True Vertical Depth

When boreholes are not completely vertical, the measured depth (MD) and true vertical depth (TVD) of the borehole are different. The measured depth is the total distance travelled along the borehole from the starting point (usually the collar) to the bottom of the borehole. The measured depth is used in combination with azimuth and inclination or dip information to calculate the true vertical depth, the actual depth below the starting point where a measured value should be displayed.

Inclination or dip and azimuth are indicators of how far off vertical a borehole is. When recorded in a collars table, the azimuth and inclination or dip apply to the entire borehole length. When recorded in a survey table, the azimuth and inclination or dip apply from the depth to the next recorded depth.

Inclination is the angle the borehole is oriented in degrees and varies from 0 to 180. 0 indicates vertical pointing down, 90 indicates horizontal, and 180 indicates vertical pointing up. Negative and positive inclination values are treated the same for depth calculations. Negative inclination values are treated differently for well path calculations in the cross section and map views. A negative inclination changes the direction (azimuth) to the opposite of the similar positive inclination. For example, the azimuth value of 90 and inclination of 45 describes the same orientation as the azimuth value of 270 and inclination of -45 degrees. Both combinations describe an eastward direction at 45 degrees down from the horizontal plane.

Dip is an alternate method of calculating the angle the borehole is oriented. Dip is oriented in degrees and varies from -90 to 90. -90 indicates vertical pointing down, 0 indicates horizontal, and +90 indicates vertical pointing up.

Azimuth is the compass orientation of the well's deviation, in degrees and varies from 0 (true vertical north) to 360. Azimuth values are measured relative to true north (not grid north in the local coordinate system).

In the borehole view properties or cross section view properties, when the Depth Method is set to True Vertical Depth, the individual log depths are calculated using the azimuth and inclination or dip values. These values can be set from a depth, interval, survey, or collars table. There are three ways to enter the inclination or dip and azimuth data:

A single set of azimuth and inclination or dip data can be entered for each borehole in the collars table. The azimuth and inclination or dip data would apply to all points throughout the borehole.
A deviation survey could be imported for each borehole into a survey table. The deviation or survey data does not have to have the same sample interval as the wireline or depth/interval data.
Azimuth and inclination or dip data can be entered for each data point in the borehole directly in the depth or interval table as additional columns with the rest of the borehole data.

Once inclination or dip data is entered into a data table, the Hole Inclination Column or Hole Dip Column and Hole Azimuth Column need to be specified for each log in the borehole view or cross section view in the Property Manager. When the inclination or dip and azimuth data is entered in the table and the columns are specified for each log in the view window, the view window can be set to true vertical depth measurements. Click on the View | Display | View Properties command. The view properties are listed in the Property Manager. Change the Depth Method from Measured Depth (the default) to True Vertical Depth. All the logs in the view will update with the selected depth method.

Depth logs are the only log types that have their own depth method selection, so you can set the depth scope of the depth log independently of the depth method for the view. This allows you to create one depth log showing measured depth and another showing true vertical depth side-by-side. To set the depth log to true vertical depth, click on the depth log in the Object Manager or view window to select it. In the Property Manager, set the Depth Scope to Hole True Vertical Depth.

Example True Vertical Depth

To create boreholes in true vertical depth requires having data for the borehole that includes azimuth and inclination or dip. The azimuth and inclination or dip values can be in a survey table, a collars table, or in the depth table. Once the azimuth and inclination or dip data are entered, the view window and logs can all be set to display true vertical depth. This example imports two tables: a survey table with inclination or dip and azimuth and a depth table. A line/symbol log is created from the depth table. Two depth logs are then created: one display true vertical depth and one displays measured depth.

Click the File | New Project command or click the button to open a new project.
Open the data tables:
1. Click the File | Open Multiple command.
2. In the Open Data dialog, click on the Example Data.xls file in the Samples directory. By default, the Samples directory is located at C:\Program Files\Golden Software\Strater 5\Samples.
3. Click Open.
4. Select the Deviation Survey sheet.
5. Press and hold the CTRL key on the keyboard and select the Depth sheet.
6. Click OK.
7. For the Depth table:
  1. In the Specify Worksheet Column Definitions dialog, make sure that the Specify Column Header Row is checked and set to 1. Click Next.
  2. In the Specify Data Type and Column Positions dialog, make sure that the Data type to Depth (Single Depth) and that all of the columns are set correctly and click Finish.
8. For the Deviation Survey:
  1. In the Specify Worksheet Column Definitions dialog, make sure that the Specify Column Header Row is checked and set to 1. Click Next.
  1. In the Specify Data Type and Column Positions dialog, set the Data type to Survey. Make sure all of the columns are set correctly and click Finish.
Click on the Borehole 1 tab or click Window | Borehole 1 to return to the borehole view.
Create a line/symbol log by clicking the Log | Create Log | Line/Symbol command.
1. Click near the center of the log pane to position the line/symbol log.
2. In the Open dialog, select Depth in the Use Current Table list.
3. Make sure that Depth is listed in the File name option.
4. Click Open.
Set the line/symbol log to use true vertical depth measurements.
1. Click on the line/symbol log in the view window or in the Object Manager to select it.
2. In the Property Manager, click on the Line/Symbol Log tab.
3. Next to Hole Inclination Column or Hole Dip Column, click on [Unspecified] and select [From survey table]: Deviation Survey.
4. Notice that the Hole Azimuth Column automatically is set to [From survey table]: Deviation Survey. When using a survey table, the inclination or dip or dip and azimuth columns must come from the same table.
Create a depth log by clicking the Log | Create Log | Depth command.
Click on the far left side of the page in the log pane to position the first depth log.
Set the depth log to use measured depth.
1. With the depth log selected, in the Property Manager, click on the Depth Log tab.
2. Set the Depth Scope to Hole Measured Depth by clicking on the existing option and select Hole Measured Depth from the list.
3. Next to Hole Inclination Column or Hole Dip Column, click on [Unspecified] and select [From survey table]: Deviation Survey.
4. Notice that the Hole Azimuth Column automatically is set to [From survey table]: Deviation Survey. When using a survey table, the inclination or dip and azimuth columns must come from the same table.
Click the Draw | Shape | Text command.
1. Click above the depth log to position the text.
2. In the Text Editor, type Measured Depth and click OK.
3. Press ESC on the keyboard to end drawing mode.
Create a depth log by clicking the Log | Create Log | Depth command.
Click between the existing depth log and the line/symbol log to position the second depth log.
Set the depth log to use true vertical depth.
1. With the new depth log selected, in the Property Manager, click on the Depth Log tab.
2. Set the Depth Scope to Hole True Vertical Depth by clicking on the existing option and select Hole True Vertical Depth from the list.
3. Next to Hole Inclination Column or Hole Dip Column, click on [Unspecified] and select [From survey table]: Deviation Survey.
4. Notice that the Hole Azimuth Column automatically is set to [From survey table]: Deviation Survey. When using a survey table, the inclination or dip and azimuth columns must come from the same table.
Click the Draw | Shape | Text command.
1. Click above the second depth log to position the text.
2. In the Text Editor, type True Vertical Depth and click OK.
3. Press ESC on the keyboard to end drawing mode.
Click the View | Display | View Properties command to display the borehole view properties.
Click on the Measured Depth option next to Depth Method and select True Vertical Depth from the list.

The view is displayed in true vertical depth. The first depth log shows measured depth and the second depth log shows true vertical depth. The log is also shown with true vertical depth.