Page 154 - Origin and Prediction of Abnormal Formation Pressures

130 G.V. CHILINGAR, V.A. SEREBRYAKOV, S.A. KATZ AND J.O. ROBERTSON JR.

ABNORMAL PRESSURE DUE TO TEMPERATURE VARIATIONS

In the creation of abnormally high pressures, temperature does not play an appre-
ciable role in the case of nonequilibrium in undercompacted rocks. Temperature does
play a significant role in the case of well-compacted rocks, because the probability of
creating abnormally high pressure in compacted isolated rocks is significantly higher as
a result of the temperature change of the rock and its interstitial fluids.
The change of the formation pressure as a direct result of temperature variations,
ApT, may be expressed by the following equation:
Of w m ly s
APT = 9 A T (5-5)
r + r - r
where O~s and Otw are respectively coefficients of thermal expansion of solids (minerals)
and water; tip, flw and fls are respectively the coefficients of compressibility of pore
volume, water and skeletal structure for the rock and A T is the temperature change.
Inasmuch as:

Olw -r ors (5-6)
the thermal expansion of pore water cannot be compensated for by the deformation of
pore space. A change in temperature will lead to variations in the formation pressure. It
follows from Eq. 5-5 that if:
tip .qt_ /~w ~> /~s (5-7)
then an increase in temperature (A T > 0) will cause an increase in formation pressure.

ESTIMATION AND PREDICTION OF ABNORMALLY LOW PRESSURES IN BASINS IN
PERMAFROST REGIONS
The creation of abnormally low pressure in formations in regions of permafrost is
due to the following: (1) a decrease in the hydrostatic level of formation water due
to permafrost; (2) deformation of the rocks and interstitial fluids due to increase of
overburden (weight of ice); and (3) a decrease in rock volume and interstitial fluids due
to a decrease in temperature (cooling).
In compacted rocks, change of pore pressure can be estimated as follows (Dobrynin
and Serebryakov, 1989):

Ap- Aa + Apv (5-8)

where ApT is the temperature-related term defined by Eq. 5-5, Ao- is the change in the
average normal stress due to the formation of permafrost zone with thickness hi. It is
defined by the following equation:
l+v
Ao" = ~gloihi (5-9)
3(1 -v)
where v is Poisson's ratio, Pi is the density of ice, and hi is the thickness of the
permafrost.