Fracture Gradient Calculator: Eaton’s Method for Safe Mud Weight Windows

Solve Eaton's fracture gradient equation for drilling and well planning: find the maximum safe mud weight from overburden gradient, pore pressure gradient, and Poisson's ratio, compare fracture gradient across lithologies, or back-calculate Poisson's ratio from a leak-off test result. Everything updates live.

The Eaton Fracture Gradient Equation

Every drilled well must keep mud weight inside a window: heavy enough to hold back pore pressure and prevent a kick, but light enough to stay below the fracture gradient and avoid lost circulation. Eaton (1972) related fracture gradient to overburden gradient, pore pressure gradient, and Poisson's ratio with the equation FG = [v / (1 - v)] x (OBG - PPG) + PPG, where v is Poisson's ratio, OBG is the overburden gradient, and PPG is the pore pressure gradient.

This tool solves fracture gradient directly, compares fracture gradient across lithologies with different Poisson's ratios, and rearranges the equation to back-calculate Poisson's ratio from a measured leak-off test result.

Why Poisson's Ratio Drives the Fracture Gradient

In Eaton's formulation, Poisson's ratio behaves less like a fixed material property and more like a proxy for the ratio of horizontal to vertical stress in a given formation. At v = 0, the fracture gradient collapses to the pore pressure gradient itself, since there is no lateral stress transfer at all. At v = 0.5, the fracture gradient rises to match the overburden gradient, the fully coupled limit. Real formations sit between these extremes, with soft, undercompacted shales trending toward the higher end and stiffer sandstones and carbonates trending lower.

Because fracture gradient rises with Poisson's ratio, the same overburden and pore pressure can produce very different safe mud weight windows depending on lithology, which is why fracture gradient prediction is usually done zone by zone rather than with one constant value for an entire well.

Comparing Fracture Gradient Across Lithologies

The lithology comparison tab holds overburden and pore pressure gradient fixed and varies Poisson's ratio across rows, so the resulting spread in fracture gradient isolates exactly how much lithology alone can shift the safe mud weight window at a single depth.

Calibrating Poisson's Ratio From a Leak-Off Test

A leak-off test (LOT) measures the actual pressure at which a formation begins to take fluid, giving a direct field measurement of fracture gradient at that depth. Rearranging Eaton's equation lets that single measurement solve backward for the effective Poisson's ratio: v = k / (1 + k), where k = (FG - PPG) / (OBG - PPG). That calibrated value can then be carried forward to predict fracture gradient at other depths in the same formation, which is the standard workflow for building a fracture gradient profile from sparse LOT data.

Common Mistakes

Using a single Poisson's ratio for an entire well when lithology, and therefore the effective stress ratio, changes significantly with depth.

Treating fracture gradient as a hard ceiling rather than the onset of fracture initiation, real drilling margins should include a safety buffer below the calculated fracture gradient.

Mixing gradient units, psi/ft, ppg, and specific gravity all describe the same physical quantity but must be converted consistently before comparing overburden, pore pressure, and fracture gradients.

Applying a Poisson's ratio calibrated from one field or basin to a different tectonic setting, since Eaton's v reflects the local stress regime and does not transfer well between geologically distinct areas.

Who Uses This

Drilling engineers building the mud weight window and casing design for a planned well.

Geomechanics and pore pressure specialists calibrating fracture gradient models against leak-off test data.

Well planners evaluating drilling margin and lost circulation risk across zones of differing lithology.

Students and instructors working through pore pressure and fracture gradient prediction methods.

Try It Above

Use the quick solve tab to convert overburden gradient, pore pressure gradient, and Poisson's ratio directly into fracture gradient and drilling margin, move to the lithology comparison tab to see how much lithology alone shifts the safe mud weight window, or use the solve tab to back-calculate Poisson's ratio from a leak-off test result.

Related Calculators and References

Continue building your workflow with the Mohr-Coulomb Failure Calculator. For background on the equation used above, see Udo et al. (2020), Determining Fracture Pressure Gradients From Well Logs, citing Eaton's Model.