Formation Integrity Test (FIT) is the tactic to check strength of formation and a casing shoe by increasing Bottom Hole Pressure (BHP) to designed pressure. FIT is generally conducted to be certain that formation below the casing show won’t be broken while drilling the following section with higher bottom hole pressure. Normally, drilling engineers on the town will design how much formation integrity test pressure required and a unit of FIT is generally in pound per gallon unit.
Tips on how to Calculate Pressure Required for Formation Integrity Test.
Before forming formation integrity test, it is best to know pressure required for Formation Integrity Test. The formula showed below demonstrates you tips on 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 feet
Example below demonstrates tips on how to use the formula to calculate pressure required for the test:
Required FIT (ppg) = 14.5
Current mud weight (ppg) = 9.2
True Vertical Depth of shoe (ft) = 4000 TVD
Pressure required for FIT = (14.5-9.2) x 0.052 x 4000 = 1102 psi
Note: The acronyms are listed below;
psi = pound per square inch
ppg = pound per gallon
ft = feet
Formation Integrity Test (FIT) procedures are as follows;
1. Drill out latest formation few feet, flow into bottom up and collect formation samples to verify that latest formation is drilled to after which pull a drill string into the casing shoe.
2. Close annular preventer or pipe rams, line up a cement pump, and flow into through an opened choke line to be certain that surface line is fully stuffed with drilling fluid.
3. Stop the pump and shut a choke valve.
4. Regularly pump small amount of drilling fluid right into a well with constant pump strokes and really useful pump strokes are 5 to 10 strokes per minute. Record total pump strokes, drill pipe pressure and casing pressure. Pump until casing pressure reaches the pressure required for formation integrity test. Stop pumping and hold pressure for couple of minutes to verify pressure stability.
5. Bleed off pressure and open up the well. Then start drilling operation.