drilling · well control
Formation Integrity / Leak-Off Test Equivalent Mud Weight
EMW_LOT = MW + LOTp / (0.00980665 × TVD)
click formula to derive ↑
Inputs
lb/gal
psi
ft
Description
Converts a leak-off test (LOT) or formation integrity test (FIT) surface pressure into an equivalent mud weight and fracture gradient at the casing shoe — the standard way shoe strength is characterized for well control (MAASP) and mud weight program design.
Variables
| Symbol | Unit | Description |
|---|---|---|
| EMW_LOT | lb/gal | Equivalent Mud Weight at Leak-Off |
| FG | psi/ft | Fracture Gradient |
| MW | lb/gal | Mud weight in the hole at the time the leak-off or formation integrity test was performed. |
| LOTp | psi | Surface (standpipe) pressure at the leak-off point (LOT), or the target pressure reached without leak-off (FIT). |
| TVD | ft | True vertical depth of the casing shoe / open-hole test interval. |
Assumptions
- The test was executed correctly (constant low pump rate, accurate surface pressure gauge, no trapped air in the lines)
- A true leak-off point (LOT) was identified, not merely a pump-pressure inflection from another cause (e.g. a packer or tool seating)
- TVD used is the casing shoe / open-hole test interval depth, not surface or measured depth
Limitations
- An FIT (pressure reached without leak-off) only confirms the formation holds at least that EMW — it does NOT determine the actual fracture gradient, unlike a true LOT
- Leak-off pressure can be affected by mud properties (especially high-solids or high-LCM-content muds) independent of true formation strength
- A single-point test at one depth — fracture gradient typically increases with depth and should not be extrapolated far below the tested interval without additional data
Use Cases
- → Casing shoe integrity verification: Confirm the cement job and formation below a newly set casing shoe can withstand the planned mud weight before drilling ahead.
- → MAASP input determination: Use the leak-off-derived fracture gradient as the FracEMW input for the MAASP calculation at this casing shoe.
- → Fracture gradient trend calibration: Accumulate LOT results across multiple wells in a basin to build or refine a regional fracture gradient vs. depth trend.
Related Calculations
Region Notes
Permian Basin
Delaware Basin intermediate shoes commonly leak off at 15-17 PPG EMW; values notably below this in a new area should be flagged for cement job or shoe-track quality review before proceeding.
Gulf of Mexico
Shallow, weakly consolidated Gulf of Mexico shoes can leak off below 13 PPG EMW — particularly important for setting the next mud weight ceiling in deepwater wells with narrow drilling margins.
Global
Always distinguish a true LOT (leak-off observed) from an FIT (pressure target reached, no leak-off) in well records — treating an FIT as a measured fracture gradient overstates confidence in the number.
References
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