Eaton Equation for Fracture Gradient
The Eaton equation estimates the pressure that will crack the formation, setting the ceiling of the safe mud weight window.
The equation
FG = PP + (ν / (1 – ν)) (OB – PP)
- FG, PP, OB are fracture, pore, and overburden gradients in psi per foot
- ν is Poissons ratio of the rock
- The bracket is the fraction of extra overburden converted to horizontal stress
Worked example
With overburden at 1.0 psi per foot, pore pressure at 0.465, and Poissons ratio 0.4, the fracture gradient is 0.465 + 0.667 × 0.535 = 0.82 psi per foot, about 15.8 ppg equivalent. The mud window is everything between pore and fracture pressure.
When to use it
Calibrate against leak off tests whenever available: Poissons ratio is a fitting parameter here more than a rock property. Depleted sands frighten drillers precisely because falling PP drags FG down with it.
Related Calculators and Reference
- Mud Weight and Pressure Gradient Calculator: convert between ppg, SG, and psi per foot.
- Probabilistic STOIIP Calculator: volumes with uncertainty ranges.
- PetroWiki by SPE: the industry reference for petroleum engineering.