Geomechanics and FracturingIn-Situ Stress and Rock Mechanics
Fracture Gradient - Zoback and Healy Formula
Fracture gradient estimates the pressure gradient required to initiate or propagate a fracture under a stated stress model.
How engineers use this formula
Use it for geomechanics screening, mud-window checks, and fracture-pressure comparisons before detailed wellbore stability work.
Assumptions
- The selected stress model is appropriate for the local geologic setting.
- Input stress and pore-pressure values are representative at depth.
- Units are consistent across pressure and gradient terms.
Limitations
- Does not replace leak-off tests, mini-frac interpretation, or calibrated geomechanical models.
- Local depletion, anisotropy, natural fractures, and tectonic effects may change fracture pressure.
Common mistakes
- Using a generic stress model without local calibration.
- Mixing gradient and pressure units.
- Treating screening results as operational limits without review.
Default example
Using the default inputs, S_hmin equals 5,602.857726 psi.
mu_fdimensionless
0.6
S_vpsi
9000
P_ppsi
4000
Inputs
mu_f
dimensionlessFriction Coefficient
S_v
psiVertical Stress
P_p
psiPore Pressure
Outputs
S_hmin
psi
Minimum Horizontal Stress
Source and review
reviewedZoback, M.D. 2007. Reservoir Geomechanics, Cambridge University Press, Page 281.
Source