What are the differences between Formation Integrity Test (FIT) and Leak Off Test (LOT).?

You may confuse between Formation Integrity Test (FIT) and Leak Off Test (LOT).

They are different in this following aspect.

Leak Off Test – you pressure test shoe and formation until formation break down.

Let’s I explain more about it: Leak off Test is conducted in order to find the fracture pressure (fracture gradient) of formation and shoe. When conducting the LOT, you will pump drilling fluid to until you see the fracture trend of formation. Once formation is fractured, the first pressure that deviated from a trend is typically called Leak Off Pressure. We use the leak off pressure to calculate LOT.

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Formation Integrity Test (FIT) Procedure and Calculation

Formation Integrity Test is the method to test strength of formation and shoe by increasing Bottom Hole Pressure (BHP) to designed pressure. FIT is normally conducted to ensure that formation below a casing shoe will not be broken while drilling the next section with higher BHP or circulating gas influx in a well control situation. Normally, drilling engineers will design how much formation integrity test pressure required for each hole section.

The formula below demonstrates you how to calculate required FIT pressure.

Pressure required for FIT (psi) = (Required FIT in ppg – Current Mud Weight in ppg) x 0.052 x True Vertical Depth of shoe in ft

Example:
Required FIT (ppg) = 14.5
Current mud weight (ppg) = 9.2
Shoe depth TVD (ft) = 4000 TVD
Pressure required for FIT = (14.5-9.2) x 0.052 x 4000 = 1,102 psi

Formation Integrity Test (FIT) guideline is listed below: (note: this is just only guide line. You may need to follow your standard procedure in order to perform formation integrity test):

1. Drill out new formation few feet, circulate bottom up and collect sample to confirm that new formation is drilled to and then pull string into the casing.

2. Close annular preventer or pipe rams, line up a pump, normally a cement pump, and circulate through an open choke line to ensure that surface line is fully filled with drilling fluid.

3.Stop the pump and close a choke valve.

4. Gradually pump small amount of drilling fluid into well with constant pump stroke. Record total pump strokes, drill pipe pressure and casing pressure. Pump until casing pressure reaches the pressure required for formatin integrity test. Hold pressure for few minutes to confirm pressure.

5. Bleed off pressure and open up the well. Then proceed drilling operation.

Please find the Excel sheetcalculate pressure required for formation integrity test.

Ref books: Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.