What is true vertical depth in drilling?
In drilling, true vertical depth (TVD) is a fundamental measurement representing the straight-line distance downwards from the surface.
True Vertical Depth, commonly abbreviated as TVD in the petroleum industry, is the measurement from the surface to the bottom of the borehole (or anywhere along its length) in a straight perpendicular line, as described in reference materials often represented visually (e.g., by line (a) in a diagram).
Think of TVD as the depth you would reach if you dropped a plumb line straight down from the wellhead to a specific point in the wellbore. It indicates how deep a target formation is below the surface horizontally, regardless of the actual path the wellbore takes.
TVD vs. Measured Depth (MD)
It's crucial to distinguish TVD from Measured Depth (MD). Measured Depth is the actual length of the wellbore as measured along its path, following every curve and deviation.
Why the difference?
- Vertical Wells: In a perfectly straight, vertical well, the TVD and MD are the same.
- Directional or Horizontal Wells: When a well is intentionally drilled at an angle or horizontally (which is very common today), the Measured Depth will always be greater than the True Vertical Depth for any point below the surface.
This distinction is vital for:
- Geological Correlation: Matching the well log data (which is measured along MD) to seismic data or geological models requires knowing the TVD of each point.
- Reservoir Engineering: Calculating reserves, pressure, and flow rates relies on understanding the true vertical distance to the reservoir.
- Well Planning: Designing directional paths and calculating casing strings requires precise TVD values at different points.
- Completion Operations: Setting tools and equipment at specific vertical depths.
Key Differences Summarized
| Feature | True Vertical Depth (TVD) | Measured Depth (MD) |
|---|---|---|
| What it is | Straight-line distance from surface (vertical) | Length along the wellbore path |
| Measured from | Surface to point in the well (vertical) | Surface along borehole to point |
| Relationship | <= MD | >= TVD |
| Application | Geological correlation, reservoir depth, TVD maps | Drilling progress, pipe tally, logging surveys |
Calculating True Vertical Depth
TVD is typically calculated from wellbore survey data. Survey tools are run periodically in the wellbore to record its inclination (angle from vertical) and azimuth (direction from North) at various points. Software uses this data to compute the TVD at any given measured depth.
For example, if a well is drilled at a constant angle off vertical, the TVD at a given MD can be calculated using trigonometry (TVD = MD * cos(inclination angle)). In reality, wells curve, requiring more complex calculation methods.
Practical Applications
- Mapping: Geologists create subsurface maps contoured by TVD to visualize the true shape and depth of formations.
- Pressure Analysis: Formation pressure gradients are calculated based on TVD, as pressure increases with true depth due to the weight of overlying rock and fluids.
- Economic Evaluation: Reserve calculations often use TVD maps to determine the volume of hydrocarbons in place.
- Safety: Understanding the true vertical depth is important for planning interventions and understanding hydrostatic pressure in the well.
In essence, while Measured Depth tells you how much pipe is in the ground, True Vertical Depth tells you how far down you are directly below the surface. It's a critical parameter for accurately locating geological features and planning efficient drilling and production operations.
Wellbore Surveying Explained (Note: This is a placeholder link for demonstration purposes)
