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

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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)

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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

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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)

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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