Requirements to Complete a LOT (Leak-Off Test) or FIT (Formation Integrity Test)
Hole Conditions:
- The new casing shoe must be drilled out, and an additional 5 to 30 feet of formation must be penetrated.
- A sample from the new formation should be circulated up to verify the formation has been drilled.
- Circulate the hole with mud until the shakers are free of cement and cuttings.
Mud Properties:
- The mud must be clean, consistent, have low gel strengths, and have a known mud weight.
- If there is uncertainty about the mud weight consistency, the test should not be conducted until you are certain the mud properties are the same throughout the active system.
- Accurate calculation of hydrostatic pressure is crucial for the test’s validity.
- Excessive gel strengths may negatively impact the results of the LOT/FIT.
Line Up:
- LOT/FIT tests require a high-pressure, low-volume pump, typically a cement pump.
- The pump should be lined up to pump both down the drill pipe and into the annulus.
- A Blowout Preventer (BOP) must be closed to seal the annulus, and BOP side outlets opened to pressurize the choke line against a closed choke.
- Valves should be positioned to allow the recording of both drill pipe and casing pressure.
- Testing through a mud motor or non-return valve should be avoided if possible.
Instrumentation:
- The driller must be able to record cumulative pump strokes or volume pumped, drill pipe pressure, and casing pressure.
- Accurate pressure measurement requires recently calibrated pressure gauges and large-scale charts mounted on a manifold.
- In some cases, downhole pressure gauges may be used for better results, especially when using high mud weights or compressive base fluids.
Pump Rates:
- The pump rates should be slow, typically ½ barrel per minute (bbl/min) or less, to ensure accurate readings and minimize risks during the test.
These requirements ensure a successful and accurate LOT or FIT, providing essential data for well integrity and pressure management during drilling.
For more information on LOTs and other Well Control subjects, read my Well Control Manual at: https://learn-well-control.com/product/well-control-manual-by-edwin-ritchie/