Leak Off Test Calculations
In a Leak-Off test the formation below the casing shoe is considered to have started to fracture at point A on Figure 24. The surface pressure at ponit A is known as the leak off pressure and can be used to determine the maximum allowable pressure on the formation below the shoe. The maximum allowable pressure at the shoe can subsequently be used to calculate:
• The maximum mudweight which can be used in the subsequent openhole section
• The Maximum Allowable Annular Surface Pressure (MAASP)
The maximum allowable pressure on the formation just below the casing shoe is generally expressed as an equivalent mud gradient (EMG) so that it can be compared with the mud weight to be used in the subsequent hole section.
Given the pressure at surface when leak off occurs (point A in Figure 24) just below the casing shoe, the maximum mudweight that can be used at that depth, and below, can be calculated from :
Maximum Mudweight (psi/ft) Pressure at the shoe when Leak-off occurs True Vertical Depth of the shoe Pressure at surface and hydrostatic pressure of mud in well True Vertical Depth of the shoe
Usually a safety factor of 0.5 ppg (0.026 psi/ft) is subtracted from the allowable mudweight.
It should be noted that the leak-off test is usually done just after drilling out of the casing shoe, but when drilling the next hole section other, weaker formations may be encountered.
While performing a leak off test the surface pressure at leak off was 940 psi. The casing shoe was at a true vertical depth of 5010 ft and a mud weight of 10.2 ppg was used to conduct the test.
The Maximum bottom hole pressure during the leakoff test can be calculated from:
hydrostatic pressure of colom of mud + leak off pressure at surface = (0.052 x 10.2 x 5010) + 940
= 3597 psi
the maximum allowable mud weight at this depth is therefore
= 0.718 psi/ft = 13.8 ppg Allowing a safety factor of 0.5 ppg,
The maximum allowable mud weight = 13.8 — 0.5 = 13.3 ppg.