drilling · pressure
Eaton Pore Pressure (Resistivity)
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Inputs
psi/ft
psi/ft
Ω·m
Ω·m
—
ft
Description
Predicts formation pore pressure from a deep resistivity log by comparing observed resistivity to a normal compaction trend, using the Eaton (1975) exponent method's resistivity form. A sibling of the sonic-based Eaton method (eaton_pp) — same paper, same normal-compaction-trend logic, but resistivity rises with compaction where sonic slowness falls, so the ratio and the direction of the overpressure signal are inverted relative to the sonic form.
Variables
| Symbol | Unit | Description |
|---|---|---|
| PP | psi/ft | Pore Pressure Gradient |
| PP | psi | Pore Pressure |
| EMW | lb/gal | Equivalent Mud Weight |
| OBG | psi/ft | Overburden (lithostatic) pressure gradient from an integrated bulk density log or regional model — typically 0.90-1.05 psi/ft. |
| Pn | psi/ft | Normal (hydrostatic) pore pressure gradient for the basin — typically 0.433-0.465 psi/ft depending on water salinity. |
| Rn | Ω·m | Shale resistivity expected for a normally compacted, normally pressured section at this depth, read from a calibrated normal resistivity trend line. Fitted independently from the sonic normal trend (DTn) — resistivity and sonic normal trends are calibrated against different log responses and are not interchangeable. |
| Robs | Ω·m | Observed shale resistivity from a deep resistivity log at this depth. Values lower than the normal trend (Rn) indicate undercompaction and overpressure — the opposite direction from the sonic method, since resistivity rises with compaction while sonic slowness falls. |
| n | — | Empirical exponent in the Eaton resistivity equation. The default of 1.2 is Eaton's (1975) commonly cited value — distinct from the sonic method's n=3.0 default — and should be recalibrated to local offset wells with MDT/RFT data where possible. |
| TVD | ft | True vertical depth of the point of interest, used to convert the predicted pressure gradient into an absolute pressure and equivalent mud weight. |
Assumptions
- The normal resistivity compaction trend (Rn) has been correctly established from offset wells or regional shale trends in normally pressured sections — fitted independently from any sonic normal trend
- The lithology is predominantly shale — the Eaton resistivity method is calibrated for shale and is less reliable in sands, carbonates, or evaporites
- The Eaton resistivity exponent n is appropriate for the basin; the default n=1.2 should be recalibrated against direct pressure measurements (MDT/RFT) where available
Limitations
- Resistivity-derived pore pressure is an estimate, not a measurement — always calibrate against direct pressure data (MDT, RFT, kicks, mud weight history) where available
- Hydrocarbon effect (resistivity increases in a hydrocarbon-bearing zone independent of pressure) can produce a spurious high-resistivity reading that mimics normal or under-pressure
- The method assumes a single compaction mechanism (disequilibrium compaction); other overpressure mechanisms (fluid expansion, lateral transfer, hydrocarbon generation) may not follow the same resistivity-pressure relationship
Use Cases
- → Cross-check against sonic-based Eaton prediction: Compare the resistivity-based and sonic-based (eaton_pp) pore pressure estimates at the same depth as an independent confirmation, since the two logs respond to compaction through different physical mechanisms.
- → Pre-drill pore pressure prediction where sonic is unavailable: Build a pre-drill pore pressure profile from offset well resistivity logs when sonic data is sparse or of poor quality.
- → Real-time geopressure monitoring while drilling: Continuously update the pore pressure estimate using LWD resistivity data to detect developing overpressure ahead of the bit.
- → Mud weight window design: Establish the safe mud weight window between pore pressure and fracture gradient for the planned well path, cross-checked against the sonic-based estimate.
Related Calculations
Region Notes
Gulf of Mexico
Classic Eaton application area — the resistivity form is the traditional cross-check against the sonic form in Plio-Pleistocene shale sections below major sand bodies.
Haynesville
Severely overpressured shale gas play; the default n=1.2 often under-predicts pressure here, similar to the sonic form's n=3.0 needing upward recalibration after MDT calibration.
References
Primary source
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