for the tie-on line
  • This would be desirable if you are side-tracking this well with a rig that has a RTE 1ft greater than the original rig.
If we choose to Preserve TVD’s the surveys will change to the below:
  • The MD and TVD values have remained the same
  • The TVDss values have changed by 1ft
  • This would be desirable if you discover that you have the wrong RTE referenced and want to reference the correct RTE without adjusting the survey depths.