CO 2 -sequestration in abandoned coal mines

1. 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. CO2-sequestration in abandoned coal mines Kris Piessens & Michiel Dusar Royal Belgian Institute of Natural Sciences Geological Survey of Belgium

2. 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? Southern coal fields in Belgium 1850 1900 1950 2000 industrial mining CMM AMM CH4 CO2 ? 5 km

3. 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. CH4 CH4-storageat Anderlues overburden: < 10 m Pmax = 0.35 MPa Phydrostatic >>> Preservoir total depth: 1100 m

4. 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. CO2 CH4-storagetowards CO2-sequestration ? overburden: < 10 m Preservoir > Phydrostatic total depth: 1100 m

5. 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. Anderlues ? overburden: < 10 m 11 MPa total depth: 1100 m

6. 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. Beringen coal mine: world champion B02-DIN1045 k < 10-9 m/s βw28 > 2 MN/m² 570 - 665 m B15-DIN1045

7. 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. Hydrostatic and geothermal gradient

8. 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 Residual space CO2 in solution Beringen P = 130 % Phydro 850 m

9. free CO2 Residual space CO2 in solution Beringen

10. free CO2 Residual space CO2 in solution Beringen

11. free CO2 Residual space CO2 in solution Beringen

12. 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. free CO2 Residual space CO2 in solution Beringen Additional residual volume adsorbed CO2

13. 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. Additional Ascertained Free: 1.7 MTon 1.7 MTon Solution: 0.15 MTon 0.15 MTon Adsorption: 0.5 MTon 0.8 MTon 2.4 MTon 2.7 MTon Total capacity of Beringen

14. 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. Capacity: reservoir pressure

15. 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. Campine coal field: sequestration capacity

16. 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. CO2-sequestration in coal mines:early opportunity ? Advantages: 1. Location: on land and near CO2-producers 2. Limited research: seal 3. Environmental: CO2-recycling - clean energy: EAMM, ECBM, CO2-geothermics - pollution prevention 4. Economic: (E)AMM, ECBM...

17. 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 CH4 CO2

18. 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

19. 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. CH4 Benefits (CO2)

20. After thermal depletion and complete filling of the reservoir, true enhanced coalbed methane recovery from around the sequestration sites may become possible. Benefits CO2 CH4 CO2