Formation Integrity Test (FIT)
The Formation Integrity Test (FIT) is essentially a test of formation strength to a predetermined value. FIT is carried out by gradually increasing the bottom hole pressure after drilling out the casing shoe and a few feet of formation. The intent of FIT is not to pressurize the wellbore until the formation fractures but to test the strength of the formation to a pre-established limit. Since it is carried out below the casing shoe, it also tests if any cement channeling can compromise the well's integrity.
Since the maximum mud weight is limited by the fracture gradient, FIT is useful to know what maximum mud weight can be used for drilling. FIT is also used as a basis for casing depths and well control options in case of influx.
Stress Caging - An effective wellbore-strengthening approach