Pressure Gradient
Rate at which fluid pressure rises with depth (psi per foot).
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Example
SG=1.07 brine: G = 0.463 psi/ft
How it works
This formula converts a fluid’s specific gravity into its pressure gradient in imperial oilfield units. The constant 0.433 comes from the hydrostatic pressure of a column of water, since water creates about 0.433 psi of pressure per foot of depth at standard conditions. Multiplying by specific gravity scales that water gradient up or down for denser or lighter fluids.
When to use it
Use this when estimating how much pressure a static column of drilling mud, brine, oil, or other fluid adds with depth. It is especially useful for well control, mud weight checks, and quick hydrostatic pressure calculations in petroleum engineering.
Worked example, step by step
Given: SG = 1.25 Step 1: Write the formula: G = 0.433 × SG Step 2: Substitute the value: G = 0.433 × 1.25 Step 3: Calculate: G = 0.54125 psi/ft Step 4: Round appropriately: G ≈ 0.541 psi/ft Final answer: The pressure gradient is about 0.541 psi per foot.
Common mistakes to avoid
- Using fluid density in lb/gal or lb/ft³ without first converting to specific gravity
- Forgetting that the result is a gradient in psi per foot, not total pressure
- Using this shortcut for flowing systems or gas columns, where hydrostatic behavior is not the only effect
Did You Know?
Fresh water's gradient is 0.433 psi/ft; salty formation brine is a bit higher.
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