In the GESTCO project (Tongeren and Laenen, 20012001 - P. C. H. van Tongeren and B. LaenenCoalbed methane potential of the Campine Basin (N. Belgium) and related CO2-sequestration possibilitiessee more; Bergen and Wildenborg, 20022002 - F. van Bergen and T. WildenborgInventory of storage potential of Carboniferous coal layers in the Netherlandssee more; May, 20032003 - F. MayCO2 storage capacity in unminable coal beds in Germany - GESTCO Project reportsee more) and in the EUEuropean Union GeoCapacity project (Wójcicki et al., 20072007 - A. Wójcicki, V. Hladik, V. Kolejka, M. Hamor-Vido, E. Hegedus, C. S. Sava, M. Car, M. Markic, R. Martinez, A. Arenillas, I. Suarez and G. GeorgievAssessment of potential for CO2 usage in coal fieldssee more; Vangkilde-Pedersen et al., 2008) two approaches to storage capacityThe accumulated mass of CO2 that can be stored environmentally safely, i.e., without causing leakage of CO2 or native reservoir fluids or triggering geologic activity that has a negative impact on human health or the environment estimations for coal bed methane fields were presented.
In the first approach, it was assumed adsorptionThe adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface capacity of coal beds, where methane reserves occur, was 33 m3 of CO2Carbon dioxide per tonne of COal (e.g. May, 20032003 - F. MayCO2 storage capacity in unminable coal beds in Germany - GESTCO Project reportsee more). This is the most optimistic estimation of coal bed storage capacityThe accumulated mass of CO2 that can be stored environmentally safely, i.e., without causing leakage of CO2 or native reservoir fluids or triggering geologic activity that has a negative impact on human health or the environment, of minimal ash content and moisture and high methane content.
The second approach takes into consideration CO2Carbon dioxide to CH4 exchange ratio (Bergen and Wildenborg, 20022002 - F. van Bergen and T. WildenborgInventory of storage potential of Carboniferous coal layers in the Netherlandssee more; Vangkilde-Pedersen et al., 2008; Tab. 2-6). The exchange ratio depends mainly on the methane content and CO2Carbon dioxide to CH4 adsorptionThe adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface/desorption potential, which means that usually one molecule of CO2Carbon dioxide can be exchanged for 1.5 to 6 molecules of CH4 depending on pressure, depth (Fig. 2-8) and coal maturity/rank(coal) Quality criterion for coal. The following relationships were used in the EUEuropean Union GeoCapacity assessment and the GESTCO project, in order to calculate the CO2Carbon dioxide to CH4 exchange potential. In most cases, un-mineable coal beds at 1-2 km depth were considered and in some cases injectionThe process of using pressure to force fluids down wells Scenarios were evaluated (practical/matched capacities assessed):
S = PGIP × ρCO2Carbon dioxide × ER
PGIP = Vcoal daf × ρcoal × CCH4 × CF × RF
The symbols are explained in Tab. 2-6, daf stands for dry ash free.
| Tab. 2-6: Parameters used in the static capacity assessment of coal beds (modified after Vangkilde-Pedersen et al., 2008). | Parameter | S | PGIP | rCO2Carbon dioxide | ER | Vcoal daf | rcoal | CCH4 | CF* | RF* | | | Storage(CO2Carbon dioxide) A process for retaining captured CO2Carbon dioxide, so that it does not reach the atmosphereThe layer of gases surrounding the earth; the gases are mainly nitrogen (78%) and oxygen (around 21%) capacityThe accumulated mass of CO2Carbon dioxide that can be stored environmentally safely, i.e., without causing leakage(in CO2Carbon dioxide storage) The escape of injected fluid from the storage formationA body of rock of considerable extent with distinctive characteristics that allow geologists to map, describe, and name it to the atmosphereThe layer of gases surrounding the earth; the gases are mainly nitrogen (78%) and oxygen (around 21%) or water column of CO2Carbon dioxide or native reservoirA subsurface body of rock with sufficient porosityMeasure for the amount of pore spaceSpace between rock or sediment grains that can contain fluids in a rock and permeabilityAbility to flow or transmit fluids through a porous solid such as rock to store and transmit fluids fluids or triggering geologic activity that has a negative impact on human health or the environment (realistic/ effective) | Producible gas in place | CO2Carbon dioxide density at reservoirA subsurface body of rock with sufficient porosityMeasure for the amount of pore spaceSpace between rock or sediment grains that can contain fluids in a rock and permeabilityAbility to flow or transmit fluids through a porous solid such as rock to store and transmit fluids conditions | CO2Carbon dioxide to CH4 exchange ratio | Coal volume dry, ash, free | Average coal bed density | Methane content in pure coal | Completion(well) Refers to the cementing and perforating of casingA pipe which is inserted to stabilise the borehole of a well after it is drilled and stimulation to connect a well bore to reservoir factor, i.e. "working" part of the coal bed) | Recovery factor, i.e. recoverable percentage of gas in place | | Typical values | Mtonnes to tens of Mtonnes (individual fields and small basins), hundreds of Mtonnes (large basins) | Bcms to tens of Bcms | 0.6-0.8 g/ccm (1-2 km depth range) | 1.5-6 (brown coal/lignite - higher than hard coal, pressure increases the ratio) | Millions to billions cubic meters | 0.9-1.4 g/cm3 | 2.5-50 m3/t (most often 5-10 m3/t) | Up to 0.6 (60%) (a vertical wellManmade hole drilled into the earth to produce liquids or gases, or to allow the injectionThe process of using pressure to force fluids down wells of fluids, in a horizontal one possibly higher) | 0.2-0.8 (20-80%) | * If both parameters are 1, this is theoretical capacity; otherwise if values are realistic - effective/realistic capacity is obtained. |
| 
Fig. 2-8: Simulated distribution of CO2Carbon dioxide - CH4 exchange ratio of hard coal beds in Germany (May, 20032003 - F. MayCO2 storage capacity in unminable coal beds in Germany - GESTCO Project reportsee more). |