Page 78 - Petroleum Geology

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MECHANICAL ASPECTS OF PORE WATER IN SEDIMENTARY ROCKS

As a permeable sedimentary rock compacts during burial, the reduction of
porosity is accompanied by the expulsion of a commensurate part of its pore
water. As the load is applied, it is borne first by the water which, by flowing
away, transfers it to the grain-to-grain contacts. A less permeable rock com-
pacting under the same conditions takes longer to compact to mechanical
equilibrium because it takes longer for the commensurate amount of pore
water to escape. The loading creates a potential (energy) gradient in the water,
which will flow, if it can, to positions of smaller potential; the incremental
load borne by the water increases the pressure in the water.
Geologists may therefore be in intuitive agreement with Terzaghi (1936)
who postulated that the total load on sediment is borne partly by the solid
framework and partly by the pore fluid:
s=u+p (3.13)

where S (= ybw z) is the total vertical component of overburden pressure, u
is the effective stress transmitted through the solid matrix, and p is the pore-
fluid pressure (which Terzaghi called the neutral stress). He found that it is
the effective stress, u, that compacts a sedimentary rock, and that this quan-
tity is the difference between the total stress and the pore pressure (Fig. 3.12).
Equation 3.13 is called Terzaghi’s relationship.
Hubbert and Rubey (1959, p. 142) introduced a useful parameter A, which
is the ratio of pore-fluid pressure to total overburden pressure:
x = p/s. (3.14)

Fig. 3-12, The effective (intergranular) stress is the difference between the total stress due
to the overburden and the pore-fluid pressures.
Fig. 3-13. The effective stress, a, in a mudstone at depth 2 is equal to that of a normally
compacted mudstone at depth zc.