Page 130 - Petrophysics

PERMEABILITY 103

Factors affecting the magnitude of permeability in sediments are:

(a) Shape and size of sand grains: If the rock is composed of large and
flat grains uniformly arranged with the longest dimension horizontal,
as illustrated in Figure 3.7, its horizontal permeability (kH) will be very
high, whereas vertical permeability (kv) will be medium-to-large. If the
rock is composed of mostly large and rounded grains, its permeability
wiU be considerably high and of same magnitude in both directions,
as shown in Figure 3.8. Permeability of reservoir rocks is generally
lower, especially in the vertical direction, if the sand grains are small
and of irregular shape (Figure 3.9). Most petroleum reservoirs fall in

Large, Flat Grains

Horizontal Permeability, kH = 2,000 mD
Vertical Permeability, kv = 800 mD
Figure 3.7. Effects of large3at grains on permeability [13].

Large, Rounded Grains

Horizontal Permeability, kH = 2,000 mD
Vertical Permeability, kv = 1,500 Md
Figure 3.8. Effects of large rounded grains on permeability [131.