Page 97 - Origin and Prediction of Abnormal Formation Pressures

~] 8 A. GUREVICH, G.V. CHILINGAR, J.O. ROBERTSON AND E AMINZADEH

It is obvious that for flat areas or, more precisely, for areas with a nearly flat water
table, p* can be obtained just by subtraction of hydrostatic pressure from the actually
measured pressures.

SOME MAJOR FACTORS OF UNDERGROUND FLUID FORCED CONVECTION AND
CHARACTERISTICS FOR CORRELATION
Characteristics and parameters for correlation with the forced convection component
of formation pressure should be selected on the basis of thorough causal analysis of
each phenomenon and its impact on pressure changes. It is convenient in many cases
to select separately characteristics (1) of rock properties that provide a possibility of
compression and pressure change resulting from an external influence, and (2) of such
an influence. For example, porosity characterizes the ability of pore space to be reduced
under the impact of external influence, and overburden weight is such an influence.
Pressure increase is a superposition of (1) a rock ability to respond to external influence,
(2) external influence, and (3) permeability. The first two characterize the pressure
increase and the third one characterizes pressure dissipation. There is a possibility, using
characteristics of these three types, to form a kind of generalized parameter similar to
the similitude criterion and correlate it with the forced convection pressure component.
This possibility should be thoroughly explored and tested on some regional data.
A preliminary analysis of several major factors is given below.

Compaction (~'granular sediments

Compaction of granular sediments is a complex process strongly dependent on the
intensity of external influences (Athy, 1930; Hedberg, 1936; Dickinson, 1953; Weller,
1959; Mukhin, 1965; Rieke and Chilingarian, 1974; Chilingarian and Wolf, 1975, 1976;
Gurevich, 1980; and many others). It has two major mechanical components (Gurevich,
1980): (1) compaction due to increase in the effective stress, and (2) compaction due
to changes in sand matrix strength caused by external influences at a constant or
decreasing effective stress. Alternating stress and temperature changes and especially
vibrations weaken bonds between grains and help their rearrangement at points of
highest stress. The third component is a physicochemical one: filling of pore space with
secondary minerals. The relative role of each one of these three contributions to porosity
reduction varies with the type of geologic formation, intensity of tectonic deformation,
and geochemical processes including migration of mineral solutions. It is obvious that
porosity and permeability of a sediment change in time quite unevenly.
Three of the above-mentioned components of consolidation pertain to sands. Com-
paction of clays involves both of the mentioned mechanical components and, to a
lesser degree, the geochemical, but is more complicated because other processes are
also involved. Clay minerals, especially montmorillonite, have large quantities of water.
This water partially is represented by bound water adsorbed on the surfaces of clay
minerals and contained in-between aluminosilicate layers. The density of bound water
is approximately equal to up to 1.4 g/cm 3 immediately near the clay mineral surface.