Determination of Coal Permeability Using Pressure Transient Methods
Experimental Setup
Robert McLendon, Hema Siriwardane1, Igor Haljasmaa, Grant Bromhal, Yee Soong, and Gino Irdi
National Energy Technology Laboratory (NETL)
1WVU Dept. of Civil and Environmental Engineering
Experimental Conditions
Upstream
Abstract
p (t )
Coalbed methane is a significant natural resource in the Appalachian region. It is believed that coalbed
methane production can be enhanced by injection of carbon dioxide into coalbeds. However, the influence
of carbon dioxide injection on coal permeability is not yet well understood. Competitive sorption of carbon
dioxide and methane gases onto coal is a known process. Laboratory experiments and limited field experience
indicate that coal will swell during sorption of a gas and shrink during desorption of a gas. The swelling and
shrinkage may change the permeability of the coal. In this study, the permeability of coal was determined by
using carbon dioxide as the flowing fluid. Coal samples with different dimensions were prepared for laboratory
permeability tests. Carbon dioxide was injected into the coal and the permeability was determined by using
pressure transient methods. The confining pressure was variedto cover a wide range of depths. The
permeability was also determined as a function of exposure time of carbon dioxide while the confining stress
was kept constant. CT scans were taken before and after the introduction of carbon dioxide. Results show
that the porosity and permeability of the coal matrix was very low. The paper presents experimental data and
theoretical aspects of the flow of carbon dioxide through a coal sample during pressure transient tests. The
suitability of the pressure transient methods for determining permeability of coal during carbon dioxide
injection is discussed in the paper.
0
Coal Sample
Downstream
Fixed Volume
∂p
⎛ k ⎞ ∂2 p
⎜ ⎟ 2 = (α + φβ )
∂t
⎝η ⎠ ∂x
∂p Q
=
∂t βV
• Fractured coal samples were
used to study the swelling
behavior of coal
• Confining pressure up to
50 MPa was used
Results
p(0, t ) = p (t )
• Permeability of coal sample used in the
experimental program is very low
(less than 0.001 mD)
0
k = Permeability
α = Pore volume compressibility
β = Fluid compressibility
φ = Porosity
η = Viscosity
V = Fixed Volume (down stream)
Q = Flow Rate
• Permeability decreased with
sorption of carbon dioxide
indicating swelling of coal matrix
Test #S02, Event #151
11
Objectives
P upstream
10.8
P downstream
10.6
• Determine permeability changes
caused by sorption of CO2
Pressure (MPa)
• Gain basic knowledge concerning CO2
sequestration in geologic formation
Jacketed
Sample
10.4
10.2
10
9.8
9.6
9.4
9.2
9
0
5
10
15
20
Time (sec)
25
30
35
Pressure Pulse Used in the Test
Temp.
controlled
Core holders
11
10.5
∂ P
2 ∂P
=a
2
∂x
∂t
2
9
Permeability of CO2 in coal with induced crack
4
Initial and Boundary Conditions
P(0, t ) = P0 (t)
For t > 0 and x = L
∂P( L, t ) Q
=
βV
∂t
P upstream
P downstream
8
0
aspike liqCO2
sine6 liqCO2
aspike gasCO2
sine6 gasCO2
3
Permeability, mD
For t > 0 and x = 0
10
9.5
8.5
3.5
P (0, 0) = Pi
P (x, 0) = P0
Test #S02, Event #152
11.5
Pressure (MPa)
Mathematical
Formulation
Photo-micrograph of
a Coal Sample
20
40
60
80
100
120
Time (sec)
Sine Wave Pressure Pulse
Fractured Coal Sample
2.5
Conclusions
2
1.5
CT Scan of a Fractured
Coal Sample
1
0.5
• Permeability of coal appears to decrease with Carbon Dioxide exposure time
• Pressure transient methods provide satisfactory results when carbon dioxide
is used with coal samples
0
0
20
40
60
80
100
120
140
Exposure time in hours
160
180
200
220
• Decrease in fracture permeability is an indication of coal swelling as a result
of sorption of carbon dioxide on coal