8.1 Depth Index Type
Three types of depth measurements are used in Well Log Viewer.
- Depth: Measured depth along the well bore, starting at the Drill floor (or KB)
- True_Vertical_Depth (TVD). A projection of the measured depth to the vertical, but still referred to the Drill Floor
- True_Vertical_Depth_Datum (TVD_Datum). Shifting the True Vertical Depth to a permanent datum (PDAT). For an offshore job, the permanent datum would be the surface of the Geodetic Datum or Mean Sea Level.
A depth index type (either Depth, TVD or TVD_Datum) is assigned to each dataset inside Well Log Viewer.
The depth index type of a datset is determined during data loading as follows:
- by default it will be assigned as "Depth", which means it is measured depth along the wellbore.
- If the depth curve name starts with "TVD", it will be assigned as "True Vertical Depth"
- If the depth curve name starts with "TVD" and ends with either "SRD" or "SS", it will be assigned as "True Vertical Depth Datum"
Having the correct depth index type assigned to a dataset is important since it determines which data ia allowable for certain processing modules. For example, if doing TVD depth conversion, then the allowable input data is of type "Depth". Or when doing Two Way Time conversion, the allowable input data is of type "True Vertical Depth Datum"
Log data that is stored in LAS files will mostly be of type "Depth", however, it can also be in TVD and there will no indications inside the LAS, other than perhaps the file name, that this is the case. If a LAS file contains TVD data, but the depth curve name does not start with TVD, it can be useful to manually edit the LAS file to change the name of the depth curve, so that it is loaded with the correct depth index type.
Once the data is in Well Log Viewer and has a correct depth index type assigned, it will be maintained correctly through subsequent processings.
8.2 Switching between TVD and TVD_Datum
The difference in the depth value between types TVD and TVD_Datum is the value of the parameter APD. The APD parameter can be updated in the Wellsite data editor.
Right clicking on the Depth Index curve name of a TVD dataset gives an option to toggle between TVD and TVD_Datum, through the APD parameter. Below is shown the Menu item that appears after right clicking on the Depth Index curve.
8.3 Converting Logs to a TVD depth Index
TVD depth conversion is available in the Log Processing menu.
Two options for TVD computation are:
- If the UTM coordinates of the well are filled, then the UTM coordinates of the well trajectory are computed as Northings and Eastings.
- If the UTM coordinates of the well are left blank or are set to zero, then only the TVD and NS/EW are computed
A pre-requisite step for computing the Northings and Eastings (option #1 above), is to go to the Wellsite Data Editor and confirm the UTM values and "Log datum above PDAT" (APD), since the same values are used for TVD conversion. Editing the Wellsite Data is discussed earlier in section 5. Although not displayed in the Wellsite data editor, the Grid Convergance angle is also computed during the UTM computation, and stored internally with the UTM values.
Converting logs from a Measured Depth index to a TVD depth index is a two step process.
The first step is to take a series of Deviation and Azimuth measurements which may be sampled sparsely and irregularly along along the wellbore trajectory and compute a TVD value at each of these Deviation/Azimuth depth measurements.
Ideally, these Devi/Azim measurements should extend all the way up to the surface; this way, the (X, Y, Z) tie-in will be (0, 0, 0). If the Devi/Azim measurements only start half way down the well, then a TVD tie-in value is required, together with the X and Y coords of the top measuremnts of the deviation survey.
The Step-1 process is shown below. The left panel shows the deviation survey, the central panel shows the processing options for the TVD computation. The third panel shows the results from the computation, which are appended to same file as the input data.
If the Northing/Easting Well Coordinates are left blank or are zero, then the UTM well coordinates will not be computed and only the TVD and NS and EW offsets from the wellhead are computed.
The second step is to take the TVD result of the deviation survey computed above, and use this to re-index an MD indexed log dataset to a TVD depth index. This is shown in the figure below. First the "Re-index a dataset to TVD" process is selected. The deviation survey dataset is selected in the first drop down list and the dataset to converted to TVD index is selected in the second drop-down list. Finally the TVD curve from the deviation survey is selected, and the Compute button then clicked.
The depth range of the deviation data must be greater than the depth range of the open hole log data.
There are two conversion options. The first options simply comnverts the dataset to TVD without any re-sampling or interpolation; that is, it will map the MD values direct to TVD depth. If the input logs are at 6 inch sample rate and the well deviation is 45 degrees, then the resulting TVD sample rate will be 6 * Cos(45) = 4.24". However, since the well devaition is never constant, then the effective sampling rate will be variable. Since no interpolation is done, the log curve values will be unchanged.
The second option (default) will re-sample the data to a TVD sample rate that has the same value as the MD sample rate. For example, a 6" MD rate goes to a 6" TVD rate. An un-avoidable consequence of this method, is that there will be a slight loss in resolution of the TVD converted data.
Step #1 processing. Computing TVD depth from INCL/AZIM
Step #2 processing. Converting log data to TVD depth.