Page 44 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF ABNORMAL FORMATION PRESSURES 27
0.013
0.012
0.011

~:~ 0.010 f1"
a;
I
0,009
'~ 0.008
0,007

~..T..: 0.006
>: o~
0,005
0.004
i=~ 0.003
~ 0
0.002
!:
0 0.001
0.000
0 1000 2000 3000 4000 5000 6000
Pressure, p, psia
Fig. 2-3. Difference between the formation volume factor of gas-saturated pure water and that of pure water
at various temperatures. Correction for the formation volume factor (EV.F.) is F.g-F.gas saturated pure water -
F.V.F.purewater. (Modified after Frick, 1962, fig. 22.16, p. 22.21.) F.V.F. = volume occupied at reservoir
conditions divided by the volume occupied at standard conditions at the surface (60~ and 1 atm pressure).

If the buoyant force Fb is equal to the weight of fluid displaced by the grains
-- ~b) (2-6)
Fb -- Wb -- ~fgb(l
and inasmuch as
Vb--A.D (2-7)

where A is the total cross-sectional surface area and D is the depth,
and the pore pressure, pp:

pp -- yfD (2-8)
then:
Fb -- pp. A (1 - ~b) (2-9)
Eq. 2-9, derived by Rieke and Chilingarian (1974), is in close agreement with the
view of Terzaghi (1926) that the uplift force, due to pore pressure, is proportional to the
surface porosity (also see Laubscher, 1960). Surface or boundary porosity is the ratio of
the pore area to the gross area, along the surface, A. It can also be shown that surface
porosity on a plane surface is the same as volumetric porosity. Hubbert and Rubey