Page 123 - Petrophysics

96 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

and fenestral porosity, as well as leached grains and leached anhydrite
crystals, are unconnected vug types. Vugs and cavities can be connected
by intergranular pore channels or by fractures. Visual evaluation of
fracture-connected porosities in core samples is complicated by the
possibility of fractures induced by the coring operations [lo]. Based
on these observations, Lucia proposed a field classification of carbonate
porosity as follows [9]: (1) for fine particle size (d, less than 20 pm),
the displacement pressure, PD, is greater than 70 psia; (2) for medium
particle size (20 .c d, .c 100 pm), the PD is in the range of 15-70 psia;
(3) for large grains (d, > 100 pm), the displacement pressure is
less than 15 psia. The term PD is the extrapolated displacement
pressure, which is determined from the mercury capillary-pressure
curves discussed in Chapter 5. Figure 3.5 shows the relationship
between PD and the average grain size as a function of the intergranular
porosity for nonvuggy rocks with permeability greater than 0.1 mD.
This relationship is the basis for dividing particle size into the three
groups.

FLUID SATURATION

The porosity of a reservoir rock is very important because it is a
measure of the ability of that rock to store fluids (oil, gas, and water).

018

04 16 21
0 P 40 60 8)mm
PfJerage-a-
Figure 3.5. Relationsbip between displacement pressure and particle size for
nonvuggy rock, with k>O.1 mD [9J