How Datums Are Handled in the EDM™ Database
EDM Database
A discussion of how the EDM database handles datums follows.
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Datum terms are defined below, and are grouped by the Properties dialog in which they are found.
In Project Properties---
System Datum: The System Datum is set in the Project Properties/General dialog, and represents absolute zero. It is the surface depth datum from which all well depths are measured, and all well depths are stored in the database relative to this datum. Usually the System Datum is Mean Sea Level, Mean Ground Level, or Lowest Astronomical Tide, but it can also be the wellhead, rig floor, RKB, etc. If you want to use a different system datum name other than the ones distributed originally with the database, you can type the new system datum name in the system datum description field; the name you entered will not be added to the system datum drop-down list, but it will be preserved for use.
Any system datum option listed in the drop down list, or entered by the user, is used as a reference label only and it does not have any controlling effect on the elevation input field (except Mean Sea Level).
Elevation: The Elevation is set in the Project Properties/General dialog, and represents the elevation above (positive) or below (negative) Mean Sea Level. If Mean Sea Level is selected as the System datum, Elevation is grayed out.
In Well Properties---
Depth Reference Datum(s): The Depth Reference Datum represents zero MD. It is sometimes known as the local datum, and is measured as an elevation from the System Datum. You can define one or more Depth Reference Datums for a well in the Depth Reference Tab (Well Properties Dialog). For each Depth Reference Datum, you must specify the elevation above or below the System Datum.
The selected default Depth Reference datum in the list box will be the viewing datum in all applications (the viewing datum can be changed ‘on the fly’ only in OpenWells® and COMPASS™ software.)
You can’t delete or change the elevation of a Depth Reference datum once it is referenced by a Design.
Offshore check box: Check to indicate that this is an offshore well; leave unchecked to indicate a land well.
Subsea check box: (offshore well) Check to indicate that this offshore well is subsea.
Ground Elevation: (land well) This is the elevation of the ground above the System Datum; it is set in the Depth Reference Tab (Well Properties Dialog).
Water Depth: (offshore well) This is the total depth of the column of water (MSL to mudline); it is referenced to Mean Sea Level.
Mudline Depth: (only for offshore subsea well) This is the depth below system datum (MSL/LAT etc.) of the wellhead flange.
Wellhead Depth: (subsea well) This is the distance from the wellhead to the system datum, and is used in some calculations where this is the hanging depth for casing leads when set. To determine wellhead depth:
Wellhead Depth (to rig floor) = Depth Reference Datum + Wellhead Depth
Wellhead Depth (set in the Well Properties/General dialog) is positive for offshore subsea and negative for wellheads above MSL (i.e., onshore or offshore platform). So, it does not matter in the above calculation whether it is offshore or subsea. Depth Reference Datum is always positive. Both wellhead depth and wellhead elevation are distances from the system datum to the flange.
Wellhead Elevation: (platform and land wells) This is the height above system datum (MSL/LAT) of the wellhead flange (surface casing). It may happen that for some land wells using ground level as the system datum that the user may have to enter a negative value because the wellhead 'cellar' is often below the ground.
Air Gap (calculated) This is the distance from the system datum to the rig floor, and is used in some calculations for hydrostatic head. Air Gap must always be positive. To calculate air gap, the application uses:
Air Gap (offshore wells) = Datum Elevation - Elevation (relative to System Datum)
Air Gap (land wells) = Datum Elevation - Ground Elevation (relative to System Datum)
Elevation is set in the Project Properties/General dialog. Ground Elevation is set in the Well Properties/ Depth Reference dialog.
In Design Properties---
Depth Reference Information: From the drop-down list of defined Depth Reference datums, select the datum you want to reference for this Design.
Procedure: Configuring Reference Datum Levels For Your Well
Project Properties, General Tab - Define the System Datum to be used for the entire Project.
Project Properties/General dialog - In the Elevation field, enter the depth offset that the System Datum is above/below Mean Sea Level. If your System Datum is below Mean Sea Level, this number will be negative; otherwise enter a positive value. If the System Datum is Mean Sea Level, Elevation is grayed out. This system datum offset is used by some of the engineering applications to calculate true air gap relative to actual mean sea level.
Well Properties, Depth Reference Tab - If the Well is offshore:
Activate the Offshore check box and enter the Water Depth below the System Datum.
If the well is subsea, activate the Subsea check box and enter the Wellhead Depth below the System Datum.
Well Properties dialog, Depth Reference tab - If the well is a land well, verify that the Offshore check box is deactivated. Enter the Ground Level elevation above the System Datum.
Well Properties dialog, Depth Reference tab - Define the Reference Datum Level(s) to be used, such as RKB or Rig floor for specific rigs or work units. Type the elevation above the System Datum in the Elevation field and specify the effective Date for the datum. Enter a description where necessary. This may be useful for other engineers to identify the correct datum level.
(WELLPLAN™ software Only) If a Design was created using one Depth Reference datum, and the Depth Reference datum is changed, then when the Design is opened any depths that become negative will be changed to zero, and all depth-related properties will be adjusted accordingly.
(StressCheck™ and CasingSeat™ software Only) When you create a design and save it for the first time, the EDM database keeps track of the Depth Reference Datum that was set at the time. This "original" Depth Reference Datum is not displayed; however, if you or someone else changes the Depth Reference Datum in the Well Properties dialog, and you then attempt to open that design, a warning message will appear. You are warned that you are trying to change to a datum that is different from the datum in which you originally saved the data, and any calculations will be invalid unless you change your inputs. You are given the choice to open the design/case in the original datum, or to convert to the new datum. If you choose to convert your data, the data will be adjusted. However, the change is NOT saved to the database until you save the design, at which time the new datum becomes the "original" datum.
How this works:
If datum is same as original datum: If you open a design or case where the Depth Reference Datum (set at the Design level) is the same as the datum the data was originally saved in, the design/case will open normally.
If datum is different than the original datum: If you open a design or case where the Depth Reference Datum (set at the Design level) is different from the original datum, the following occurs:
The application checks to see if the well is a slant hole. If positive inclination exists in well paths whose depths would become negative after the datum shift, the program cannot make the adjustments; a message pops up to inform you of this. Click Open to open the design in the original datum; if you click Cancel, the design will not open at all.
For wells other than slant holes, the program will issue this message: "The currently selected design datum is different to the datum with which the design was created. The application will then attempt to adjust the data, but some data might be shifted or removed. If you open the design, we strongly suggest that you review your input data; any changes will not be saved to the database until you explicitly save your data. Please select "Open" to review the design using the datum with which it was created."
If you want to open the Design with the original elevation, select Open. If you want to convert the data to the new elevation, select Adjust. Open is the default.
If you enter "Open": Data is loaded to the original design datum, but the Depth Reference Datum set in the Design will NOT change to match the original datum.
If you enter "Adjust": Well Explorer loads the data to the new Wellbore datum and attempts to adjust the data; however, some data may be shifted or removed. The program will resolve the deltas in the first depths of column data (strings, well paths, columns, etc.) to adjust for the new gap and read zero depth on the first line.
Note: Once you open the design you should review your input data; remember that the changes will not be saved to the database until you explicitly save your data.
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