Leak-Off Test Calculator: Shoe EMW and MAASP from LOT or FIT

Drilling Engineering
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Shoe EMW (ppg)
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Shoe gradient (psi/ft)
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MAASP with current mud (psi)

Leak-Off / Formation Integrity Test Result

Generated by calculatgeek.com LOT / FIT Calculator

Leak-off test chart showing surface pressure rising linearly with volume pumped until the leak-off point where the curve rolls over, with formulas for shoe equivalent mud weight and MAASP
A leak-off test pressures the open formation below a new shoe until the pressure-volume line departs from linear; that point converts to the shoe strength every later calculation depends on.

Every casing string ends with a small ritual. Drill out the shoe, expose a few feet of fresh formation, close the BOP, and pump slowly against the closed well while watching pressure against volume. The line climbs straight while the wellbore stretches elastically; the moment it bends over, the rock beneath the shoe has started accepting fluid, and that pressure is the leak-off point. Convert it to an equivalent mud weight at the shoe and you have the single number that governs everything about the next hole section: the heaviest mud you can run, the biggest kick you can safely shut in on, and the ceiling for every ECD and surge excursion.

The arithmetic

Shoe EMW = test mud weight + LOP / (0.052 x shoe TVD)

And once drilling resumes with whatever mud is in the hole, the same shoe strength converts into the maximum allowable annulus surface pressure:

MAASP = (shoe EMW – current mud weight) x 0.052 x shoe TVD

MAASP is the working number on the rig floor during a well control event: it is how much casing pressure the weakest known point can take before breaking down. Notice its uncomfortable property, which the chart in this calculator draws explicitly: every increase in mud weight shrinks MAASP one-for-one in hydrostatic terms, so a shoe that looked comfortable at spud can be nearly consumed by the time the section reaches TD with heavy mud.

Worked example

  • LOT below a 5,000 ft TVD shoe with 9.5 ppg mud leaks off at 800 psi surface pressure
  • Shoe EMW = 9.5 + 800/(0.052 x 5000) = 12.58 ppg, a gradient of 0.654 psi/ft
  • Drilling ahead with 10.0 ppg mud: MAASP = (12.58 – 10.0) x 0.052 x 5000 = 670 psi
  • Weight up to 11.5 ppg later in the section and MAASP drops to about 280 psi

LOT versus FIT, and honest caveats

A leak-off test finds the limit; a formation integrity test deliberately stops short of it, pressuring only to a predetermined value that the well plan requires and proving the shoe is at least that strong. FITs are standard where breaking the formation would be costly or where the shoe strength is already well known regionally. The trade is informational: after a FIT you know a floor, not a ceiling, so kick tolerance built on a FIT value is conservative by construction. Interpretation caveats worth carrying: the classic straight-line-then-departure picture assumes a reasonably tight cement job, and channelling behind casing shows up as an early, mushy departure that understates true rock strength; slow pumping (typically a quarter to half a barrel per minute) matters because pumping fast adds friction to the gauge reading; and the leak-off pressure approximates the minimum horizontal stress plus some tensile contribution, which is why repeat tests after a breakdown often level out at a slightly lower, more repeatable value.

Related Calculators and Reference

Shoe strength from this test is exactly what the kick tolerance calculator consumes, the predicted value before any test comes from the fracture gradient calculator, and the circulating pressures that must stay below it are computed by the ECD calculator. The PetroWiki entry on leak-off tests covers procedure and interpretation.