Page 250 - Advanced Mine Ventilation

228 Advanced Mine Ventilation

smaller than the micropores in the coal matrix. At greater depth, the gas may be in a
“condensed, liquid-like state” [2]. The volume of gas retained in coal is dependent on
the rank, temperature and pressure, and the depth of the coal seam. The microscopic
surface of coal is large; a ton of coal has a surface area of approximately 2218 million
2
2
ft (200 Mm ). Thus, one cubic foot of coal can store two to three times the amount of
gas contained in a typical sandstone reservoir for natural gas of the same volume but at
higher pressure.
Gas content measurement methods are classified as (1) conventional and (2) pres-
surized desorption techniques. In the conventional technique, coal cores or drill cut-
tings are retrieved from the core holes and immediately put in a sealed container to
measure the desorbed gas. This method suffers from uncertainty in the estimate of
gas lost during sample retrieval and handling. To eliminate this problem, the pressur-
ized core desorption technique has been developed. In this technique, gas loss is mini-
mized by sealing the coal samples while they are in the core hole. Both methods
provide positive proof of gas presence. Desorbed gases are chemically analyzed to
determine the composition and calorific value of coalbed methane.

14.1.1 The Direct Method of Gas Content Measurement

This technique was originally developed by Bertard and Kissell [3,4]. It was further
improved by Diamond and Schatzel [5] and it became the “ASTM standard practice
for determination of gas content of coal” [6]. In this technique, the desorbed gas
from the coal sample is measured first. Next, the cumulative gas production is plotted
against the square root of time to determine the lost gas. Finally, a small, weighted
portion of coal sample is crushed in a hermetically sealed mill to get the residual
gas. The total gas content is the sum of the three components: (1) desorbed gas, (2)
estimated lost gas, and (3) residual gas.

14.1.2 Desorbed Gas
After coal cores or drill cuttings are put in a hermetically sealed container, called a
desorption canister, the desorbed gas is measured periodically. In the first few days,
readings may be taken every hour, but later a measurement once a day is sufficient.
The general layout of the experimental setup is as shown in Fig. 14.1 [5].
The desorption canister is about 18 in. tall, with a 4 in. internal diameter. It is equip-
ped with a pressure gage and a valve to let the desorbed gas out. The desorbed gas is
measured by water displacement in a graduated glass cylinder 4 in. in diameter and
12 in. high. The glass cylinder is connected to a leveling water reservoir, and the
gas volume measurement is taken when the water levels in the cylinder and leveling
reservoir are the same. The precision of the measurement is about 4% [7].
The desorption process typically extends to 4e6 weeks. It is stopped when gas
3
desorption is less than 10 cm /day. The cumulative gas production is plotted on a
0.5
graph paper against (time) to determine the lost gas component of the total gas con-
tent (discussed in the next paragraph). The desorbed gas is periodically analyzed using
a gas chromatograph to determine its composition and calorific value.