Page 18 - Origin and Prediction of Abnormal Formation Pressures

Chapter 1

INTRODUCTION TO ABNORMALLY PRESSURED FORMATIONS

E.C. DONALDSON, G.V. CHILINGAR, J.O. ROBERTSON JR. and V. SEREBRYAKOV

INTRODUCTION
There are wide variations in the subsurface formation fluid pressures due to a variety
of hydraulic and tectonic phenomena. Variations of interstitial fluid pressure from the
hydrostatic pressure of the subsurface fluids are labeled as abnormal formation pressure.
The hydrostatic pressure is equal to the vertical height of a column of water extending
from the surface to the formation of interest:

Ph -- Vw X h (1-1)
where Ph is the hydrostatic pressure in lb/ft 2, Vw is the specific weight of water in lb/ft 3,
and h is the height of the column of water, in ft. The hydrostatic pressure gradient, Gh,
in psi/ft, is equal to:
Vw
Gh = (1-2)
144
If the specific weight of water is 62.4 lb/ft 3, the Gh -- 0.433 psi/ft (0.10 kg cm -2 m-l).
The specific weight of water is a function of the salinity of the water, temperature,
and content of dissolved gases. Therefore, there is a general variation in the hydrostatic
pressure gradient at different locations and the average estimated hydrostatic pressure
gradient is usually taken as 0.465 psi/ft (0.074 kg cm -2 m-l); this corresponds to water
with a salinity of 80,000 parts per million (ppm) of sodium chloride at 77~ (25~
(Dickinson, 1953). In the presence of a normal hydrostatic pressure gradient, there is
fluid communication (vertical) between the formations. The coexistence of normal and
abnormal formation pressures in the same geologic environment can occur if one or
more of the formations are impermeable to the vertical hydraulic communication.
The average total overburden (lithostatic) pressure gradient resulting from the com-
bined pressure of the rocks (grain-to-grain or rock matrix stress) and their interstitial
fluids are taken as 1.0 psi/ft (0.231 kg cm -2 m-l):
Pob -- Pe + Pp (1-3)

where Pob is the total overburden (lithostatic) pressure which increases with depth, Pe is
the stress exerted through the grain-to-grain contacts, and pp is the pressure of the fluids
present in the pore spaces of the rocks. The hydrostatic, fluid pressure gradient cannot
exceed the pressure gradient of the total overburden load. Thus, any reservoir with a
hydrostatic gradient between 0.465 and 1.0 psi/ft is considered to have an abnormally
high pressure. Actually, as pointed out by Swarbrick and Osborne (1998), when the