Page 353 - Advanced Mine Ventilation

330 Advanced Mine Ventilation

Using Eq. (20.2)

2 2
707:8 ð0.015Þ 40 500 50
q ¼
1; 000
0:02 520 ln
0:25
¼ 1.22 MMCFD

Let us drill 84 wells at 80-acre spacing (2.78 BCF per well) to have an initial pro-
duction of 102.48 MMCFD. CBM wells continue to produce for a long time. A life of
30 plus years is not uncommon in a reserve that will not be mined. Hence, let us as-
sume an annual decline of 3%.
Table 20.1 shows the production rate and cumulative production for the first 5 years
only. This is a typical life of a well in a working mine.

20.4.2.2 Horizontal Well Production Forecast
Steady-state gas production from horizontal wells will be discussed by the author in a
book to be published in 2019 in great detail [5]. For the current project, let us assume
the coal seams of the project have a specific gas production of 15 MCFD/100 ft [4].
Hence, we need to drill 17 wells with two laterals of 5000 ft in four coal seams with
an average thickness of 10 ft each.
Each lateral of 5000 ft will produce 750 MCFD and hence each well will produce 6
MMCFD. With 17 wells, the total production will be 102 MMCFD. The gas produc-
tion rate and cumulative gas production per year will be much the same as shown in
Table 20.1.
It is deliberately kept close for a fair comparison of the two techniques.

20.4.3 Gas Gathering and Processing

The CBM as produced cannot be sold without processing owing to the presence of im-
purities. Table 20.2 shows the typical composition of CBM. Gas pipeline (buyers)

Table 20.1 Gas Production in Five Years of the Project

Year of Production Gas Rate (MMCF/Year) Cumulative Production (MMCF)
1 36,893 36,893
2 35,786 72,679
3 34,712 107,391
4 33,671 141,062
5 32,661 173,723