BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
Chapter 12
PRESSURE PREDICTION AND CONTROL
All sedimentary rocks are porous to some degree. The pore spaces are filled with fluids –
liquids, gases or a combination – which are under pressure. A fundamental function of
drilling fluids is to control these downhole pressures as the failure to do so can result in
catastrophic consequences, both in terms of HSE and financial.
FORMATION PRESSURE
Overburden is the combined weight of the rock grains and the fluids contained within the pore
spaces. For example, a rock grain density of 2.65 g/cm 3 combined with a pore fluid density of
1.07 g/cm 3 and 34% porosity would result in an average density of 2.12 g/cm 3 [(2.65 × 0.66) +
(1.07 × 0.34) = 2.12 g/cm 3 ] would exert an overburden pressure of approximately 0.92 psi/ft.
(2.12 × 8.34 × 0.052 = 0.92 psi/ft). Overburden with normal pore pressure varies from
approximately 0.84 psi/ft near the surface to 1.0 psi/ft at 20,000 ft in the Gulf Coast area.
The relationship between overburden pressure gradient, pore pressure gradient, and rock grain
pressure gradient can be expressed as follows,
where,
P o
P o
= P f + P c
= overburden pressure gradient, psi/ft
P f = fluid pressure (pore pressure) gradient, psi/ft
P c = rock grain pressure gradient, psi/ft.
Normally Pressured Formations
Pore pressures in normally pressured zones are equivalent to the average hydrostatic pressure
exerted by the fluid contained in the pore spaces above the depth of interest. In the Louisiana
and Texas Gulf Coast area, empirical data indicates a normal pressure gradient of
approximately 0.465 psi/ft. This gradient corresponds to a fluid weighing approximately 9.0
lb m /gal or one having a salinity of approximately 80,000 mg/L Cl – . It should be noted that this
is an average pressure gradient. Obviously, near surface freshwater formations could have a
pressure gradient near that of freshwater (0.433 psi/ft or 8.34 lb m /gal).
In the Mid-Continent areas most sediments were deposited in the presence of freshwater.
Consequently, normal pore pressure is approximately equal to the gradient of freshwater (0.433
psi/ft or 8.34 lb m /gal in equivalent fluid weight).
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-1