Well control · field units

MAASP and leak-off test calculations

MAASP, the maximum allowable annular surface pressure, is the highest casing pressure you can hold at surface before the formation at the casing shoe starts to break down. In field units, MAASP (psi) = (maximum allowable mud weight − mud weight in the hole) × 0.052 × casing shoe TVD (ft). The maximum allowable mud weight comes from the leak-off test: LOT EMW (ppg) = test mud weight + leak-off pressure ÷ (0.052 × shoe TVD). This page sets out both calculations in oilfield units (psi, ppg, ft) with one worked example, a table of MAASP against mud weight, and the limits of the number.

Reference material only. Well-control operations must follow your company's approved procedures, the rig's own kill sheet, and the training standards of IADC WellSharp or IWCF. DrillX calculators are advisory.

1. What a leak-off test measures

After casing is set and cemented, the crew drills out the shoe and a short length of new formation, usually around 10 to 15 ft, then closes the well and pumps mud slowly while plotting surface pressure against volume pumped. At first the plot is a straight line. The leak-off pressure is the point where the line bends, because the formation has started to take fluid. That surface pressure, added to the hydrostatic pressure of the mud in the hole, is the strength of the weakest exposed formation, which is normally assumed to be at the shoe.

A formation integrity test (FIT) uses the same set-up but stops at a planned pressure without reaching leak-off. A FIT proves the shoe can carry the mud weights planned for the next section; a leak-off test (LOT) measures what the shoe can actually carry. Either result can be used to set MAASP, but a FIT value gives a lower, more conservative limit because the real leak-off point was never reached.

2. The formulas

The constant 0.052 converts pounds per gallon to a pressure gradient in psi per foot. LOT EMW is also called the maximum allowable mud weight: a static column of mud at that weight would, on its own, bring the shoe to leak-off. Round LOT EMW down to the next 0.1 ppg so that the limit stays on the safe side of the measurement.

3. Worked example

Assume a leak-off test run after drilling out a casing shoe, with these readings:

Two checks are built into these numbers. With the test mud still in the hole, MAASP equals the leak-off pressure itself, 1,144 psi, as it should. And the fracture pressure of 3,640 psi equals a 14.0 ppg column to the shoe: 0.052 × 14.0 × 5,000 = 3,640 psi.

4. MAASP falls as mud weight rises

MAASP is what is left of the shoe strength after the mud column has taken its share. For the same shoe (5,000 ft TVD, 14.0 ppg LOT EMW), each extra 0.1 ppg of mud weight removes 26 psi of MAASP:

This is why MAASP has to be recalculated and re-posted every time the mud weight changes, and why a kill sheet shows two values: one for the original mud and a lower one for the kill mud.

5. What MAASP does and does not tell you

• It assumes mud of one weight from surface to shoe. The formula is a static calculation with a full column of the stated mud above the shoe.
• It matters most while the influx is below the shoe. During a kill, once a gas influx has been circulated up past the shoe, the fluid column above the shoe is lighter than mud. Surface casing pressure can then rise above the calculated MAASP without the pressure at the shoe exceeding its fracture pressure. How this is handled is set by the operator's well-control policy and training.
• It is a formation limit, not an equipment limit. Casing burst rating and wellhead and BOP rated working pressure are separate limits and must be checked separately.
• It ignores circulating friction. While pumping, annular pressure loss adds to the pressure at the shoe; see equivalent circulating density (ECD).

6. Mistakes to avoid

• Using measured depth instead of TVD for the shoe in a deviated well. Every 0.052 × MW × depth term needs TVD.
• Not recalculating MAASP after weighting up or cutting back the mud.
• Rounding LOT EMW up. Round down to the next 0.1 ppg.
• Treating a FIT pressure as a leak-off pressure. A FIT gives a tested limit, not the formation's measured strength.
• Running the test with uneven mud weight in the hole. Circulate and condition the mud first so that the weight in and out is the same, otherwise the hydrostatic term is wrong.
• Reading the leak-off point from a coarse rig gauge. Use a calibrated, low-range gauge and plot pressure against volume pumped.

How DrillX helps

DrillX includes a MAASP from leak-off test calculator alongside hydrostatic pressure, ECD, annular pressure loss, triplex pump output, annular capacity and velocity, buoyancy and string weight, and a Driller's and Wait & Weight kill sheet. Values update as you type, in field units, and in English or Arabic. To see how MAASP sets the largest kick a well can take, see kill mud weight and kick tolerance. DrillX is in final testing; email info@drillx.app for a demo or early access, or see what DrillX does.

Sources and further reading

• International Association of Drilling Contractors, WellSharp well-control training: iadc.org/wellsharp
• International Well Control Forum, well-control certification and standard kill sheets: iwcf.org
• American Petroleum Institute, RP 59, Recommended Practice for Well Control Operations: api.org