Page 257 - Advanced Mine Ventilation

234 Advanced Mine Ventilation

Some interesting conclusions can be drawn from data in Table 14.2 and the existing
gas content and reservoir pressures of these coal seams. The Hartshorne and Pocahon-
3
tas coal seams are deep (1500e2500 ft depth). Their gas contents are 550e650 ft /t
and reservoir pressures are 500e650 psi. This indicates that these coal seams are still
near their saturation points and have not lost much gas. They are potentially good re-
serves and will yield high rates of gas production as they have in the field. The Pitts-
burgh and Illinois coal seams, on the other hand, are relatively shallow (at 1000 ft
3
depth). The measured gas content is typically 100e200 ft /t, and the reservoir pressure
is less than 200 psi. This shows that a considerable amount (50%e60%) of their orig-
inal gas content has been lost and, therefore, these reservoirs would be low producers.
These observations are, in fact, confirmed by actual gas production data.

14.2 Coal Matrix Permeability

Permeability is a property of a porous rock such as coal and is a measure of the capac-
ity of the medium to transmit fluids. It depends on the driving pressure differential, the
area of the specimen, and the viscosity of the fluid.
Mathematically, it can be written as

Q k dp
u ¼ ¼ (14.7)
A m dx

2
where u is the average fluid velocity in cm/s; A is the cross-sectional area in cm ;k is
the permeability of the medium in darcy; m is the viscosity of gas/liquid in centipoise;
and dp is the pressure gradient in atm/cm.
dx
A negative sign indicates that fluid flows in the direction of the declining pressure
gradient. As most mineable coal seams are shallow (less than 3000 ft in depth), the
fluid can be assumed to be noncompressible. Integrating Eq. (14.7) for the length of
the specimen, L,

Q Z L k Z P 1
dx ¼ dp (14.8)
A o m P 2

or

kA
Q ¼ ðP 1 P 2 Þ (14.9)
mL
In an experiment to measure k, all the parameters in Eq. (14.9) are known, and
hence permeability can be easily determined.
A cube of coal 1 cm on a side will have a permeability of 1 darcy, if a fluid of 1 cp
viscosity flows between the back and front faces of the cube at a rate of 1 cc/s under a

pressure differential of 1 atm at 68 F. Converted to SI units, 1 darcy is equivalent to