geomechanics · fracture pressure
Mud-Window Plot (Pore Pressure vs. Fracture Gradient)
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Inputs
psi/ft
psi/ft
μs/ft
μs/ft
—
—
psi
ft
ft
Description
Plots Eaton's (1975) sonic pore-pressure gradient and Eaton's (1969) poroelastic fracture-gradient correlation together across a depth track, shading the safe mud-weight window between them — the same window mud_weight_window computes as a single scalar comparison, visualized here as a continuous profile from surface to a plotted depth. Both curves reuse the exact same formulas as the standalone Eaton Pore Pressure and Fracture Gradient (Eaton 1969) calcs (shared via a common utility module, not re-derived), applied uniformly across the depth range rather than re-evaluated per real log row — a single representative pressure regime, not a multi-zone profile.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Pp_EMW | lb/gal | Pore Pressure (EMW at TD) |
| FG_EMW | lb/gal | Fracture Gradient (EMW at TD) |
| ΔEMW | lb/gal | Mud Weight Window at TD |
| OBG | psi/ft | Overburden (lithostatic) pressure gradient — used to derive both Eaton's pore-pressure curve and the depth-varying overburden stress Sv(z)=OBG×z feeding the fracture-gradient curve. One input serves both curves, unlike the standalone Fracture Gradient (Eaton 1969) calc's separate Sv input. |
| Pn | psi/ft | Normal (hydrostatic) pore pressure gradient for the basin — typically 0.433-0.465 psi/ft depending on water salinity. |
| DTn | μs/ft | Sonic slowness expected for a normally compacted shale at this depth, read from a calibrated normal compaction trend line. |
| DT | μs/ft | A single representative observed sonic slowness for the plotted interval. This v1 plots one pressure regime across the whole depth range (the same simplification the standalone Eaton Pore Pressure calc's scalar inputs already carry) — it does not re-evaluate DT at each depth from real log rows. |
| n | — | Empirical exponent in the Eaton equation. The default of 3.0 was calibrated to Gulf Coast sonic data — recalibrate to local offset wells with MDT/RFT data when possible. |
| 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. |
| z_max | ft | Bottom of the plotted depth interval (the chart sweeps from surface to this depth). Also the depth at which this calc's own scalar outputs are evaluated. |
| D_shoe | ft | Planned or as-run casing shoe depth, marked on the chart as a horizontal reference line. A direct value the user already knows — this calc does not compute a casing seat depth. |
Assumptions
- A single representative pressure regime (fixed OBG, Pn, DTn, DT, n, ν, σt) applies uniformly from surface to the plotted depth — not a multi-zone or per-log-row profile
- Vertical wellbore, uniaxial-strain burial history for the fracture-gradient curve — the same assumption Fracture Gradient (Eaton 1969) itself carries
- The normal compaction trend and Eaton exponent are appropriate for the whole plotted interval, not recalibrated at different depths
Limitations
- Not a real multi-zone or LAS-log-driven depth profile — a genuine per-row survey/log-based depth track (varying DT, lithology, or stress regime with depth) is a larger, separately-scoped future capability, not built here
- Inherits every limitation of the two underlying calcs: Eaton's (1975) sonic method is calibrated for shale and needs MDT/RFT recalibration; Eaton's (1969) fracture-gradient correlation ignores tectonic strain and is one of several competing correlations, not a universally accepted formula
- Does not add any operational safety margin (trip margin, kill margin, ECD allowance) on top of the raw window — matches mud_weight_window's own explicit scope
Use Cases
- → Pre-drill casing program planning: Visualize how the safe mud-weight window narrows or widens with depth across a planned interval, to sanity-check candidate casing shoe depths before spudding.
- → Communicating a mud program to non-specialists: A depth-track chart of the window (versus a single scalar comparison) is the conventional way this information is reviewed in well planning meetings.
- → Cross-check against mud_weight_window at a single depth: Compare this chart's curves at any one depth against mud_weight_window's own scalar bounds for that same depth, using matching inputs.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring intervals often show only mild overpressure — a normal trend calibrated to the Delaware Basin shale baseline is essential before trusting the plotted window at depth.
Gulf of Mexico
Shallow, unconsolidated deepwater sections often show a window that narrows sharply near the mudline — worth checking explicitly on this chart before finalizing a shallow casing string.
Global
Always validate both curves against real offset-well data (MDT/RFT, LOT/FIT) before finalizing a mud weight program from this predictive, single-regime profile alone.
References
Primary source
Primary source
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