CO 2 -sequestration in abandoned coal mines Kris Piessens & Michiel Dusar Royal Belgian Institute of Natural Sciences Geological Survey of Belgium In order to reduce the emission of the most important green house gas, CO 2, 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 industrial miningCMMAMMCH 4 CO 2 ? 5 km One of these options is CO 2 -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 CO 2 -sequestration sites?

CH 4 CH 4 -storage at Anderlues overburden: < 10 m total depth: 1100 m P max = 0.35 MPa P hydrostatic >>> P reservoir 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.

CO 2 CH 4 - storage towards CO 2 - sequestration ? overburden: < 10 m total depth: 1100 m P reservoir > P hydrostatic Storing CO 2 instead of CH 4 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 β w28 > 2 MN/m² k < m/s 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 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 CO 2 Beringen Residual space CO 2 in solution P = 130 % P hydro 850 m CO 2 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 CO 2 Beringen Residual space CO 2 in solution

free CO 2 Beringen Residual space CO 2 in solution

free CO 2 Beringen Residual space CO 2 in solution

free CO 2 Beringen Residual space CO 2 in solution Additional residual volume adsorbed CO 2 An increased reservoir volume in response to reservoir pressurisation doubles this amount. CO 2 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 AdditionalAscertained Free:1.7 MTon Solution:0.15 MTon Adsorption:0.8 MTon0.5 MTon 2.7 MTon 2.4 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 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 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 CO 2 is anually produced and vented. These emissions can be stored in the Campine coal mines for at least 15, and possibly 30 years.

CO 2 -sequestration in coal mines: early opportunity ? Advantages: 1. Location: on land and near CO 2 -producers 2. Limited research: seal 3. Environmental: CO 2 -recycling - clean energy: EAMM, ECBM, CO 2 -geothermics - pollution prevention 4. 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 CO 2 CH 4 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 CO 2 CH 4 After injection started, coal bed methane may be gained from the mining influenced zone around the mine. At this stage, the net balance is CO 2 - mitigation, because larg amounts of CO 2 will be injected.

Benefits (CO 2 ) CH 4 When certain pressure conditions are met, the reservoir may be used to produce low-enthalpy geothermal energy using the concept of CO 2 - geothermics. The produced energy is very green because is requires a very limited amount of energy input.

Benefits CO 2 CH 4 CO 2 After thermal depletion and complete filling of the reservoir, true enhanced coalbed methane recovery from around the sequestration sites may become possible.