Page 67 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 43

1.4.4
Casing Design [ ]

Having defined the hole size at TD, once the casing points are
defined then the sequence of hole and casing sizes can be seen. The
actual sizes chosen may depend on available casing stocks, standard
company sizes, rig equipment, or other factors.

1.4.4. Pore Pressures and Fracture Gradients

Knowledge of how the pore pressures and fracture gradients are like-
ly to change with depth is fundamental to a safe casing design. Discussed
here are ways of establishing the likely trends at the casing design stage.
If the pores in all formations from the depth of interest to the sur-
face were hydraulically connected, the pore pressure of the fluids with-
in the formations would be determined by the hydrostatic gradients of
those fluids. This would be termed “normally pressured.” In the
absence of more accurate data, a generally accepted average gradient of
formation fluid is 0.465 psi/ft; fresh water is 0.433 psi/ft.
The overburden stress (that is, the stress caused by the weight of all
the materials above a depth of interest) is supported partially by the pore
pressure and the rest by grain-to-grain contact within the formations.
The overburden stress divided by the depth will give the overbur-
den gradient. In areas of low tectonic activity, the overburden stress is
generally accepted to be at around 1 psi/ft. In tectonically active areas it
may be as low as 0.8 psi/ft.
Conditions may exist that give pressures that are higher than nor-
mal (termed “abnormally pressured”) or may be lower than normal
(termed “subnormally pressured”) when compared to formation fluid
hydrostatic. In predicting where abnormal pressures may occur, two
conditions are necessary:

There must be an impermeable barrier above the abnormal or sub-
normal zone. In normal depositional basins, a layer of clean shale
(i.e., no sand within it) will commonly form this barrier.
There must have been a mechanism for creating the abnormal or
subnormal pressure regime.

Mechanisms of abnormal pore pressure generation. Abnormal
pore pressures can be created by several different mechanisms.
Understanding of the mechanisms is important in identifying where
these may occur.

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