drilling · hydraulics
Hydrostatic Pressure
Ph = 0.0519 × MW × TVD
click formula to derive ↑
Inputs
lb/gal
ft
Description
Computes the hydrostatic pressure exerted by a static column of drilling fluid using the fundamental fluid-statics relationship P = ρ·g·h. This is the baseline pressure against which formation pressure, kick tolerance, and equivalent circulating density are evaluated.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Ph | psi | Hydrostatic Pressure |
| MW | lb/gal | Drilling fluid density. 1198 kg/m³ (10 lb/gal) is roughly the density of fresh water-based mud; typical drilling muds range 838-2636 kg/m³ (7-22 lb/gal). |
| TVD | ft | True vertical depth to the point where hydrostatic pressure is being evaluated. |
Assumptions
- Mud column is static (no circulation) — circulating pressure (ECD) is higher due to friction losses
- Mud density is uniform throughout the column (no significant gel strength/barite sag effects)
- TVD is used, not measured depth — for deviated wells, hydrostatic pressure depends only on vertical height
Limitations
- Does not include annular friction pressure losses — use an ECD calculation when circulating
- Assumes a single, uniform-density fluid column; a U-tube with different fluids (e.g., during a displacement) requires a multi-segment calculation
- Temperature and pressure effects on mud density (especially for synthetic-based muds at depth) are not modeled here
Use Cases
- → Kick pressure / overbalance calculations: Compare hydrostatic pressure to formation pressure to determine overbalance or underbalance and assess kick risk.
- → ECD estimation: Use static hydrostatic pressure as the baseline to which annular friction losses are added to estimate equivalent circulating density.
- → Casing seat selection: Evaluate hydrostatic pressure at candidate casing setting depths against pore pressure and fracture gradient to select casing seats.
Related Calculations
Region Notes
Permian Basin
Typical drilling fluid densities of 1080-1320 kg/m³ (9.0-11.0 ppg) are common in the upper hole sections, increasing to 1440-1680 kg/m³ (12-14 ppg) through pressured Wolfcamp/Bone Spring intervals.
Gulf of Mexico
Deepwater wells must account for the seawater + riser margin separately from the mud column below the wellhead — this calculation covers only the mud column itself.
Global
Always cross-check the calculated hydrostatic gradient (psi/ft or kPa/m) against the planned mud weight program — a mismatch often indicates a unit-conversion error in the mud weight input.
References
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