Page 91 - Origin and Prediction of Abnormal Formation Pressures

72 A. GUREVICH, G.V. CHILINGAR, J.O. ROBERTSON AND E AMINZADEH

Forced convection of ground fluids
The forced convection is caused by any factor or combination of factors that change
compression of fluids as a result of changes in volumes of rock solids, interparticle
voids, and fluids themselves. These processes act locally, producing pressure deviations
directly in the volume where they are manifested. Then these deviations are transmitted
to the neighboring volumes of fluid-saturated rocks by piezoconductivity by means of
mass transfer.
A list of physical and physicochemical processes known today (and actually long
ago) that can theoretically cause a forced convection, may be presented as follows
(Gurevich et al., 1987).
(1) Mechanical deformations of rocks with a change in porosity and, thereby, a change
in the fluid pressure:
(a) Elastic compression (or expansion) of the grains and of the entire rock frame with
increasing (or decreasing) overburden load.
(b) Irreversible decrease in porosity of granular sedimentary rocks due to addition of
overburden or reduction in fluid formation pressure or/and a reduction in the rock
frame strength. The strength of the rock frame is influenced by the following: (1)
diffusion of dislocations in crystal structure, which favors slow slippage of grains
relative to each other; (2) pulsating (alternating) temperature changes which create
non-uniform stresses in an inhomogeneous frame and tend to rupture bonds between
grains; (3) seismic-type vibrations which reduce the force of friction upon passage of
the tension phase of the seismic wave; (4) steady tangential and especially alternating
geodynamic stresses; (5) alternating stresses from lunar and solar tides; and so on. As
the end result, the rate of irreversible compaction sometimes may reach the maximum
not at the maximum rate of overburden increase, but rather in periods of fast uplift
when impact on the frame is intensified and the strength of the rock frame is reduced,
though overburden load may remain constant or even decrease.
(c) Geodynamic stresses (compression and tension) that arise during periods of active
tectonic movements and act on the volume of void space. Effects on intergranular
capacity are usually insignificant, but fracture capacity, due to high compressibility
of fractures, is strongly affected.
(d) Flow of salt (plastic deformation) under the influence of overburden and geo-
dynamic load, into the void space of intersalt porous rocks. The salt flow rate will
be increased by vibration. This process constantly tends to equalize the pore fluid
pressure in these porous rocks and the stress in the salt.
(2) Influence of mass transfer fluxes:
(a) Passage of fluxes of fluids through barogeochemical and thermogeochemical
barriers, leading to dissolution or precipitation of solids from solution, and to
mixing or differentiation of fluids. Dissolution of a certain component of rocks
in a fluid moving through the rocks in the direction of increasing solubility of this
component usually leads to a decrease in the total volume and a reduction in pressure.
Precipitation of the substance during fluid movement in the direction of a decreasing
solubility below the actual concentration of this component leads to an increase in
pressure.