Page 244 - Advanced Mine Ventilation

222 Advanced Mine Ventilation

13.4.3 Gas Wetness Index

It is defined on a ratio of

ðC 2 H 6 þ C 3 H 8 þ C 4 H 10 þ C 5 H 12 Þ
ðCH 4 þ C 2 H 6 þ C 3 H 8 þ C 4 H 10 þ C 5 H 12 Þ
Typically the index is above 3% for thermogenic methane. If less then 3%, the
methane is of biogentic origin.

13.4.4 The d 13 C and dD Isotropic Ratios

Isotopes are different forms of the same element. Carbon, for example, has three natu-
12
14
13
14
rally occurring isotopes: C (carbon-12), C (carbon-13) and C (carbon-14). Cis
radioactive and gives out beta ray that has been used for respirable dust measurement,
but its concentration in coal is low, on the order of 1 10 10 percent in atmospheric
carbon dioxide. Most carbon is bout 99% 12 C and 1% 13 C.
Similarly, there are three different forms of hydrogen. Tritium or hydrogen-3 is
radioactive and is formed in the upper atmosphere by cosmic rays. The stable isotopes
2
1
are H (hydrogen-1) and H (hydrogen-2).
In a compound like methane, there can be three kinds of carbon and three kinds of
hydrogen present. By measuring the relative concentration of these isotopes, we can
characterize CBM from different sources.
Stable isotope compositions are generally expressed using the d (delta) notation.
Thus,
13 12 13 12

13 C= C sa ð C= CÞ st
d C ¼ 1; 000 (13.1)
sa 13 12
ð C= CÞ
st
and
2 1 2 1
ð H= HÞ ð H= HÞ st
sa
dD sa ¼ 2 1 1; 000 (13.2)
ð H= HÞ
st
sadmeans sample of methane.
stdstandard.
For carbon, the standard is Vienna Pee Dee Belemnite.
For hydrogen, the standard is Vienna Standard Mean Ocean Water.
Table 13.7 shows the range of all the above indices for biogenic and thermogenic
methane [5,6].

13.4.5 Differentiating Between Thermogenic Coalbed Methane
and Natural Gas

The geochemical fingerprint of a thermogenic gas is dependent on the type of organic
material from which the gas was originally formed [7]. Most CBM originated from