CO2-sequestration in abandoned coal mines

Kris Piessens & Michiel Dusar

Royal Belgian Institute of Natural SciencesGeological Survey of Belgium

In order to reduce the emission of the most important green house gas, CO2, one has several options to meet the Kyoto targets. If we want to stabilise the temperature increase, deep cuts have to be made of about 50 % or more. The

number of options that allow such reductions is limited.

Southern coal fields in Belgium

1850 1900 1950 2000

industrial mining CMM AMM CH4 CO2 ?

5 km

One of these options is CO2-sequestration, but the subsurface of a country such as Belgium

lacks large aquifers. Can the former coal mines, that were used for storage of natural gas, be

converted into CO2-sequestration sites?

CH4

CH4-storage at Anderlues

overburden: < 10 m

total depth: 1100 m

Pmax = 0.35 MPa

Phydrostatic >>> Preservoir

One of the former storage sites (injection was stopped in 2000) is the Anderlues colliery. It is a deep mine, up to 1100 m, but with a very shallow top (10 m). Therefore the reservor

pressure was kept low. To keep the mine from flooding (resulting in loss of reservoir

volume), it has to be kept dry by pumping. In the case of Anderlues it was not the mine itself that was pumped, but the adjacent aquifers.

CO2

CH4-storage towards CO2-sequestration ?

overburden: < 10 m

total depth: 1100 m

Preservoir > Phydrostatic

Storing CO2 instead of CH4 does not pose problems, but sequestrating it does. It is clear that it is not possible to keep the mine dry by pumping for a thousand years or longer, which is

the duration of a sequestration project.

One way to by-pass this problem is to raise the reservoir pressure until it is as high as in the host rock. From this moment on, formation water

can not longer enter the reservoir.

Anderlues ?

total depth: 1100 m

overburden: < 10 m

11 MPa

For Anderlues this means that the reservoir pressure has to be raised to close to 11 MPa. In view of the very shallow seal of Anderlues, this

is impossible. Anderlues is therefor not a suited sequestration site.

Beringen coal mine: world champion

B15-DIN1045

B02-DIN1045

βw28 > 2 MN/m²k < 10-9 m/s

570 - 665 m

Anderlues is a very peculiar reservoir because of its very shallow seal. The mines in the

northern (Campine) coal field are also exceptional, this time because they are very well sealed, down into the coal mine. Note the concrete plug of 100 to 200 m thick, and the clay plugs at levels of impermeable strata.

Hydrostatic and geothermal gradient

hydrostaticgradient

CO2-staticgradient

B

A

depth

pressure

C

H O CO in solution

hydrostatic pressure

2

2

COoverpressure

2

A

B

C

It is very important that the mines are sealed down to the exploitation levels, because the most critical point (the highest amount of overpressure) is the shallowest point of the

reservoir.

free CO2

Beringen

Residualspace

CO2 in solution

P = 130 % Phydro

850 m

CO2 is stored in different ways in a coal mine. For Beringen the free-space storage, and to a minor degree also solution storage, amounts to

about one-third of the total capacity.

free CO2

Beringen

Residualspace

CO2 in solution

free CO2

Beringen

Residualspace

CO2 in solution

free CO2

Beringen

Residualspace

CO2 in solution

free CO2

Beringen

Residualspace

CO2 in solution

Additionalresidualvolume

adsorbed CO2

An increased reservoir volume in response to reservoir pressurisation doubles this amount. CO2 will also adsorb onto coal, but for Beringen the

largest part of these reserves may not participate in this process as they are located

at deeper (unmined) levels.

Total capacity of Beringen

Additional Ascertained

Free: 1.7 MTon1.7 MTon

Solution: 0.15 MTon0.15 MTon

Adsorption: 0.8 MTon0.5 MTon

2.7 MTon2.4 MTon

The ascertained capacity refers to the capacity calculations for which all parameters are

suffiently known to allow error estimates. This is a very conservative estimate, as the real

capacity may well be twice as much.

Capacity: reservoir pressure

0 kg

1 10^ 9 kg

2 10^ 9 kg

3 10^ 9 kg

4 10^ 9 kg

5 10^ 9 kg

6 10^ 9 kg

7 10^ 9 kg

8 10^ 9 kg

9 10^ 9 kg

10 10^ 9 kg

1 050.0 1 000.0 950.0 m 900.0 m 850.0 m 800.0 m 750.0 m 700.0 m 650.0 m 600.0 m 550.0 m

0

5

10

15

20

25

30

35

40

45

Additional adsorption storage potential:

Additional solution storage potential:

Additional free-space potential:

Adsorption storage potential:

Solution storage capacity mine water:

Free-space storage capacity coal mine:

Overpressure at 550 m

The total capacity relies heavily on the reservoir pressure, but as shown by this graph,

the relation is not linear due to the non-linearity of underlying equations. The current evaluation assumes that an overpressure of 30 % is realistic, but this remains to be verified.

Campine coal field: sequestration capacity

MTonm

mMTon

V

VMTon

Beringen

collieriesall 15³9.5

³3.327.27.2

MTonm

mMTon

V

VMTon

Beringen

collieriesall 27³9.5

³3.3255

The capacity of Beringen can be extrapolated to the total capacity of the seven abandoned Campine coal mines. In the industrial area around Antwerp, about 1 MTon of pure CO2 is

anually produced and vented. These emissions can be stored in the Campine coal mines for at least

15, and possibly 30 years.

CO2-sequestration in coal mines:early opportunity ?

Advantages:

1. Location: on land and near CO2-producers2. Limited research: seal3. Environmental: CO2-recycling

- clean energy: EAMM, ECBM, CO2-geothermics

- pollution prevention4. Economic: (E)AMM, ECBM...

The total capacity of coal mine reservoirs is relatively small compared to e.g. aquifer

storage, but it is significant. The smaller size and on-land location may even be benefits for a

pilot or early-opportunity projects.

Benefits

CO2

CH4

There are also other benefits, both economic and environmental. Currently the abandoned mines are

reservoirs for high-quality mine gas. The necessary extraction of this gas can be

developed as an economic activity.

Benefits

CO2

CH4

After injection started, coal bed methane may be gained from the mining influenced zone around

the mine. At this stage, the net balance is CO2-mitigation, because larg amounts of CO2 will be

injected.

Benefits

(CO2)

CH4

When certain pressure conditions are met, the reservoir may be used to produce low-enthalpy geothermal energy using the concept of CO2-

geothermics. The produced energy is very green because is requires a very limited amount of

energy input.

Benefits

CO2

CH4CO2

After thermal depletion and complete filling of the reservoir, true enhanced coalbed methane

recovery from around the sequestration sites may become possible.