Page 57 - Origin and Prediction of Abnormal Formation Pressures

40 G.V. CHILINGAR, J.O. ROBERTSON JR. AND H.H. RIEKE III
PRESSURE, psi
0 500 1000 1500
0.[ I I l I I I IIli~
~\ 25 50 75 1 O0
I~\ Kg/Sq cm
I \\ '~ / 0.77 psi/ft or O. 18 kg/sq cm/m

500' = '" "-~ ,
k " '~, '~,Xx\ - Total overburden pressure gradient ~'
|\ \\ \ \ \ I,'/70.91 si/ftorO.21 kg/sqcm/m
p
~.200m ~\ ~\\\ / /
~\ ~\\~ \ \~.X\/ Unconfined &
~\
~_\'~ \ \\\\\ unconsolidated
! ~\ ~,..'\'~ \ ', X_ Aquifer
,ooo,: \ ",),
, Nydrostatic pressure gradient~ ~ s,,,\ ~ V'.~.
\
E 0.43 psi/ft or O. I kg/sq cm/m \~, ~_~ ,, \~'~
'/
" OOm \ ,,,
15oo' , / _~ Impermeable Caprock"xX.

'~ ;/ Confined & ' \ \ '~ ,ntergranulal Prissure gaden t
\
'~'
r
i
\
0.4_8 p~si/f) or O. 11 kg/sq cm/m
_',
\
\
~\\unconsolidated '\ ~ ~""---~~
aquifer ~ ~ '~ '~ t~,
2000, _60~3 ~ /
r Curve 2c
Fig. 2-9. Hydrostatic, geostatic and skeletal load changes as influenced by changes in fluid levels and
pressure in unconfined and confined aquifers. Curves 1, la, lb -- hydrostatic pressure gradient; curves 2, 2a,
2b, 2c -- intergranular pressure gradient; curves 3, 3a, 3b -- total overburden pressure (geostatic) gradient.
Sp. gr. solids -- 2.7; sp. gr. water - 1; porosity, 4) = 35%. (Modified after Allen and Chilingarian, 1975, fig.
2, p. 50.)

point and will increase with depth, at a normal hydrostatic gradient; therefore, Curve
l b is parallel to Curve 1. The skeletal structure at a depth of 1500 ft assumes the full
overburden load, and the intergranular gradient shifts from Curve 2 to Curve 2c, because
of the loss of hydrostatic pressure support.
Fig. 2-10 illustrates the changes in skeletal loading if the pore pressure at the top
of the confined zone at 1500 ft is raised to become equal to the lithostatic (geostatic)
pressure. The hydrostatic pressure gradient (Curve 1) would become equal to the
geostatic gradient (Curve 3) at 1500 ft (Curve 1 shifts to Curve la) and the intergranular
pressure (Curve 2) is reduced to zero (Curve 2 shifts to Curve 2a). The entire weight of