Page 280 - Advanced Mine Ventilation

Premining Degasification of Coal Seams 257

Table 15.2 Specific Gas Emissions for Coal Seams

Specific Gas
Production
Coal Seam Depth (ft) Rank (MCFD/100 ft)
Pittsburgh 500-1000 High Vol. Bituminous 15.00
Pocahontas No. 3 1400e2000 Low Vol. Bituminous 8.00
Blue Creek/Mary Lee 1400e2000 Low Vol. Bituminous 9.00
Pocahontas No. 4 800e1200 Medium Vol. Bituminous 5.00
Sunnyside 1400e2000 High Vol. Bituminous 9.00

The specific production goes down with time and is about 3e4 MCFD/100 ft at the
time of plugging just prior to mining through the wells.
Thakur [1] also derived mathematical expressions for gas flow from horizontal
boreholes. It is given by Eq. (15.2).

Q ¼ At n (15.2)

where Q is the cumulative production in MCF; A is the initial production in MCFD
(months); t is the time in day (months); n is a characteristic of the coal seam.
Eq. (15.2) can also be written as follows:

lnQ ¼ lnA þ nlnt (15.3)

A plot of ln Q against ln t yields a straight line. The characteristic “n” varies from
0.8 to 1.00.

15.2.2.2 Gas Production From Vertical Wells
Production from a vertical well can be estimated by Eq. (15.2) but another estimate is
provided by Eq. (15.4).

2 2
707:8kh p p w
e
q ¼ (15.4)
m z Tlnðr e = r w Þ

Where q ¼ cubic ft/day at 60 F and 14.67 psia; k ¼ permeability in darcy;
h ¼ thickness in ft; p e ¼ pressure at external radius, r e ;p w ¼ pressure at the well
radius, r w ; m ¼ average viscosity; z ¼ average compressibility factor; T ¼ temperature
in degree Rankine (Fahrenheit þ 460).