Original synthetic study
The error from using measured depth for hydrostatic pressure
Using measured depth in a hydrostatic calculation overstates the fluid head whenever MD exceeds the true vertical height. The relative error is MD/TVD − 1 for the same uniform fluid.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
A controlled synthetic depth error
This study fixes fluid density at 10 ppg and TVD at 8,000 ft. Only the incorrectly substituted MD changes. The correct fluid head is therefore always 4,160 psi, while the wrong answer increases with the path length.
The study does not simulate a well survey or estimate a trajectory. Each row is a deliberate input substitution showing the consequence of a common dimensional mistake.
Normalize the error to the correct result
At 12,000 ft MD, the wrong pressure is 6,240 psi. Its 2,080 psi excess is 50% of 4,160 psi. Dividing by the wrong result would instead give 33.3%, which answers a different normalization question.
A vertical well gives MD/TVD = 1 and zero substitution error. That limiting case can conceal the problem in a worksheet that is later reused for a deviated well.
Preserve measured length for its proper tasks
MD is still needed for annular volume and frictional path length. Replacing MD with TVD everywhere would fix the hydrostatic input and introduce different errors elsewhere.
Download the table to reproduce both the correct and incorrect columns. The experiment isolates depth choice; it makes no claim about survey uncertainty, compressibility or temperature corrections.
Explore the example
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Substituted MD (ft) = 10000. Correct head (psi): 4160; Wrong head (psi): 5200; Overstatement (%): 25.
- Correct head (psi)
- Wrong head (psi)
Worked example
Deliberately wrong depth substitution
Correct TVD 8,000 ft and uniform mud 10 ppg. Only the incorrectly substituted measured depth varies.
Relation and unit basis
Pcorrect = 4,160; Pwrong = 0.052 × 10 × MD; error = (MD/8,000 − 1) × 100%
MD belongs to path geometry, not the hydrostatic vertical height. The wrong result is intentionally labelled.
1. Define the baseline
Substituted MD (ft) = 10000
Correct TVD 8,000 ft and uniform mud 10 ppg. Only the incorrectly substituted measured depth varies.
2. Select the relation
Pcorrect = 4,160; Pwrong = 0.052 × 10 × MD; error = (MD/8,000 − 1) × 100%
MD belongs to path geometry, not the hydrostatic vertical height. The wrong result is intentionally labelled.
3. Substitute the values
Wrong = 0.52 × 10,000 = 5,200; error = (5,200 − 4,160)/4,160 = 25%
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
Correct head (psi) = 4160; Wrong head (psi) = 5200; Overstatement (%) = 25
A vertical-well limiting case can conceal this mistake because MD and TVD coincide there.
Result and interpretation
Correct head (psi) = 4160; Wrong head (psi) = 5200; Overstatement (%) = 25
A vertical-well limiting case can conceal this mistake because MD and TVD coincide there.
The flat correct-pressure line is the control. The rising line is intentionally wrong and is labelled as such in the data.
Study data and reproduction
Original synthetic teaching calculation. Equations and fixed controls are stated here; these are not field measurements.
| Substituted MD (ft) | Correct head (psi) | Wrong head (psi) | Overstatement (%) |
|---|---|---|---|
| 8,000 | 4,160 | 4,160 | 0 |
| 9,000 | 4,160 | 4,680 | 12.5 |
| 10,000 | 4,160 | 5,200 | 25 |
| 12,000 | 4,160 | 6,240 | 50 |
| 16,000 | 4,160 | 8,320 | 100 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. At MD 10,000 ft and TVD 8,000 ft, what is the hydrostatic overstatement?
Show the worked answer
The wrong result is 5,200 psi, 1,040 psi or 25% above the correct 4,160 psi.
2. Reproduce the baseline Correct head (psi) without rounding intermediate values.
Show the worked answer
Wrong = 0.52 × 10,000 = 5,200; error = (5,200 − 4,160)/4,160 = 25%. Correct head (psi) = 4160. A vertical-well limiting case can conceal this mistake because MD and TVD coincide there.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Substituted MD (ft) changes from 8000 to 16000. Other controls remain as stated in the premise. Correct head (psi) changes from 4160 to 4160.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. MD belongs to path geometry, not the hydrostatic vertical height. The wrong result is intentionally labelled. A vertical-well limiting case can conceal this mistake because MD and TVD coincide there.
Before using the result
- Use the correct result as the error denominator.
- Keep density and TVD fixed across this experiment.
- Retain MD for geometry and path-length calculations.
A vertical-well limiting case can conceal this mistake because MD and TVD coincide there.
Continue learning
Start with the broader workflow guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.