Page 260 - Advanced Mine Ventilation

Reservoir Properties of Coal Seams 237

Figure 14.6 Permeability versus closure pressure.

The closure pressure, P C , is 1050 psi. It is roughly equal to the lower principal, hor-
izontal stress (s h ) and is linearly related to the permeability of the coal matrix as shown
in Fig. 14.6 for US coal seams.
Mathematically,

(14.12)
ln k ¼ a þ bP C
where k is the permeability in md; a and b are constants for the coal seam; and P C is the
closure pressure in psi.
Analyzing the data in Fig. 14.6,

ln k ¼ 5:1 0:001485 P C (14.13)

For example, let us assume, P C ¼ 1500 psi.
Hence,

ln k ¼ 5:1 0:001485 1500

or k ¼ 17:6md

Permeability of coal decreases with depth. Thakur [1] calculated the relationship
between permeability and the depth of coal seam as

D
k ¼ k O e (14.14)
a
where k O is the assumed permeability of 100 md at 100 ft depth; a is a characteristic for
the coal basin with a value of 700e1000 ft.