Drilling guides › Leak-off test, formation strength and MAASP
Leak-off test, formation strength and MAASP
After drilling out a casing shoe, a leak-off test (LOT) or formation integrity test (FIT) shows how much pressure the formation below the shoe can take. That limit sets the MAASP used during every well control event in the next hole section.
The shoe matters because it is normally the weakest exposed point in the well. Deeper rock is usually stronger, and everything above the shoe is behind steel and cement. If a kick is shut in and the pressure at the shoe goes beyond what the rock can hold, mud is lost into the formation, the hydrostatic falls and the well can go from a controlled kick to an underground blowout.
How the test is run
The shoe track and a short section of new hole, typically 10 to 20 ft, are drilled. The hole is circulated clean so the mud weight is even all round, the bit is pulled back inside the shoe, and the well is shut in. A cement unit or a mud pump then pumps at a slow, steady rate, often a quarter to half a barrel per minute, while pressure and volume pumped are recorded every quarter or half barrel. Plotted against volume, pressure first rises in a straight line as the mud and casing compress. The point where the line starts to bend over is the leak-off point. Pumping stops soon after, and the pressure is watched as it falls back, and the volume that flows back is measured.
Equivalent mud weight from the test
The formula adds the pressure applied at surface to the hydrostatic of the mud that was in the hole during the test, then turns the total into a mud weight at the shoe. This is why the test mud weight is used, not whatever mud is in the hole later.
Worked example
Leak-off at 1,250 psi surface pressure, 10.0 ppg test mud, shoe at 5,000 ft TVD:
MAASP
The maximum allowable annular surface pressure is the casing pressure that would bring the shoe to its tested limit with the current mud in the hole.
Worked example
Drilling ahead with 10.5 ppg mud:
If the mud weight is raised to 11.6 ppg, the MAASP falls to about 834 psi. Update it on the kill sheet and the choke panel every time the mud weight changes.
MAASP falls as mud weight rises because the heavier column already uses up more of the shoe's strength before any pressure is added at surface. The shoe strength itself has not changed; only the room left for surface pressure has.
Second worked example
Shoe at 7,800 ft TVD, 11.0 ppg test mud, leak-off at 1,620 psi. The next section will be drilled with 12.0 ppg mud.
Many companies round the EMW down, here to 14.9 ppg, which gives a more cautious MAASP of about 1,176 psi.
LOT or FIT?
- A leak-off test pumps until the pressure line bends away from a straight line: the formation has started to take fluid.
- A formation integrity test stops at a planned pressure, proving the shoe can take at least the EMW the next section needs, without breaking it down.
An FIT is chosen when the well plan only needs to prove a certain strength and there is no benefit in finding the real limit, or when breaking down the rock could harm a weak shoe or a reservoir.
Worked example: FIT surface pressure
The next section needs to prove 13.5 ppg EMW at a shoe at 4,200 ft TVD, testing with 10.2 ppg mud.
If the shoe holds 721 psi at surface for the agreed time, it has passed.
Metric example
Shoe at 1,500 m TVD, 1.20 sg test mud, leak-off at 95 bar: EMW = 1.20 + 95 ÷ (0.0981 × 1,500) ≈ 1.85 sg (1.84 sg rounded down). With 1.35 sg mud, MAASP = (1.84 − 1.35) × 0.0981 × 1,500 ≈ 72 bar.
Kick tolerance
Kick tolerance is the largest kick (usually in barrels, at a given intensity) that can be shut in and circulated out without exceeding the shoe strength. It depends on the LOT EMW, the mud weight, the hole and string geometry and the expected influx. Your well plan or drilling engineer calculates it; keep it in mind when the MAASP gets small.
Is the leak-off pressure the fracture pressure?
Not quite. Leak-off is where the rock first starts to take fluid. If pumping goes on, pressure usually peaks at a higher breakdown pressure, then drops to a lower propagation pressure as the fracture grows. After the pump stops, the pressure at which the fracture closes again is lower still. Well control uses the leak-off figure because it is the first sign of losses, and because a shoe that has been broken down may never hold its original strength again.
Common mistakes
- Using the current mud weight instead of the test mud weight in the LOT EMW.
- Using measured depth for a shoe in a deviated well.
- Not updating MAASP after a weight-up, so the choke operator works to a limit that is hundreds of psi too high.
- Pumping too fast, so the bend in the curve is hidden and the shoe is broken down further than needed.
- Testing with uneven or gelled mud, which makes the hydrostatic part uncertain.
Good practice
- Use the same pump, gauge and rate for every test, and plot the points live rather than afterwards.
- Agree the maximum test pressure before starting, so a FIT does not turn into an accidental LOT.
- Write the LOT EMW, test mud weight and shoe TVD on the kill sheet, and post the current MAASP at the choke panel.