Page 352 - Advanced Mine Ventilation

Economics of Coal Mine Degasification 329

GIP ¼ A H r Gc (20.1)

where A is the area of the lease in acres, H is the thickness of the coal seams that will be
produced, r is the coal density in tons/acre-ft, and Gc is the gas content of coal seams
3
(ft /ton).
Each coal seam in the lease is evaluated separately, and a composite gas isopach
map is prepared that shows the gas content contours of the lease in MMCF/acre or
BCF/section. A section is equal to one square mile (640 acres). Areas with higher
values of BCF/acre are generally very profitable to produce.

20.4.2 Production Techniques and Gas Production Forecast
At present, there are only two techniques for commercial CBM production:

1 Vertical wells with multiple completions.
2 Horizontal wells drilled from surface into multiple coal seams.
In active coal mines, there is a third source of gas that is gas recovered from long-
wall gobs. It can range from 5 to 10 MMCFD in moderately gassy coal fields to 20 to
30 MMCFD in highly gassy coal fields. This is almost “free” gas because the cost of
production is very small compared with the cost of production from virgin coal.
Thakur [4] has discussed these techniques in great detail in a recent publication.
Reference should be made to the original work for greater details.

20.4.2.1 Vertical Well Production Forecast
The steady-state production from a vertical well is estimated from Eq. (20.2) [4].

2 2
707:8kh P P w
e
q ¼ mzT ln r e= r w (20.2)

where q ¼ cubic feet/day at 60 F and 14.67 psia, k ¼ permeability in darcy,
h ¼ thickness of each coal seam in feet, P e ¼ pressure at external radius (r e ),
P w ¼ pressure at the well radius (r w ), m ¼ gas viscosity, z ¼ compressibility factor
(can be assumed 1.0 to a mineable depth of 2500 ft), and T ¼ temperature of coal seam
in degrees Rankine (Fahrenheit þ 460).
For example, let us assume the following:
k ¼ 0.015 darcy (15 md)
h ¼ 40 ft
m ¼ 0.2 cp
z ¼ 1.0
T ¼ 520 R
r e ¼ 1000 ft (length of the fracture created)
r w ¼ 0.25 ft
P e ¼ 500 psi
P w ¼ 50 psi