geomechanics · fracture pressure
Safe Mud Weight Window
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Inputs
psi
psi
psi
ft
Description
Defines the safe mud-weight window bounded below by pore pressure (kick margin) and above by fracture gradient — the standard convention used throughout well planning and real-time drilling for choosing a mud weight that neither takes a kick nor fractures the formation. This calc adds one refinement to the textbook two-bound framing: it also surfaces Critical Mud Weight's shear-failure/breakout threshold as a second candidate lower bound, since in weak or highly anisotropic-stress rock that threshold can sit above pore pressure and become the actual governing constraint, not just a separate check run on the side.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Pp | psi | Lower Bound — Pore Pressure |
| Pw_crit | psi | Lower Bound — Collapse Threshold |
| Pw_min | psi | Governing Lower Bound |
| FG | psi | Upper Bound — Fracture Gradient |
| MW_min | lb/gal | Governing Lower Bound (EMW) |
| MW_max | lb/gal | Upper Bound (EMW) |
| Pp | psi | Pore pressure at the depth of interest — the primary, well-control lower bound, typically the PP output of Drilling's Eaton Pore Pressure calc. |
| Pw_crit | psi | Secondary, shear-failure/breakout-based lower bound — the Pw_crit output of the Critical Mud Weight (Wellbore Breakout) calc. In stronger rock this is often below Pp (pore pressure governs); in weak or highly anisotropic-stress intervals it can exceed Pp (collapse governs instead). |
| FG | psi | Upper bound — the FG output of the Fracture Gradient (Eaton 1969) calc. |
| TVD | ft | True vertical depth of the point of interest, used only to express the window bounds as equivalent mud weight. |
Assumptions
- Pp, Pw_crit, and FG all represent the same depth/interval
- Pw_crit (if supplied) already reflects the correct rock-strength and stress inputs for this interval — this calc does not recompute it, only compares it against Pp
- No trip margin, safety margin, or ECD/surge allowance is added on top of either bound — this is the theoretical window, not an as-planned operational mud weight
Limitations
- Does not add any operational safety margin (trip margin, kill margin, ECD allowance) on top of the raw bounds — operational practice typically drills somewhat inside this theoretical window, not at its edges
- Uses Fracture Gradient (Eaton 1969)'s predictive correlation, not a measured LOT/FIT — calibrate the upper bound against Drilling's LOT EMW where available
- Does not model window narrowing with hole angle/deviation — Critical Mud Weight's own vertical-well-only limitation carries through to this calc's collapse-threshold bound
Use Cases
- → Pre-drill mud weight program planning: Establish the full safe mud-weight window for a casing section before spudding, using predictive pore-pressure and fracture-gradient correlations.
- → Weak-rock / high-anisotropy screening: Check whether the shear-failure threshold, not pore pressure, is actually the binding lower-bound constraint — most likely in weak shale with significant horizontal stress anisotropy.
- → Casing seat depth selection: Identify where the window narrows to zero or near-zero across a well path to flag candidate casing setting depths.
Related Calculations
Region Notes
Permian Basin
Wolfcamp intervals with notable stress anisotropy are a common case where the collapse threshold sits meaningfully above pore pressure — worth checking explicitly rather than assuming pore pressure always governs.
Gulf of Mexico
Shallow, unconsolidated deepwater sections often have a narrow window dominated by a low fracture gradient near the mudline — the upper bound is frequently the tighter constraint here, not the lower one.
Global
Always validate both bounds against real offset-well data (MDT/RFT for pore pressure, LOT/FIT for fracture gradient) before finalizing a mud weight program from predictive correlations alone.
References
Primary source
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