Enhanced mine gas drainage
Luke Connell
Advanced Coal Technology, CSIRO
27 June 2012
Nature of gas storage in coal
• Coal physical structure
• Has a dual porosity• Macropores – the cleat system
• Micropores – the coal matrix
• Most of the porosity and surface area is in the matrix (eg 85%)
• Cleat systems
• Two main sets - orthogonal • Face cleats
• Butt cleats
• perpendicular to bedding plane
matrix
Plan view
Laubach et al 1998
Cross – section
Total porosity is low
Majority of gas is adsorbed to solid structure
Most surface area is within the matrix
Gas adsorption in coal
• Quantity stored is a function of pressure, temperature
• Coal adsorbs more CO2 than CH4, capacity for nitrogen is low
• typically 2 molecules of CO2 for each CH4 molecule
• 4 CO2 or 2 CH4 for every N2
0
5
10
15
20
25
30
0 1000 2000 3000 4000 5000 6000
Pressure (kPa)
Ga
s c
on
ten
t (m
3/t
on
ne
)
CO2
CH4
N2
Bulli seam
Initial state – pressure
maintains a certain mass of
gas adsorbed
Fluid pressure lowered in cleat/
fracture system – pressure difference
between cleat and matrix
gas adsorbed
within matrix
water filled cleats
pressure difference between
cleat - matrix
Pressure lowered, gas desorbs and diffuses
through matrix to cleat – water and gas
flow within cleats
matrix
Primary coal seam gas recovery
Two-phase flow system around the well
• Keffective = Krelative . K
Gas drainage effectiveness
• Drainage
• a complex function of borehole spacing, gas flow, gas desorption with pressure
• Key reservoir properties - permeability, adsorption isotherm
• Limited by the ability to lower the pore pressure
Pressure drawdown and gas desorption
• Drawing the reservoir
pressure down from
4 MPa to 1 MPa
• CH4 14 m3/t to
8.8m3/t
• CO2 25 m3/t to 19
m3/t – much lower
pressures required
0
5
10
15
20
25
30
0 1000 2000 3000 4000 5000 6000
Pressure (kPa)
Gas c
onte
nt
(m3/t
onne)
CO2
CH4
N2
Limited by the ability to lower the pore pressure within the
coal
• largest changes in gas content occur at low
pressure
Enhanced coal seam gas recovery
• Primary coal seam gas drainage
• Pressure drawdown and gas desorption
• Limited by the ability to reduce the reservoir pressure
• Drainage a function of well spacing, reservoir properties and
drainage lead time
• Enhanced recovery using gas injection
• A contrasting gas (i.e. not the coal seam gas) is injected into the
coal seam and this acts to displace the coal seam gas
Initial state – pore fluid pressure
maintains a certain mass of
gas adsorbed
Methane partial pressure lowered in cleat/
fracture system by injection of contrasting gas,
for example nitrogen
Methane adsorbed
within matrix
water filled cleats
Gas partial pressure difference between
cleat - matrix
Methane pressure lowered, gas desorbs and diffuses
through matrix to cleat – water and gas
flow within cleats
matrix
Enhanced recovery process
Example: Enhanced gas drainage using nitrogen
Adsorbed gas content Gas phase composition
Methane
Nitrogen
Methane
Nitrogen
Methane gas drainage rate
Enhanced rate
Primary
Simulation results calculated with SIMEDWin
Enhanced coal seam gas drainage
• Potential advantages• Since ECBM relies on gas partial pressure difference to displace gas in
place• Ultimate drainage can be much higher than primary recovery
• Also the injected gas acts to maintain the reservoir pressure and increase gas drainage rates
• Injecting a weakly adsorbing gas (i.e. nitrogen) will increase the permeability through coal shrinkage with decreased total gas content
• Candidate gases for injection• Weakly adsorbing gas – nitrogen
• Gas mixtures – nitrogen & carbon dioxide - for open cut purposes
• Pure or high percentage CO2 not appropriate – much higher gas contents than in place methane, well known problematic gas for mine drainage, associated with low permeability, lower outburst threshold
• Additional costs of ECBM• Well costs (dedicated injection well)
• Sourcing the injection gases and their compression/injection
Enhanced recovery field trials
• N2 injection
• Tiffany trial San Juan Basin
• 1998-2002 (intermittent injection)
• There was an 5x increase in the methane gas rate in response to
N2 injection – due to combined effects of methane displacement,
pressure maintenance and permeability enhancement
Enhance recovery field trials
• Yubari trial – JCOAL
• Vertical injection and production wells 66m separation in target
seam at 900m depth
• Short period of N2 injection after longer duration CO2 injection
• Also N2 breakthrough at production well
0.0
100.0
200.0
300.0
400.0
500.0
600.0
700.0
800.0
8-May-06 13-May-06 18-May-06 23-May-06 28-May-06
Time
Ga
s r
ate
(m
3/d
ay)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
Inje
ctio
n r
ate
(to
nn
es/d
ay)
Gas production
Nitrogen injection
Gas injection and production rates during N2 injection
0.01
0.1
1
10
100
0 2 4 6 8 10 12 14 16 18
Reservoir pressure (MPa)
k / k
0
100% CO2
98% CH4 , 2% CO2
100% N2
0.01
0.1
1
10
100
0 2 4 6 8 10 12 14 16 18
Reservoir pressure (MPa)
k / k
0
100% CO2
98% CH4 , 2% CO2
100% N2
Nitrogen enhanced mine drainage
• Russell Packham as part of PhD at
UNSW
• Bowen basin coal mine
• Surface to inseam wells
• An existing nitrogen membrane plant
used for goaf inertisation was available
for periods within the year
• Injection into one horizontal well while
production maintained in neighbouring
wells
• Virgin gas content ~7m3/t
See Packham et al 2011 Int. J. Coal Geol. and Packham et al. 2012 in press
Injection well
Gas drainage rates
Gas drainage predictions from modelling
Packham trial
Primary
N2 constant
N2 start
N2 start
N2 6 monthly
Start of primary recovery
Enhanced drainage and permeability
• Series of reservoir simulations
comparing primary and
enhanced drainage
• 100m spaced horizontal wells
• 600m deep seam at
hydrostatic pressure and gas
saturated
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600 700 800
Frac
tio
n o
f in
itia
l gas
in p
lace
Time (days)
Drainage of CH4 rich coals
Enhanced drainage
Primary
0.5 md
0.25 md
Start N2 injection
Producer
Injector
Producer
Injector
Initial gas content 50:50 CH4:CO2
SIMEDWin
Time DAYS
700650600550500450400350300250200150100500
GA
S P
ER
ME
AB
ILIT
Y, X
-WA
RD
. (m
D)
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600 700 800
Frac
tio
n o
f in
itia
l gas
in p
lace
Time (days)
Drainage of C02 rich coals
Primary
Enhanced
0.5 md
0.25 md
Start N2 injection
Matrix shrinkage increasing permeability
CSIRO. Capturing fugitive emissions from open cut
Enhanced mine drainage for reducing open
cut fugitives
• 130m deep seam with reservoir properties from Hunter Valley coal sample; produced gas is used in a power station
• Differential net present value after 4 years between the no drainage case and enhanced drainage or no drainage and primary drainage
• For the no drainage case the seam gases become fugitive and incur an emissions penalty
• There is a positive business case for enhanced drainage above an emissions penalty of $20/tonne CO2e.
• In contrast primary drainage never reaches breakeven and so is not feasible compared to allowing the gases to become fugitive
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
0 10 20 30 40 50 60
CO2 Penalty, $/t
Dif
f N
PV
@ $
60/M
Wh
, $M
M/M
Mt
FG150 FG200 FG250 FG300
PRI 150 PRI 200 PRI 250 PRI 300
From ACARP C17055
Conclusions
• Enhanced drainage acts to maintain gas drainage rates
and increase the proportion of gas recovered
• Nitrogen is a lower adsorbing gas than CH4 and CO2
• Enhanced drainage with nitrogen also would increase the
permeability through coal shrinkage
• Coals rich in CO2 may have the greatest benefit because
of the low pressure drawdowns required to meet safe
mining thresholds
• Initial reservoir permeability will still play an important
role
• Trials are needed to evaluate this process
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