Page 92 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF FORMATION FLUID PRESSURE DISTRIBUTIONS 73

(b) Piezo-convection effect. In the convective rise of gas, changing places with
the liquid that it displaces, the gas enters a region of lower pressure, whereas the
liquid enters a region of higher pressure. But because of the great difference in
their compressibilities (and the lower the pressure, the greater this difference), the
expansion of gas must be much greater than the compression of the liquid that has
arrived at the former site of gas. If the volume in fluid-saturated rocks in which this
convective redistribution of fluids takes place is poorly permeable or, especially, if
it is surrounded by poorly permeable rocks, the outflow of fluid compensating this
difference in new volumes will proceed very slowly. Therefore, a significant (local)
increase in pressure will arise, by means of which, through additional compression
of both liquid and gas, the greater expansion of the gas will be compensated, thus
preserving the overall balance of volume. This effect will be the greatest in vertically
fractured zones in poorly permeable rock formations.
(3) Effects of temperature change:
(a) Thermoelastic effect. Inasmuch as the coefficient of thermal expansion is greater
for fluids than for rock and, therefore, for the pore space, an increase in temperature
will increase the pressure, and a drop in temperature will lower the pressure.
(b) Changes in the state of aggregation, i.e., ice/water and water/vapor phase
transitions as the temperature passes through a critical point. Also included here is
the transition of bound water to free water as the temperature is raised (with an
increase in pressure), as well as the reverse transition of free water into bound water
when temperature is lowered (with a decrease in pressure).
(c) Precipitation of salts from saturated solutions, when the temperature is lowered
and the salt solubility is reduced, generally leads to a decrease in pore volume and an
increase in pressure. Dissolution of salts, when temperature is raised, usually leads to
an increase in the pore volume and a drop in pressure.
(d) Generation of oil and gas by thermocatalytic transformation of organic matter and
the decomposition of carbonates to form carbon dioxide (in quantities greater than
the quantities that can be dissolved in the pore water), accompanied by the formation
of new fluid and an increase in pressure.
(e) Dehydration of minerals. Transformation of montmorillonite to illite, gypsum
to anhydrite, and analogous transformations of other minerals lead to increases in
pressure.
(4) Chemical transformations of substances that are not initiated by temperature
increase, such as radiochemical decomposition of water and hydrocarbon molecules,
thermochemical decomposition of hydrocarbon molecules, synthesis of molecules of
resins and asphaltenes, and dolomitization of limestone. These phenomena have not
been yet evaluated rigorously in terms of the overall change in the volume of the system
and its impact on pressure.

This, in brief, is the list of basic thermodynamic phenomena (physical, physicochem-
ical, and chemical in character) leading to changes in the specific amount and degree
of compression of a fluid in the intergranular space of rocks. It must be emphasized
that even though the nature of these mechanisms and individual processes and their
qualitative patterns are rather clear in most cases, it may still be extremely difficult to