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 :
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.
Institute of Petroleum Engineering, Heriot-Watt University