Leak-Off Test (LOT), Formation Integrity Test (FIT) and MAASP: The Engineer's Guide with Numbers
Key takeaways
- An LOT pressures the formation below the shoe until the plot departs from a straight line. A FIT tests the formation to a pre-set pressure without breaking it down.
- Test equivalent mud weight: EMW = MW + LOP ÷ (0.052 × shoe TVD), rounded down.
- MAASP = (EMW − current MW) × 0.052 × shoe TVD. It changes whenever mud weight changes and must be recalculated.
- Worked example: shoe 6,000 ft, 10.0 ppg mud, LOP 1,250 psi gives EMW 14.0 ppg and MAASP 780 psi with 11.5 ppg mud.
- MAASP is an approximate limit built on assumptions; never treat it as absolute, especially when gas is above the shoe.
Educational content only: procedures and operating values vary between companies and wells. Always follow your company’s approved procedures, your supervisor’s instructions and the permits in force at your site.
Why test the formation below the shoe?
After the casing is run and cemented, the weakest exposed point in the well is usually the formation just below the casing shoe. Rocks generally get stronger with depth, and the shallower formations are now isolated behind casing. If a kick is taken and the well shut in, pressure rises throughout the well; if shoe pressure exceeds what the formation there can withstand, it fractures and we may lose mud. Worse, fluid may find a path outside the casing to other formations or to surface: an underground blowout.
So we need two numbers: the strength of the formation at the shoe, and the maximum pressure we can allow at the annulus surface without reaching it. The first comes from the test; the second is the MAASP, the Maximum Allowable Annular Surface Pressure.
LOT or FIT: what is the difference?
| Item | LOT: leak-off test | FIT: formation integrity test |
|---|---|---|
| Purpose | Find the pressure at which the formation starts to take fluid | Prove the formation holds a specific pressure needed by the programme |
| Stop point | When the plot departs from a straight line | When the target pressure is reached |
| Information obtained | The actual leak-off limit | A guaranteed minimum only |
| Risk to the formation | May weaken it slightly if overshot | Lower, since leak-off is not reached |
| When preferred | Exploration wells and accurate window definition | Development wells with known data |
Typical leak-off test procedure
Procedures vary between companies. The following is the sequence commonly described in the literature, not a substitute for the approved well programme:
- After the casing is cemented and tested, drill out cement and shoe track, then a few feet of new formation (often about 10 to 20 ft, per programme).
- Circulate to clean the hole and condition the mud, and measure mud weight accurately: it goes straight into the calculation.
- Pull the bit inside the casing and close the BOP on the pipe.
- Pump slowly at a constant low rate with a high-accuracy pump, usually the cement unit, recording pressure against volume every quarter or half barrel.
- Plot as you pump. At first the line is roughly straight because mud, pipe and casing compress elastically.
- When the line starts to bend, i.e. pressure rises less than expected per barrel, the leak-off pressure (LOP) has been reached. Stop pumping.
- Shut in and watch the pressure decline for several minutes, then bleed off and measure the volume returned, comparing it with the volume pumped.
- Record the result, calculate EMW and MAASP, and post them on the rig floor and on the kill sheet.
A FIT follows almost the same sequence, but pumping stops at the pre-calculated target surface pressure, which is held for a set period.
How to read the plot
- Straight section: elastic compression of the system; its slope depends on the volume of mud in the well and its compressibility.
- Departure point (LOP): the start of mud entering the formation; this is the point used in the calculation.
- If pumping continues: pressure may peak then fall. The formation has now actually fractured and been weakened more than necessary.
- Returned volume: if most of the pumped volume comes back on bleed-off, the behaviour was mostly elastic.
- Erratic plot: may indicate air in the lines, a surface equipment leak or poor cement behind the casing. Do not accept the result before understanding the cause.
Formulas
EMW (ppg) = MW (ppg) + LOP (psi) ÷ (0.052 × Shoe TVD (ft))
Fracture pressure at shoe (psi) = LOP + 0.052 × MW × Shoe TVD
MAASP (psi) = (EMW − Current MW) × 0.052 × Shoe TVD
Rounding rules: the IADC WellSharp formula sheet specifies rounding the test equivalent density down, and MAASP down as well, while kill mud weight is rounded up. The logic is the same in every case: always round in the safe direction. Check the official sheet of the body whose exam you are taking; details may differ.
Full worked example
Shoe at 6,000 ft TVD, mud weight during the test 10.0 ppg, LOP read at surface 1,250 psi.
EMW = 10.0 + 1,250 ÷ (0.052 × 6,000) = 10.0 + 4.006 = 14.006 → 14.0 ppg
Hydrostatic at shoe = 0.052 × 10.0 × 6,000 = 3,120 psi
Fracture pressure at shoe = 3,120 + 1,250 = 4,370 psi (≈ 0.728 psi/ft)
After some days of drilling, mud weight was raised to 11.5 ppg:
MAASP = (14.0 − 11.5) × 0.052 × 6,000 = 780 psi
Then to 12.0 ppg:
MAASP = (14.0 − 12.0) × 0.052 × 6,000 = 624 psi
Note how MAASP fell from 1,250 psi with 10.0 ppg mud to only 624 psi with 12.0 ppg. Failing to recalculate means the crew is working to an unsafe number.
FIT example
Shoe at 8,000 ft TVD, mud 10.8 ppg; the programme requires the formation to hold an equivalent of 13.5 ppg.
Test surface pressure = (13.5 − 10.8) × 0.052 × 8,000 = 1,123.2 ≈ 1,123 psi
Pump slowly to 1,123 psi at surface and watch that pressure holds. If it does, the test passes: the formation holds at least 13.5 ppg, but its real limit remains unknown.
MAASP during a well kill
MAASP as calculated above assumes the annulus above the shoe is full of current mud. While circulating a kick out, this changes:
- While gas is below the shoe: surface pressure compared with MAASP is a useful indicator.
- Once gas passes above the shoe: light gas replaces some of the mud above the shoe, so surface pressure rises without shoe pressure rising by the same amount. Exceeding MAASP on the gauge at this stage is therefore not direct evidence that the shoe has broken down.
- After kill mud is pumped: annulus density changes and MAASP is recalculated with the kill mud weight.
This does not mean ignoring the number. Decisions in these situations belong to the supervisor and follow company procedure. On wells with narrow margins some companies set an operating limit a defined safety margin below the calculated MAASP and require the supervisor to be told as soon as it is approached. But it shows why the engineer must understand the assumptions behind every number.
Factors that affect the test result
The test result is not an absolute property of the rock: it reflects both the rock and the procedure, and several factors must be understood when interpreting it:
- Pump rate: fast pumping can raise pressure through line friction and mask the departure point; very slow pumping can allow gradual leakage that makes the plot hard to read. Hence a low, constant rate set by the programme.
- Mud properties: high gel strength mud needs extra pressure to start moving, and non-homogeneous mud makes the real density differ from the measured one.
- Length of open hole below the shoe: the more formation exposed, the more likely a point weaker than the shoe exists, and the result reflects that point rather than the shoe itself.
- Cement quality behind the casing: poor cement at the shoe can let mud channel upward behind the casing, showing an early "leak-off" unrelated to rock strength.
- Temperature and time: mud temperature changes after circulation stops alter its volume slightly and affect shut-in monitoring readings.
From test to well design
The result is not used only for MAASP. The engineer also uses it to calculate kick tolerance, the largest gas influx that can be taken, shut in and circulated out without breaking down the shoe. It also sets the maximum mud weight and ECD allowed in the next section, and shows whether the target depth can be reached before another casing string. If the result comes in lower than expected, the team may decide on a remedial cement job, a change to the mud programme or a shallower next casing point.
Recording and communicating the result
Record in the drilling report: shoe MD and TVD, open-hole length, mud weight and properties, pump rate, the pressure-versus-volume table, LOP, EMW, returned volume and current MAASP. Then post the MAASP clearly on the rig floor and at the choke control panel, update it whenever mud weight changes, and brief the crew at the shift handover meeting.
IWCF-style practice questions
- Shoe at 5,200 ft TVD, mud 10.2 ppg, LOP 950 psi. What is the EMW? Solution: 10.2 + 950 ÷ 270.4 = 13.71, rounded down to 13.7 ppg.
- In the worked example above, after mud is raised to 12.4 ppg, what is the MAASP? Solution: (14.0 − 12.4) × 312 = 499.2, rounded down to 499 psi.
- Why not use shoe MD in these calculations? Because hydrostatic pressures depend only on vertical depth.
- What do you do if the plot departs much earlier than expected? Stop pumping and find the cause (a surface leak or poor cement, for example) before accepting the result. A remedial squeeze cement job may be needed.
Common mistakes
- Forgetting that LOP is a surface pressure and that hydrostatic must be added to obtain fracture pressure at the shoe.
- Using the current mud weight instead of the mud weight during the test to calculate EMW.
- Not updating MAASP after a mud weight change.
- Pumping on past the departure point, weakening the formation for no benefit.
- Testing with non-homogeneous mud or with air in the lines.
Summary for the engineer
The test at the shoe gives you the ceiling of the window, and MAASP translates that ceiling into a number the crew reads on the gauge. Remember that every number here depends on a gauge reading and a measured mud weight, so measurement accuracy is part of safety. Calculate carefully, round in the safe direction, recalculate with every mud weight change, and understand when the number stops being meaningful. See the hydrostatic pressure article, the lesson on the kill sheet and the lesson on BOP equipment.
Test your understanding
Glossary terms
Related lessons
- Kill Sheet Step-by-Step: Driller's Method or Wait and Weight?
- Formation Pressure: Normal, Subnormal, and Abnormal
- Well Pressure Fundamentals
- BOP Equipment: Types and Closure Methods
Sources
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