geomechanics · fracture pressure
Fracture Gradient (Eaton 1969)
Shmin = Pp + (Sv − Pp) × [ν/(1−ν)] ; FG = Shmin + σt
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Inputs
psi
psi
—
psi
ft
Description
Estimates minimum horizontal stress and fracture gradient using Eaton's (1969) poroelastic (uniaxial-strain) correlation, which derives the horizontal-to-vertical effective stress ratio directly from Poisson's ratio. This is a predictive correlation for use before any direct measurement exists — always calibrate against an actual leak-off test [lot_emw] where available.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Shmin | psi | Minimum Horizontal Stress |
| FG | psi | Fracture Gradient |
| FG_EMW | lb/gal | Fracture Gradient (EMW) |
| Sv | psi | Total vertical (overburden) stress at the depth of interest — typically the output of the Overburden Stress calc. |
| Pp | psi | Pore pressure at the depth of interest — typically the output of the Eaton Pore Pressure calc. |
| ν | — | Poisson's ratio of the rock interval — typically 0.25-0.35 for shale and somewhat lower for well-cemented sandstone. Strongly controls the predicted stress ratio. |
| σt | psi | Rock tensile strength, added to Shmin to estimate true fracture initiation (breakdown) pressure. Often assumed negligible (0) as a conservative simplification when not measured. |
| TVD | ft | True vertical depth of the point of interest, used only to express the result as an equivalent mud weight. |
Assumptions
- Vertical wellbore, uniaxial-strain (no horizontal strain) burial history — no tectonic strain term is included
- Sv and Pp are already known with reasonable confidence (typically from the Overburden Stress and Eaton Pore Pressure calcs)
- Poisson's ratio ν represents the specific rock interval of interest (shale and sandstone ν can differ meaningfully within the same well)
Limitations
- Eaton's (1969) method is one of several historical fracture-gradient correlations, not a universally accepted single formula — Matthews & Kelly (1967) uses a different, empirically-fit coefficient and can give a materially different answer in the same interval; neither should be treated as definitively "correct" without local calibration
- Ignoring tectonic strain entirely makes this method least reliable in compressional or otherwise tectonically active basins — it was originally calibrated on Gulf Coast (tectonically quiescent) data
- Assuming σt=0 is conservative (underestimates true breakdown pressure) but is not a substitute for measured rock tensile strength when precision matters
- A predictive correlation, not a measurement — always validate against an actual leak-off test or formation integrity test [lot_emw] once available for the well
Use Cases
- → Pre-drill mud-weight window planning: Predict fracture gradient before spudding, when no leak-off test data yet exists, to set a preliminary upper mud-weight bound and candidate casing-shoe depths.
- → LOT/FIT calibration check: Compare the predicted fracture gradient against an actual measured LOT EMW [lot_emw] to assess (and locally recalibrate) the correlation's accuracy for the basin.
- → Kick-tolerance and MAASP fracture-margin estimation: In exploration wells lacking offset leak-off data, use the predicted fracture gradient as the FracEMW input to MAASP [maasp] or Kick Tolerance [kick_tolerance].
Related Calculations
Region Notes
Permian Basin
Wolfcamp shale intervals are commonly modeled with ν in the 0.25-0.35 range in published geomechanical studies; predicted fracture gradients should always be checked against actual Delaware/Midland Basin LOT/FIT data per well rather than relied on alone.
Gulf Coast
Eaton's (1969) correlation was originally developed and calibrated on Gulf Coast sediments — it performs best in this kind of normally-compacted, tectonically quiescent setting.
Global
Eaton's method is least reliable in compressional or otherwise tectonically active regions, where horizontal tectonic strain (ignored here entirely) can dominate the actual minimum horizontal stress.
References
Primary source
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