Page 45 - Origin and Prediction of Abnormal Formation Pressures

28 G.V. CHILINGAR, J.O. ROBERTSON JR. AND H.H. RIEKE III

4000
,,4,,,-, ' \~e "~ x,~4 o
(1)
11)
.ff \ \ '~ California - Ventura Field
O. "~_ ~ ~ " pressures near crest
_,ro~ \
~.... \ b..\
(1) 8000 9 Q . ~ w
1=1 \ ~ ~ ~ Ventura Field
\ ~ ,0 ~ D-7 Zone
,, "~, \
\
Texas -- Louisiana
Gulf~oast Fields
12,000 Wyoming '\
Church Buttes
\ \
\
0 4000 8000 12000 16000
Pressure, psi

Fig. 2-4. Relationship between the formation fluid pressure and depth in several abnormally pressured pools.
Specific weight of water = 62.4 lb/ft 3 (+144 in2/ft 2 = 0.433 psi/ft). (Redrawn from Watts, 1948, fig. 2, p.
194; in Rieke and Chilingarian, 1974, fig. 10, p. 26.)

(1959; also see Hubbert and Rubey, 1960), however, showed that the pore pressure,
pp, is common to both the water and the clay and acts over the whole of any surface
passed through the porous solid, with the surface porosity being in no way involved (see
experimental results of Rieke and Chilingarian, 1974, p. 6). On assuming that all pores
are filled with water, at a depth, D, the total overburden pressure, pt, resulting from the
weight of overlying water and solids can be expressed by the following equation:

Pt- [y~(1 -4>) + y'wr (2-10)
where Vs is specific weight of the sediment grains (lb/ft3), 05 is fractional porosity, and
Vw is specific weight of water (lb/ft3). Inasmuch as the effective pressure (grain-to-grain
stress), Pe, is equal to the difference between the total overburden pressure and the pore
pressure [Pe - Pt - Pp], and pore pressure at a depth D is equal to VwD, then:
Pe -- [ys(1 --q~) + Ywq~- yw]D (2-11)
or:

Pe -- D[(1 -~b)(ys- Yw)] (2-12)
Brandt (1955) introduced an 85% correction factor (n) into the pp term to take
into account the "fact that the internal fluid pressure does not wholly react against