Page 79 - Drilling Technology in Nontechnical Language

70 Drilling Technology in Nontechnical Language Second Edition

Knowing the depth of the formation and the pressure it will bear, the
formation strength gradient can be calculated. In this example, it will be
pressure divided by depth, or 730 psi ÷ 1,000 ft = 0.73 psi/ft (5,032

kPa ÷ 305 m = 16.5 kPa/m). Now it is possible calculate a figure that
the driller on the rig must always know—the maximum surface pressure
that can be exerted on the well with the particular drilling mud density
in the hole. It is called the maximum allowable annular surface pressure
(MAASP) and is very easy to calculate:

MAASP = (Formation strength gradient – Mud gradient) ×
Vertical depth

With the 0.5 psi/ft (11.3 kPa/m) mud in the hole, the MAASP will
be 230 psi (1,586 kPa), which was the leakoff pressure at the end of the
test. However, if the mud gradient is increased later on while drilling,
the MAASP will reduce. So at a mud density gradient of 0.6 psi/ft (13.6
kPa/m), MAASP must be recalculated and will be as follows:

(0.73 psi/ft – 0.6 psi/ft) × 1,000 ft = 130 psi

or

(16.5 kPa/m – 13.6 kPa/m) × 305 m = 885 kPa

If the well drills into an overpressured formation and takes a kick, the
maximum pressure limit at the surface is 130 psi (885 kPa). If this pressure
is exceeded, the formation just under the shoe is likely to fracture.

Drilling the rst intermediate hole section
After drilling out the surface casing shoe and a bit of new formation,
the formation strength was tested. The well was circulated to a new
mud system.
The next chapter will discuss drilling a well accurately to a target that is
not directly below the surface location. However, the example exploration
well here is a vertical well (with the target directly under the rig), so there

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