Drilling engineers work with three main pressures: hydrostatic pressure, overburden pressure and formation pressure. This article explains the first two with their formulas, units and worked examples. For other basic terms, see key pressure definitions.
1. Hydrostatic Pressure
Hydrostatic pressure is the pressure due to the unit weight and vertical height of a static column of fluid. It is expressed as follows.
In field units
PH = 0.052 × ρ × D
- PH = hydrostatic pressure, psi
- ρ = fluid density, pounds per gallon (ppg)
- D = true vertical depth, ft
Example: a 10 ppg mud at 10,000 ft gives PH = 0.052 × 10 × 10,000 = 5,200 psi.
In metric units
PH = ρ × D / 10
- PH = hydrostatic pressure, kg/cm²
- ρ = fluid density, g/cc
- D = well depth, meters
Example: a 1.2 g/cc mud at 3,000 m gives PH = 1.2 × 3,000 / 10 = 360 kg/cm².
2. Overburden Pressure
Overburden pressure is the pressure exerted by the total weight of the solids and fluids in the formation above a given depth. It is closely related to formation pressure, which is the pressure of the fluids in the rock pores.
In metric units
σob = ρb × D / 10
- ρb = average bulk density for the interval, g/cc
- D = depth of sediment, meters
In field units
σob = 0.433 × ρb × D
- ρb = bulk density, g/cc
- D = depth, ft
- σob in psi
The bulk density is given by ρb = ρg × (1 − φ) + ρfl × φ, where ρg is grain density, ρfl is fluid density and φ is porosity. Rearranged, porosity is φ = (ρg − ρb) / (ρg − ρfl). Bulk density can be obtained from the density, resistivity and gamma ray logs.
One of the most important parts of well planning is estimating bulk density for the various formations drilled. It can be computed from seismic data, shale density or sonic logs.
Overburden Gradient
The overburden gradient is defined as σob / D (psi/ft). Bulk density is a function of depth. As overburden increases with depth, porosity decreases through compaction, which raises the bulk density. The fracture pressure that limits the mud weight is measured in the field with a leak-off test.
| Pressure | Source | Formula (field units) |
|---|---|---|
| Hydrostatic | Weight of a fluid column | 0.052 × ρ(ppg) × D(ft) |
| Overburden | Weight of rock and fluids above | 0.433 × ρb(g/cc) × D(ft) |
Frequently Asked Questions
Why is the constant 0.052 used?
It converts mud weight in ppg and depth in feet directly to pressure in psi.
Does hydrostatic pressure depend on the hole shape?
No. It depends only on fluid density and true vertical depth, not on hole diameter or inclination.
Key Takeaways
- Hydrostatic pressure: PH = 0.052 × mud weight × depth.
- Overburden pressure comes from the weight of rock and fluid above, and its gradient is about 1 psi/ft in many basins.
- Both pressures set the safe mud-weight window during well planning.
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