Page 116 - Petrophysics 2E

POROSITY 89

SOLUTION

The bulk volume of the core sample is:

The pore volume is:

v, = - (V,,t - Vdq) = 453 - 425 = 26.17 cm3
1
Y 1.07

Using Equation 3.1, the porosity of the core is:

=
@ = 3 26.17 = 0.173 or 17.3%
vb 150.80

FACTORS GOVERNING THE MAGNITUDE POROSITY
OF
In an effort to determine approximate limits of porosity values, Fraser
and Graton determined the porosity of various packing arrangements of
uniform spheres [l]. They have shown that the cubic, or wide-packed
system, has a porosity of 47.6% and the rhombohedral, or close-packed
system, has a porosity of 25.9%. The porosity for such a system is
independent of the grain size (sphere diameter). However, if smaller
spheres are mixed among the spheres of either system, the ratio of pore
space to the solid framework becomes lower and porosity is reduced [ 21.
Figure 3.1 shows a three-grain-size cubic packing. The porosity of this
cubic packing is now approximately 26.5%.
The porosities of petroleum reservoirs range from 5% to 40% but
most frequently are between 10% and 20%. The factors governing the
magnitude of porosity in clastic sediments are:
(a) Uniformity of groin size: Uniformity or sorting is the gradation of
grains. If small particles of silt or clay are mixed with larger sand grains,
the effective (intercommunicating) porosity will be considerably reduced
as shown in Figure 3.1. These reservoirs are referred to as dirty or shaly.
Sorting depends on at least four major factors: size range of material,
type of deposition, current characteristics, and the duration of the
sedimentary process.
(b) Degree of cementation or consolidation: The highly cemented sand
stones have low porosities, whereas the soft, unconsolidated rocks
have high porosities. Cementation takes place both at the time of