Page 256 - Advanced Mine Ventilation

Reservoir Properties of Coal Seams 233

Figure 14.4 A plot of P/V against P.

The term “b” is experimentally found to be equal to 1/P L , where P L is the charac-
teristic pressure that corresponds with V m /2 on the gas isotherm.
Thus, Eq. (14.5) can be written as

P P L P
¼ þ (14.6)
V V m V m
If the isotherms shown in Fig. 14.3 are replotted with P/V on the y axis and P on the
x axis, a straight line is obtained. The plot for the Pocahontas #3 seam is shown in
Fig. 14.4. The slope of the line is 1/V m , from which V m can be determined. The inter-
cept on y axis is P L /V m , from which P L can be determined.
Calculated values of P L and V L for all gas isotherms shown in Fig. 14.3 are shown
in Table 14.2.

Table 14.2 Calculated V L and P L Values for US Coal Seams
3
Coal Seam V L (ft /t) P L (psi)
Hartshorne 788 205
Pocahontas #3 646 158
Pittsburgh 443 170
Castlegate 409 229
Illinois #6 353 273