drilling · well control
Maximum Allowable Annular Surface Pressure (MAASP)
MAASP = 0.00980665 × (FracEMW − MW) × TVD
click formula to derive ↑
Inputs
lb/gal
lb/gal
ft
Description
Calculates the maximum surface annular (casing) pressure that can be tolerated during well control before the formation at the last casing shoe fractures. MAASP is the standard pre-determined limit rig crews monitor shut-in casing pressure against during a kick.
Variables
| Symbol | Unit | Description |
|---|---|---|
| MAASP | psi | MAASP |
| FracEMW | lb/gal | Fracture gradient at the casing shoe, expressed as an equivalent mud weight — typically from a leak-off test (LOT) or formation integrity test (FIT) at the shoe. |
| MW | lb/gal | Mud weight currently in the hole. 1198 kg/m³ (10 ppg) is roughly water-based mud; typical range 7-22 ppg. |
| TVD | ft | True vertical depth of the last casing shoe — the shallowest, typically weakest, open-hole point below surface casing. |
Assumptions
- Fracture gradient at the casing shoe is known with reasonable confidence (from a leak-off test, formation integrity test, or offset well data)
- The casing shoe is the weakest point in the open hole below it — no shallower, weaker zone is exposed
- Current mud weight is uniform throughout the wellbore at the time of evaluation
Limitations
- MAASP assumes a single, known fracture gradient at the shoe — uncertainty in the FIT/LOT-derived fracture gradient propagates directly into MAASP
- Does not account for an additional weak zone uphole of the shoe (e.g. a shallow gas zone or unconsolidated interval) that could fracture before the shoe does
- A static calculation — does not include friction pressure from circulating, which reduces the effective margin while pumping
Use Cases
- → Well control kill-sheet preparation: Calculate MAASP before spudding the next hole section so the rig crew has a pre-determined surface pressure limit to monitor against during any kick.
- → Casing seat depth selection: Evaluate MAASP at candidate shoe depths to ensure the selected casing string provides adequate kick tolerance margin for the planned mud weight program.
- → Real-time well control monitoring: Compare observed SICP against the pre-calculated MAASP during a kill operation to detect when the casing shoe is approaching its fracture limit.
Related Calculations
Region Notes
Permian Basin
Delaware Basin intermediate casing shoes are commonly set with FIT-derived fracture EMW of 15-17 PPG; with 10-11 PPG drilling mud through the Wolfcamp, this typically leaves a MAASP of roughly 500-1000 psi at 9,000-12,000 ft TVD shoe depths — track this margin closely given the basin’s narrow mud weight windows.
Gulf of Mexico
Deepwater MAASP calculations must also account for the seawater/riser margin and a typically much higher fracture gradient near the mudline that increases rapidly with the first few thousand feet of burial.
Global
MAASP should be recalculated any time the mud weight changes or a new FIT/LOT establishes a revised fracture gradient at the shoe — it is not a fixed well constant.
References
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