LOT/FIT Graphs and the Pressure Window

Two tests with different endpoints

A formation integrity test, or FIT, demonstrates that an exposed formation and shoe system can sustain a predetermined test pressure without evidence of unacceptable leak-off. It normally stops at that target. It establishes a tested lower bound for integrity, not the exact fracture pressure. A leak-off test, or LOT, seeks the point where fluid begins entering the formation as pumping continues. Leak-off can involve opening a fracture, existing fractures or permeable paths. Treating every LOT endpoint as an exact natural fracture pressure overstates what was observed.

The well programme defines the objective, test limits, hole preparation and acceptance criteria. The exposed interval, fluid density, fluid condition, gauge location and line-up must be known. Pump slowly at the specified rate, record pressure against volume or time, and observe the volume returned on controlled bleed-off. A hesitation procedure uses pauses; a continuous procedure pumps without those pauses. Use the authorised method and its interpretation rather than changing methods midway because the graph is inconvenient.

Read the shape before selecting the number

At first, pressure generally rises approximately linearly with pumped volume as the closed system compresses and expands elastically. A persistent deviation from that line can indicate leak-off. A single erratic point may instead be a pump, gauge or line-up problem. Check the record and the test conditions. The largest pressure observed is not automatically the correct pressure to use for the allowable pressure calculation; the approved interpretation may select an earlier leak-off point and include a margin.

Consider this simplified LOT record for a vertical shoe at 4,000 ft with 10.0 ppg test mud:

The first four points follow a 200 psi/bbl trend. The later increments are smaller. If the approved interpretation selects 400 psi as the usable leak-off pressure, use 400, not the peak of 470. This training table demonstrates interpretation; an actual acceptance decision needs the complete test record, equipment checks and the responsible supervisor's approval.

Convert the accepted pressure into a fluid-density limit

Hydrostatic pressure at the shoe during this test is 0.052 × 10.0 × 4,000 = 2,080 psi. Adding the accepted 400 psi surface pressure gives a test-derived shoe pressure of 2,480 psi. The corresponding equivalent mud weight is 2,480 ÷ (0.052 × 4,000) = 11.923 ppg. Equivalently, add 400 ÷ 208 = 1.923 ppg to the 10.0 ppg test density. This is the test-derived limit before any programme-required reduction or allowance.

If the current homogeneous mud above the shoe is 10.8 ppg, its shoe head is 2,246.4 psi. The corresponding static MAASP is 2,480 − 2,246.4 = 233.6 psi. Increasing the mud density to 11.3 ppg reduces this simplified static MAASP to 129.6 psi. Recalculate when the fluid above the weak point changes. The mud at the bit cannot be used as a substitute for the actual fluid profile above the shoe.

A FIT example and its limitation

Suppose another programme specifies a FIT to an equivalent density of 12.0 ppg at the same 4,000 ft shoe with 10.0 ppg mud. The target surface pressure is (12.0 − 10.0) × 0.052 × 4,000 = 416 psi. If the test reaches and satisfies the target criteria, the system has demonstrated integrity at that pressure under the test conditions. It has not demonstrated that fracture occurs at 416 psi. Continuing until the formation breaks would change the test objective and requires authorisation.

Use the window during an operation

The lower pressure limit comes from the formation-pressure prognosis, with the relevant operational margin. The upper limit comes from the weakest exposed formation or other binding integrity limit. Both carry uncertainty. At shallow depth below a subsea wellhead, increased water depth can leave a small formation overburden contribution and a narrow operating window; the same surface pressure can be significant relative to formation strength. Use pressure at the weak point rather than depth alone to assess this risk.

Static MAASP is a useful reference for a specified fluid profile. During a kill, influx position, fluid interfaces and friction change the relationship between surface pressure and pressure at the weak point. A surface pressure above the original mud-only MAASP does not by itself reveal the complete downhole profile, and a pressure below it does not guarantee every formation is safe. Monitor the approved dynamic limits, predicted pressures, returns and volumes. A fall in returns combined with unexpected pressure behaviour requires immediate communication and the approved contingency response.