petrophysics · formation pressure
Formation Pressure Gradient
G = ρ × g (kPa/m) ≡ ρ × 0.4335 (psi/ft)
click formula to derive ↑
Inputs
g/cc
Description
Converts a fluid density to its equivalent hydrostatic pressure gradient. The gradient is the fundamental quantity used to identify fluid contacts (oil-water, gas-oil) from MDT/RFT pressure surveys and to check whether observed reservoir pressures are consistent with the expected fluid type. It is also the first step in computing absolute formation pressure from a depth-gradient pair.
Variables
| Symbol | Unit | Description |
|---|---|---|
| G | psi/ft | Pressure Gradient |
| ρ | g/cc | Density of the pore fluid in g/cc. Fresh water ≈ 1.0, NaCl brine ≈ 1.05–1.15, oil ≈ 0.7–0.85, gas condensate ≈ 0.4–0.6, dry gas ≈ 0.05–0.2 (at reservoir conditions). |
Assumptions
- Fluid is at hydrostatic equilibrium — no flow, no dynamic pressure components
- Fluid composition is uniform and its density at reservoir temperature and pressure is correctly estimated
Limitations
- Gas gradient at reservoir conditions varies dramatically with pressure and temperature — always use an EOS or standing correlation to compute in-situ gas density rather than surface density
- Mixed-fluid columns (e.g., transition zone with oil+water) have a gradient that changes with depth and requires a layer-by-layer calculation
Use Cases
- → Fluid contact identification from MDT survey: Plot MDT pressure vs. TVD; fit a straight line to each fluid phase — the slope is the gradient (in psi/ft), and the intersection of two trend lines is the fluid contact depth.
- → Sanity check on reservoir fluid type: If the log interpreter predicts oil (Sw < 0.5) but the RFT gradient is 0.45 psi/ft, the fluid is likely brine — use this cross-check before committing to net pay.
- → Overpressure detection: When the measured formation pressure gradient significantly exceeds 0.465 psi/ft (fresh water), the excess above hydrostatic defines the degree of overpressure in the formation.
Related Calculations
Region Notes
Permian Basin
Wolfcamp formation water gradient typically 0.44–0.46 psi/ft (brine density 1.01–1.06 g/cc); Bone Spring brine gradients range 0.44–0.47 psi/ft depending on salinity.
Gulf of Mexico
GOM brine gradients typically 0.465 psi/ft (1.074 g/cc); gas gradients in deepwater prospects 0.08–0.12 psi/ft; hydrate stability considerations apply at shallower depths.
Eagle Ford
Eagle Ford formation water gradient 0.45–0.47 psi/ft; oil gradient in productive intervals 0.35–0.40 psi/ft — crossing point defines GOC and OWC from MDT data.
North Sea
North Sea Brent Group brine: 0.455–0.470 psi/ft; oil: 0.36–0.41 psi/ft. Gas-water contacts in the Frigg and Statfjord fields are textbook MDT gradient examples.
Global
Fresh water = 0.433 psi/ft = 9.80 kPa/m. This is the normal hydrostatic reference for pressure prediction. All gradients > 0.433 indicate salinity; all < 0.433 indicate lighter fluids (oil or gas).
References
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