Page 183 - Petroleum Geology

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where N is the average number of point contacts per sphere.
The closest possible packing of equal spheres is close hexagonal, with 25.95%
porosity, 12 point contacts per sphere, and a maximum value of of 30".
The maximum saturation due to pendular rings in such material is therefore
24%. Cubic packing, with 47.64% porosity, 6 point contacts per sphere, and
p = 45", can have up to 18% saturation. Figure 8-6 shows the saturations of
these two extremes as a function of p. Clearly, saturations well into the ob-
served range are geometrically possible with pendular rings.
Higher saturations can be caused by irregularities of sorting (Morrow,
1971~) and it seems likely that the considerable variation of irreducible satura-
tion found in heterogeneous reservoir sands is due to smaller pores remaining
water-filled, the oil preferentially occupying the large pores with smaller in-
jection-pressure requirements.
At the oil/water contact, the evidence of well logs and of capillary pressure
experiments suggests that the water saturation increases from irreducible to
100% over a thin transition zone (direct measurement in this zone is not pos-
sible because part of this water is mobile).
Real reservoir rocks differ from our ideal packing in two important re-
spects. The sizes and shapes of the grains vary, depending on the sorting, and

WATER SATURATION

Fig. 8-6. Water saturation in idealized granular material as a function of the angle 0 (see
Fig. 8-5) for close hexagonal packing of equal spheres (A) and cubic packing of equal
spheres (B).