Page 251 - Advanced Mine Ventilation

Reservoir Properties of Coal Seams 229

Figure 14.1 Gas content measuring apparatus.

14.1.3 Lost Gas
This portion of the total gas content is the gas that escapes from the sample during its
collection and retrieval, prior to being sealed in an airtight canister. It is estimated indi-
rectly. Most gas desorption processes from coal or shale follow a power law [8,9].
n
Q ¼ At (14.1)
3
where Q is the cumulative volume of gas desorbed in ft ; A is a characteristic of the
coal (equals initial production in gas wells); t is time in days or minutes; and n is a
characteristic of the coal or shale.
Eq. (14.1) can be expressed in its logarithmic form as

ln Q ¼ ln A þ n ln t (14.2)

The value of “n” for most coal is 0.8e1.00. Hence, a plot of ln Q against ln t yields a
straight line. The intercept on the “y” axis is equal to ln A.
In a simplified version of Eq. (14.1), (refer to Equation (14.16))

1
Q ¼ Bt 2 (14.3)
Hence, a plot of cumulative desorbed gas, Q, against (t) 0.5 yields a straight line. Here,
B is the intercept on the y axis and is a measure of the lost gas as shown in Fig. 14.2.

14.1.4 Residual Gas
Even when the coal sample in the desorption container has stopped producing gas, a
significant volume of gas is still left in the sample. It can only be retrieved and
measured by crushing the sample to very fine sizes.