Formation Integrity Test (FIT) is the method to test strength of formation and a casing shoe by increasing Bottom Hole Pressure (BHP) to designed pressure. FIT is normally conducted to ensure that formation below the casing show will not be broken while drilling the next section with higher bottom hole pressure. Normally, drilling engineers in town will design how much formation integrity test pressure required and a unit of FIT is normally in pound per gallon unit.
How to Calculate Pressure Required for Formation Integrity Test.
Before forming formation integrity test, you should know pressure required for Formation Integrity Test. The formula showed 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 feet
Example below demonstrates 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 new formation few feet, circulate bottom up and collect formation samples to confirm that new formation is drilled to and then pull a drill string into the casing shoe.
2. Close annular preventer or pipe rams, line up a cement pump, and circulate through an opened 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 a well with constant pump strokes and recommended 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 few minutes to confirm pressure stability.
5. Bleed off pressure and open up the well. Then start drilling operation.