Page 64 - Petroleum Geology

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TABLE 3-1
Variation of bulk wet mass density and bulk wet weight density of quartz sand with poro-
sity*

Porosity pbw ybw ybw ybw
(kg/m3) (kPa/m) (kgf em-' m-') (psi/ft)

0.3 5 2090 20.5 0.209 0.91
0.30 2170 21.3 0.217 0.94
0.25 2250 22.1 0.225 0.98
0.20 2330 22.9 0.233 1.01
0.15 2410 23.6 0.241 1.04
0.10 2490 24.4 0.249 1.08
* Interstitial water mass density, 1050 kg/m3; grain density, 2650 kg/m3.

Porosity and permeability are related, as we shall see in Chapter 8 on the
nature of petroleum reservoirs. Effective porosity determines the amount of
movable water and, with pore size, the resistance of the porous material to
movement of the water. Tortuosity of the pore passages increases, in general,
with compaction and this too can be related empirically to porosity. Kozeny
(1927) found by analysis and experiment that the permeability of uncon-
solidated sands varies approximately as f3/(l - f)'. Chapman (1981, p. 60)
preferred fx/(l - f)' where 3c varies from 3.5 in unconsolidated sands to about
5.3 in indurated sandstones, because this takes tortuosity better into account.
Little is known about the permeability of mudstones, but we may safely as-
sume that similar principles apply. Rearrangement of the grains of an uncon-
solidated sand that reduces the porosity from 40 to 30% reduces its perme-
ability to about 25-30% of its former value. While this is a considerable rela-
tive loss of permeability, the final permeability is usually still quite large be-
cause the original permeability may well have been two or three darcies. A
sand with 500 millidarcies (md) permeability is still very permeable.
Sandstone compaction is not as simple as that, however, and the effects
tend to be obscured by environmental influences. Maxwell (1964) and Selley
(1978) found linear trends, and Stephenson (1977) examined the effect of
temperature on sandstone compaction.
Sandstone compaction is elastic on a short time-scale, but the removal of
load does not lead to restoration of the original properties (no core would
ever be extracted from a core barrel if that were not true). Under severe load-
ing, grains may be fractured, resulting in greatly reduced porosity and perme-
ability. Diagenesis also leads to cementation and to authigenic minerals in
the pore space. Pendular cement may have very little effect on permeability
(Fuchtbauer, 1967, p. 359), but authigenic minerals, particularly clays, may
have a very adverse effect (see Plates 8-1 and 8-2 on pp. 162-163 and 166-