Page 132 - Petrophysics

PERMEABILITY-POROSITY RELATIONSHIPS 105

(d) Fracturing and solution: In sandstone rocks, fracturing is not an
important cause of the secondary permeability, except where sandstones
are interbedded with shales, limestones, and dolomites. In carbonates,
the solution of minerals by percolating surface and subsurface acidic
waters as they pass along the primary pores, fissures, fractures, and
bedding planes, increase the permeability of the reservoir rock. As shown
by Chilingarian et al. [7], horizontal and vertical permeabilities are equal
in many carbonate reservoirs.

PERM EABI LITY-POROSITY RELATIONSHIPS

Figure 3.10 shows a plot of permeability versus porosity data obtained
from a large number of samples of a sandstone formation. Even though
this formation is generally considered very uniform and homogeneous,
there is not a specifically defined trendline between permeability and
porosity values. In this case, the relationship between permeability
and porosity is qualitative and is not directly or indirectly quantitative
in any way. It is possible to have very high porosity without having
any permeability at all, as in the case of pumice stone (where the
effective porosity is nearly zero), clays, and shales. The reverse of
high permeability with a low porosity might also be true, such as
in micro-fractured carbonates. In spite of this fundamental lack of
correspondence between these two properties, there often can be found
a very useful correlation between them within one formation, as shown
in Figure 3.11.

10000
I I

? I o o o ~
9

10
0.1 0.2 0.3 0.4
Porosity, Fraction
Figure 3.10. PmeabiZity-porosity rehtiombip.