A preliminary evaluation of the CO2 storage potential in unminable coal seams of the Münster Cretaceous Basin, Germany

The Münster Cretaceous Basin is a synclinal structure containing Upper Cretaceous sediments of local thickness exceeding 1700 m, partially covered by Cenozoic deposits. The underlying basement consists of deep Carboniferous strata with interbedded hard coal seams. The Cretaceous sequence can be subdivided into the Cenomanian-Turonian-Complex, representing a brine aquifer, and the overlying 'Emscher Mergel' (clayey marl), separating the brine from shallow freshwater aquifers. This geological setting potentially offers favourable conditions for geological storage of CO₂ by physical sorption in the coal seams below the 'Emscher Mergel' cap rock formation. This storage option could be combined with the production of coalbed methane (CBM), by means of application of a CO₂-enhanced coalbed methane (ECBM) strategy. A potential storage area of 820 km² has been identified by considering local geological, hydrogeological and tectonic settings. This regional study was complemented by the analysis of carbon isotope signatures of CH₄ and CO₂ in shallow aquifers and concentration profiles of methane-in-place. Using the Coal Inventory Calculation Model (KVB-model) of the Geological Survey (GD) of North Rhine-Westphalia, Germany, coal volumes within the study area have been estimated to amount to 37.5 × 10⁹ metric tons. High-pressure CO₂ sorption isotherms measured on a selected coal sample at different temperatures were used to estimate the sorptive storage capacity of this coal at the pressure and temperature conditions in the seams. Assuming an accessibility of 40% of the total coal as well as a methane-in-place recovery efficiency of 80%, a maximum CO₂ storage potential of 160 million metric tons of CO₂ was estimated for the coal seams down to a depth of 3000 m. However, the application of ECBM is restricted due to the low permeabilities and large depth of the coal deposits.