Definition of Terms
A datum is a value that is agreed upon and known. Datums are used as a basis for calculating or measuring depth. An example of a datum is the elevation of a benchmark, or sea level, to which other measurements are corrected.
Depth Reference Datums can be found in two places in the OpenWells application; Project Properties (see page 3-34) and Well Properties (see page 3-42).
In Project Properties
System Datum
The System Datum is set in the Project Properties/General dialog, and represents zero depth reference elevation across the Project. It is the depth reference datum from which all other well depth reference elevation levels are measured. Additionally, All well depths are stored in the database relative to this datum level. Depths are not stored relative to the user's viewing datum. Usually the System Datum is Mean Sea Level, Mean Ground Level, or Lowest Astronomical Tide, but it can also be the wellhead, rigfloor, RKB, etc. A positive reference level represents a height above the system datum, a negative elevation level represents a position below the system datum.
Elevation
The Elevation is set in the Project Properties/General dialog, and represents the elevation of the system datum above Mean Sea Level. (If
Mean Sea Level is selected as the System Datum, the Elevation field is grayed out.)
In Well Properties
Depth Reference Datum(s)
The Depth Reference Datum is the reference elevation for depths displayed or entered in EDM applications. It is sometimes known as the local datum, and is measured as an elevation relative to the System Datum. You can define one or more Depth Reference Datums for a well in the Depth Reference Tab (see page 3-43) of the Well Properties Dialog. For each Depth Reference Datum the elevation above or below the System Datum must be specified. A positive reference level
represents a height above the system datum, a negative elevation level represents a position below the system datum.
The default Depth Reference datum selected in the list will be the viewing datum in all applications, the viewing datum can be changed ‘on the fly’ in OpenWells.
Offshore: Activate the checkbox to indicate that this is an offshore well; leave unchecked to indicate a land well.
Subsea: (offshore well) Activate the checkbox 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 in the 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.
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 when set. To determine wellhead depth:
Wellhead Depth (from rig floor) = Depth Reference Datum Elevation + Wellhead Depth
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 is always positive. To calculate air gap, the application uses:
Air Gap (offshore wells) = Depth Reference Datum – Elevation
Air Gap (land wells) = Depth Reference Datum – Ground Level
Elevation is set in the Project Properties/General dialog. Ground Elevation is set in the Well Properties/ Depth Reference dialog.
Procedure: Configuring Reference Datum Levels For Your Design In the Project Properties dialog, General tab the System Datum to be used for the entire Project can be defined. See “General Tab” on page 3-34.
In the Project Properties dialog, General tab the Elevation field is used to 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 dialog/Depth Reference tab (see “Depth Reference Tab” on page 3-43)
• If the Well is offshore, activate the Offshore checkbox and enter the Water Depth below the System Datum.
• If the Well is subsea, activate the Subsea checkbox and enter the Wellhead Depth below the System Datum.
• If the Well is a land well, verify that the Offshore checkbox 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 Rigfloor 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.
Unit System (Displayed)