Formation Integrity Tests

3.11 Leak-off Test. A leak-off test is made to determine the pressure at which a formation will begin to fracture. Leak-off tests are usually run after drilling a short distance below the surface casing shoe. These tests may also be made on other casing strings. A leak-off test is performed by pumping drilling fluid into the wellbore at a slow rate (typically one-half barrel per minute), with blowout preventers closed and carefully plotting the resulting pressure versus the total volume pumped. The pressure at which the plotted curve begins to flatten. i.e., when the pressure increases a smaller amount for a volume pumped, is the surface leak-off pressure. The pump should be stopped immediately. This pressure plus the hydrostatic pressure of the drilling fluid is the formation fracture pressure.

Formation fracture pressure (psi) =

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