Page 258 - Advanced Mine Ventilation

Reservoir Properties of Coal Seams 235

13 2 2
9.869233 10 m or roughly 1 mm . As a darcy is a very large unit, the perme-
ability is mostly expressed in 1/1000 of a darcy or millidarcy (md), and it has a dimen-
2
sion of L .
The above equation is valid for liquids. For gases, the volume q is introduced as
defined by Eq. (14.10):

q ¼ Q, P 1 þ P 2 (14.10)
2P b

Substituting in Eq. (14.9) and expressing K in md, the equation for gas flow can be
written as

2000 qL mP b
k ¼ (14.11)
2
A P P 2
1 2
3
where k ¼ permeability in millidarcy; q ¼ gas flow rate in cm /s; L ¼ length of the
specimen in cm; m ¼ gas viscosity in centipoise; P ¼ absolute pressure in atm;
subscript 1 ¼ upstream core; subscript 2 ¼ downstream core; b ¼ base pressure of gas
measurement.
For example, let us assume the following:
3
q ¼ 2cm /s
P 1 ¼ 2 atm
P 2 ¼ 1 atm
L ¼ 2 cm; A ¼ 3cm 2
P b ¼ 1.00 atm

m ¼ 0.018 cp at 68 F

2000 2 0.018 1:0
k ¼ 1 ¼ 8md
3ð4 1Þ 1:0
14.2.1 Measurement of Permeability

There are numerous, theoretical, laboratory-based, and field techniques available for
the measurement of coal matrix permeability. Reliable data are obtained by only field
measurements.
Field measurements of permeability can be done in a variety of ways as listed
below:
1. Drill stem testing.
2. Slug testing.
3. Injection fall-off testing.
4. Pressure-buildup and drawdown testing.
5. Minifrac(ture) of the coal seam.

The first three tests are described by Rogers et al. [12], and the pressure build up and
draw down are explained byThakur in detail [1]. The minifrac technique is described here