As mentioned earlier, a basic drilling fluid function is to control formation
pressures to ensure a safe drilling operation. Typically, as formation pressures
increase, drilling fluid density is increased with barite to balance pressures and
maintain wellbore stability. This keeps formation fluids from flowing into the wellbore
and prevents pressured formation fluids from causing a blowout. The pressure exerted
by the drilling fluid column while static (not circulating) is called the hydrostatic
pressure and is a function of the density (mud weight) and True Vertical Depth (TVD)
of the well. If the hydrostatic pressure of the drilling fluid column is equal to or greater
than the formation pressure, formation fluids will not flow into the wellbore. Keeping
a well “under control” is often characterized as a set of conditions under which no
formation fluid will flow into the wellbore. But it also includes conditions where
formation fluids are allowed to flow into the wellbore — under controlled conditions.
Such conditions vary — from cases where high levels of background gas are tolerated
while drilling, to situations where the well is producing commercial quantities of oil
and gas while being drilled. Well control (or pressure control) means there is no
uncontrollable flow of formation fluids into the wellbore. Hydrostatic pressure also
controls stresses adjacent to the wellbore other than those exerted by formation
fluids. In geologically active regions, tectonic forces impose stresses in formations and
may make wellbores unstable even when formation fluid pressure is balanced.
Wellbores in tectonically stressed formations can be stabilized by balancing these
stresses with hydrostatic pressure. Similarly, the orientation of the wellbore in highangle
and horizontal intervals can cause decreased wellbore stability, which can also
be controlled with hydrostatic pressure. Normal formation pressures vary from a
pressure gradient of 0.433 psi/ft (9.79 kPa/m) (equivalent to 8.33 lb/gal [1 kg/L]
freshwater) in inland areas to 0.465 psi/ft (10.52 kPa/m) (equivalent to 8.95 lb/gal
[1.07 kg/L]) in marine basins. Elevation, location, and various geological processes and
histories create conditions where formation pressures depart considerably from these
normal values. The density of drilling fluid may range from that of air (essentially 0
psi/ft), to in excess of 20.0 lb/gal (2.4 kg/L) (1.04 psi/ft [23.52kPa/m]).