7.1 Wellsite Data Parameters
The Geodetic Datum specifies a reference ellipsoid for locating positions on the Earths surface. The surface of the ellipsoidal is at MSL and provides the reference for elevations. Locations on the ellipsoid are given by spherical coordinates as Latitude and Longitude. A local projection to a grid gives the UTM coordinates. A Latitude/Longitude pair must also be accompanyed by a Datum name (eg, WGS-84) to be able to specify the exact positon on the Earths surface.
The Geodetic Datum and elevation information of the logging reference points are kept for each dataset loaded, and saved under each well. These can be reviewed for each dataset loaded, and edited to consistant set of parameters for each well.
The minimum parameters required to locate a well are:
- Latitude, Longitute and Geodetic Datum name.
- PDAT: Permanent Datum. This is the elevation datum which subsequent logging depth reference points are referred to.
- EPD: Elevation of Permanant Datum. This is the elevation the PDAT referred to the Geodetic datum. Typically for an offshore dataset, the PDAT would be MSL, and since MSL is about the same as the Geodetic datum, then the EPD is zero.
- LMF: Log Measured From. Is the reference point from which logs are measured from. This can be the Drill Floor.
- APD: Above Permanent Datum. The distance between the PDAT and the LMF
The UTM Easting and Northing can be computed from the Latitude/Longitude/Datum above.
Three other parameters that can be kept, but less important are the EKB, EDF and EGL.
7.2 Editing Wellsite Data
The wellsite data can be edited by right clicking on the Well name in the data-list and selecting "Edit Wellsite data".
When log data is loaded, the individual wellsite data that is extracted from each LAS/DLIS/JSON file is stored and is displayed in the section labeled "Individual File headers". This data is for information only and cannot be changed.
The "Main List" contains the data that accessed by the processing modules and is also the wellsite data that can be edited. The data in the Main List is initially derived from the first file that was loaded.
It is not a requirement that all datasets under a Well need to have the same depth index units. Since the wellsite data in the Main List comes from the first data set loaded, then elevations in the wellsite data will have units that are the same as the first dataset upon loading.
When a dataset under this Well is saved as a LAS file, the wellsite data in the Main List is used for this LAS or JSON file. If the dataset to be saved has a depth index unit that is different from the units of the wellsite data, then the wellsite data will be automatically converted to the appropriate depth when saving the data to file.
The data in the white cells can be edited directly. The data in the yellow and green cells should not be directly edited, since these will be filled automatically after perfroming the "Update Lati-Long" computation on the left.
A limited number of Geodetic Datums are included for UTM computations. Additional Geodetic Datums can be added upon request.
The units for the parameters of the wellsite data can be changed using the selector under the Main List. Changing the units is independent of changing the depth index units, and if one is changed, then the other should also be changed. If there is a mis-match between these units during later processing, then a warning message will pop up.
The figures above shown the UTM calculation before and after. The LAS file as loaded already had Lati-Long values in it, and these are shown in the first plot on the first figure on the left. If these were not available in the LAS file, then they can be entered manually here, either as a decimal value or as D-M-S triplet in the upper left.
Next the Geodetic datum calculation is allowed, the correct Geodetic datum is selected, and the "Compute UTM" is clicked.
Note that a Lati-Long value by itself does not refer to a specific location on the Earths surface unless a Geodetic datum is known or specified. And perhaps more importantly (for this calculaton), the computed UTM values will not be correct unless the correct Geodetic datum is specified