Free Online Pore Pressure Prediction Tool: Eaton Method and Mud Weight Window

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Load the demo or paste your own depth-sonic pairs. Everything runs in your browser.
Curves: hydrostatic (dashed blue), pore pressure from the Eaton sonic method (red), fracture gradient from the effective stress ratio (orange), and overburden from Gardner-density integration of the compaction trend (gray). The teal band is the safe mud weight window. Where the red curve lifts off the blue line is the top of overpressure.

Pore pressure prediction in your browser

The tool above builds the plot every well plan stands on: pore pressure, fracture gradient, overburden, and hydrostatic, all in equivalent mud weight against depth, with the safe drilling window shaded between them. Load the demo well to see a textbook overpressure ramp, or paste your own depth and sonic pairs from any well. It applies the Eaton sonic method, the most widely used pore pressure technique in the industry, with every parameter exposed: the normal compaction trend, the Eaton exponent, water salinity, and the Poisson ratio behind the fracture gradient.

Pressure-depth plot showing hydrostatic, pore pressure from the Eaton sonic method, fracture gradient, and overburden curves in ppg equivalent mud weight, with the safe mud weight window shaded between pore pressure and fracture gradient
The pressure-depth plot: pore pressure and fracture gradient bound the safe mud weight window, and a narrowing window forces a casing point.

This is the fourth tool in the Geoscience Workbench. The LAS viewer reads your logs, and the computed sonic column pastes straight into this tool; downstream, the numbers here feed directly into kill sheets, kick tolerance, and casing design.

How the Eaton method works

Shale compacts as it buries, so sonic transit time should fall smoothly with depth along the normal compaction trend. When pore fluid cannot escape fast enough, the extra load stays on the fluid instead of the grains, compaction stalls, and the sonic reads slower than the trend predicts. Eaton turned that observation into arithmetic: pore pressure equals overburden minus the normal effective stress scaled by the ratio of trend to observed sonic, raised to an exponent near 3. Overburden itself comes from integrating density with depth, here estimated from the compaction trend through the Gardner relation. The fracture gradient then follows from the effective stress ratio set by the Poisson ratio, and the gap between the two curves is the window your mud weight must stay inside.

Reading the result

  • The top of overpressure is where the red curve leaves the hydrostatic line; the tool marks it automatically.
  • The narrowest window statistic is the well planning headline: it tells you where the margin is tightest and how close to impossible the section is.
  • The Eaton exponent is the honesty knob. Three is standard for sonic, but basins calibrated against measured pressures often use different values; change it and watch the curve move.
  • The normal compaction trend drives everything. If the trend is wrong, the prediction is wrong, which is why real studies calibrate against offset well pressure measurements before anyone drills.

What this tool is for

Teaching the method, screening a prospect when all you have is a sonic log, and sanity-checking a pressure study someone handed you. It is a one-dimensional shale-based prediction: it will not see pressure regressions in isolated sands, centroid effects, or unloading, which need the Bowers method and measured data. As a first honest look at whether a well needs three casing strings or five, it does exactly what the full-price version of this plot does.

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