Page 25 - Drilling Technology in Nontechnical Language

16 Drilling Technology in Nontechnical Language Second Edition

In a stack of permeable rocks from the surface to 10,000 ft, if there are
no permeability barriers (such as a layer of salt or clean shale), the pressure

at 10,000 ft can be calculated if the pressure gradient of the fluid inside the
rock pore spaces is known. In an area where the pressure is “normal,” the

fluid in the pore spaces will average about 0.465 psi/ft.
It is possible for pressures in a formation to be much higher than is
normal for the depth (this is termed overpressured). For this to occur, two
conditions are necessary:

1. There is a pressure-tight barrier above the overpressured
formation.

2. There was a mechanism that created the higher pressure.
It is not necessary to go into the various mechanisms that might cause
overpressures under a barrier. When such an overpressured formation is
drilled into, if the formation pressure is higher than the hydrostatic pressure
of the drilling mud, the mud is pushed up the well by the pressure in the
formation. This is called a kick. If this happens, the rig crew must seal
the top of the well with a piece of equipment called the blowout preventer

(BOP) to stop more formation fluids entering the well (fig. 1–10).
Once the BOP seals the top of the well, the mud in the well is replaced

with a heavier fluid that gives a hydrostatic pressure greater than the
formation pressure. This process is called killing the well. A well that is
under pressure from a formation is said to be live.
If the well encounters a kick that is not controlled, the well will
blow out. Hydrocarbons will flow freely to the surface and may ignite,

endangering the rig and the people on it, and may result in huge costs and
pollution, as well.
It is very important to know how the pressure in the rock pores might
vary with depth in the planned well. In a known area where other wells
exist, the pressures and depths will be known, but even so, surprises may
still occur. It is vital to never be complacent about downhole pressures
or to assume that everything that is likely to happen is known, no matter
how many wells are already drilled in the area. Most of all, the situation
must never occur where a well is not strong enough to resist the pressures

encountered during a kick. If the rock fractures and allows fluid to flow
away from the well, control of the well has been lost. This is very dangerous.

_Devereux_Book.indb 16 1/16/12 2:06 PM