8.1 Depth Index Type
Three types of depth measurement indexing 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 (MD) along the wellbore.
- If the depth curve name starts with the characters "TVD", it will be assigned as "True Vertical Depth"
- If the depth curve name starts with the characters "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 depth conversion from MD to TVD, 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 it is a TVD indexed dataset. If a LAS file contains TVD data, but the depth curve name does not start with "TVD", it is useful to manually edit the LAS file to change the name of the depth curve so that it starts with the characters "TVD", then it will be 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 (Above Permanent Datum). 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 Measured Depth (MD) Logs to a TVD depth Index
TVD depth conversion is available in the Log Processing menu.
Some guidelines on how to use the options in the Processing Menu are given in the next section.
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.
Step-1
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.
If the TVD channel already exists in the loaded deviaton survey data and it is correctly referenced to the logging datum, then this step can be skipped, and you can proceed to step-2 below.
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.
Step-2
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 converts 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 $6 \times \cos{\left(45\right)} = 4.24 \text{ inch}$. 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 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.
The first conversion option is the default option, which will produce a variable sample rate data-set. If at some time later, a constant sample-rate dataset is required, then the "Resample Dataset" process can be applied.
Step #1 processing. Computing TVD depth from INCL/AZIM
Step #2 processing. Converting log data to TVD depth.
8.4 Converting TVD logs back to Measured Depth (MD)
There may be occassions when the original MD indexed logs are no longer available, and it is desired to re-produce these.
This process is available, and the deviation survey with a TVD channel that is computed in Step-1 above is required. This deviation survey should be referenced to MD depth, as it was in step-1. The log channels for conversion back to MD will referenced in TVD depth.
As discussed above, if the original MD->TVD conversion used the same output sample rate as the input logs, then some resolution will have been lost. This lost resolution cannot be reclaimed when converting back to MD. Although when doing theTVD->MD conversion, the output data is re-sampled to give the same sample rate, but the resolution has already been lost.
If, however, the original MD->TVD conversion was done to retain the resolution (which would have resulted in a variable sample rate dataset with a finer spaced sample than the input data), then converting this dataset back to MD will faithfully re-produce the original MD input logs without any loss of resolution.