Page 261 - Advanced Mine Ventilation

238 Advanced Mine Ventilation

For example, calculate the permeability at 7000 ft depth using Eq. (14.14).

1. if a ¼ 700
7000
k ¼ 100 e ¼ 0.0045 md or 4.5 md
700
2. if a ¼ 1000

7000
k ¼ 100 e ¼ 0.09 md x 90 md
1000
where md means a microdarcy.
Eq. (14.14) gives only an approximate value of permeability. It always needs to be
confirmed by a field test.

14.3 Diffusivity of Methane in Coal

Methane is held in adsorption on the surface of coal particles in a monolayer.
The flow of gases adsorbed on the coal matrix surfaces starts as soon as the
confining pressure is reduced. The process goes through the following steps:
• Diffusion of gas from coal following Fick’s law, i.e., concentration-dependent flow.
• Laminar flow of gases through the fractures in coal matrix. This follows Darcy’s law, i.e.,
pressure-dependent flow. It is controlled by permeability.
• Turbulent gas flow in horizontal boreholes and vertical wells. This is controlled by the pres-
sure gradient and by borehole/pipeline characteristics.
The net flow of gases is controlled by the first two factors: rate of diffusion and
permeability-controlled flow. The sizes of horizontal boreholes and casings are
designed to be so large that they do not impede the gas flow. Fig. 14.7 illustrates
the flow sequence [1,9].

Figure 14.7 A model of methane flow in coalbeds.