Page 155 - Origin and Prediction of Abnormal Formation Pressures

METHODS OF ESTIMATING AND PREDICTING ABNORMAL FORMATION PRESSURES 131

For geological cross-sections with compacted carbonate rocks, usually:
tip + flw >> fls (5-10)

and:
Otw >> ors (5-11)
Thus, the following equation for estimating the pore pressure in cross-sections with
permafrost and without permafrost can be derived:
[ ( )( )() ](ow)
Pa = Pn- A p
tip
Pi
1 + v
= gPw (h - hst) - 3(1 - v) tip + flw Pw hi tiP +/% AT (5-12)
where hst is the depth of the static water level.
The first term in Eq. 5-12:
H = gpw(h - hst)
represents the normal hydrostatic pressure, whereas the second term in Eq. 5-12:
(lqt-l) ) ( ~P ) (P-~w)hi

O -- gPw 3(1 - v) /~p -+- flw
indicates deformation of the rocks and fluids due to increase in the mass of ice.
(ow)
The last and most important term in Eq. 5-12:

V-- /~P -nt- flw AT
represents the pore pressure component caused by volume changes in rocks and
especially in interstitial fluids due to decreasing temperature (cooling).
Fig. 5-3 shows the graph for estimating the pore pressure in the Nepsko-Botuobin an-
ticline (eastern Siberia) based on Eq. 5-1. Using this equation, an estimated abnormally
low pressure was calculated in more than 40 wells in 23 oilfields, and then compared
to data obtained from field tests. The margin of error was usually not more than 4-5%.
Abnormally high pressure (10-20% higher than the normal hydrostatic pressure) was
present in one formation of this region (Osinskiy Horizon). But the origin of this
pressure appears to be related to the decrease in pore volume due to salt deposition in
pores and salt injection into the highly fractured zones.

FORMATION PRESSURE IN REGIONS WITH UPTHROWN AND DOWNTHROWN BLOCKS
(UPLIFT AND SUBSIDENCE OF SEDIMENTARY ROCKS)
Some tectonic processes in the Earth's crust create abnormal pressure. Most likely,
these processes involve multiple changes in the overburden, temperature, and squeezing
out of water from shales into associated sandstones. A change in the overburden
creates volumetric changes (increase or decrease in pore volume), whereas a change