Formation strength tests (FST’s - sometimes referred to as “leak-off tests” (L.O.T) or “limit tests” or “formation intake tests” (F.I.T)) measure the formation strength below the casing. FST’s, shall be done below all casing shoes (or outside milled casing windows for side-tracks) in exploration and appraisal wells.

For development wells, the FST will be done below casing shoes (or outside milled casing windows for sidetracks) above hydrostatically pressured hydrocarbon bearing and/or over pressured sections in all other wells.

Formation Strength Test Profiles

The figure below shows a typical series of graphical representations for FST's conducted in different formation types.

Leak-off Pressure (LOP)

At this pressure fluid starts leaking off into the formation or through the casing cementation.

Breakdown Pressure (FBP)

At this pressure a fracture is initiated in the formation.

Fracture Propagation Pressure (FPP)

This is the pressure required to propagate fluid further into the formation.

Instantaneous Shut in Pressure (ISIP)

At the moment the pumps are shut in there is an almost instantaneous pressure drop once injection stops. The ISIP is the pressure level after this drop.

Fracture Closure Pressure (FCP)

This is the fluid pressure at which the fracture width reduces to zero. It is a slope inflection point below the ISIP