Page 156 - Origin and Prediction of Abnormal Formation Pressures

132 G.V. CHILINGAR, V.A. SEREBRYAKOV, S.A. KATZ AND J.O. ROBERTSON JR.
Pore Pressure (pp), MPa

10 20 30 0 10 20 30
' qF] '
600 600
2Vzz: 1

A) 1200 CB)
1200

r
\\
~8oo 1800
a

2600 \ 2600
\
\
Fig. 5-3. Curves for estimating pore pressure in the formations of the Nepsko-Botuobin anticline in East
Siberia. (A) Areas with permafrost and average geothermal gradient (G2) of 0.8~ m. (B) Areas devoid
of permafrost. Figures on curves represent geothermal gradient: 1 = normal hydrostatic pressure gradient; 2
-- calculated using the following equation:
O/w
Pa = gpw(h -- 0.8hi) -- ~[To.I -F (GI - G2)h 4- G2hi]
tip "t- ~w
where tSw is the average density of formation water at depth h; hi is the thickness of the permafrost; G1
and G2 are geothermal gradients respectively before and after cooling; T0,1 is yearly average temperature
of the Earth's surface prior to cooling; /4p is coefficient of pore compressibility and flw is coefficient of
compressibility of pore water; and oe,,, is coefficient of thermal expansion of pore water. Assumptions for
dense carbonate rocks: /4p ~ flw; tip + flw >> fl~ (/4~ is compressibility of solid mineral grains of rocks);
C~w >> c~ (c~ is coefficient of thermal expansion of solid mineral grains composing the rocks); Poisson's
ratio, v = 0.25; and Pi/lSw ,~ 0.75 (,oi is average density of ice). (Modified after Dobrynin and Serebryakov,
1989, fig. 51, p. 108.)

in temperature creates volume changes in the rock's skeletal structure and interstitial
fluids. These processes operate only in sedimentary basins with aquifer systems; they
do not operate in infiltration water systems. The following equation can be used for the
estimation of abnormal pressure in regions with uplift plus erosion, or subsidence plus
sedimentation (Dobrynin and Serebryakov, 1989):
pa = Pn 4- Ap

gpw(h hst) + ( I + V ) (tip)gprAh_4_ ~C~w (5-13)
3(1 - v) /~p --~ flw /~p +/3w AT
where parameters are the same as in Eqs. 5-5, 5-8, 5-9 and 5-10; Ah is the amplitude
of subsidence or uplift and Pr is the average density of new deposits after subsidence
and sedimentation. After the first and second parts of Eq. 5-13, it is necessary to use a
minus sign in the case of uplift and erosion and a plus sign in the case of subsidence
plus sedimentation.