GEOLOGIC CONTROLS ON CO2 SEQUESTRATION AND COALBED METHANE IN INDIANA COALS

Carbon dioxide (CO₂) emissions from stationary sources exceed 66 million tonnes in Indiana each year. In order to reduce the increasing amounts of CO₂ delivered to the atmosphere, capture and permanent storage of carbon dioxide in geological formations is considered as the preferred mechanism. In southwestern Indiana, vast Pennsylvanian coal resources exist at relatively shallow depths. Several coals such as the Springfield and Danville coal members of the Petersburg and Dugger Formations respectively, are being actively mined; but deeper and lower quality beds, i.e. the Seelyville Coal Member of the Linton Formation, as well as associated organic-rich black shales, are considered to be technologically unminable in some areas. These unminable horizons represent potential sites for carbon dioxide sequestration. Additionally, they are potential sources of coalbed methane (CBM), making CO₂ sequestration an even more attractive option as CO₂ displaces methane (CH₄).

More than 20,000 records from oil wells and coal drill holes in southwestern Indiana provide a large database to delineate the best potential coal seams for carbon sequestration. Target coal beds occur at depths between 140 and 275 m (450 - 900 ft), at subcritical fluid conditions for CO₂ sequestration, with hydrostatic pressures ranging from 1.2 to 2.7 MPa (180 to 400 psi). In general, reservoir temperatures vary between 16 and 18ºC (61 - 65ºF), but geothermal anomalies present in some areas show temperature gradients of up to 2.4ºF/100 ft. Accurate determinations of formation temperatures are critical for performing adsorption isotherms, and consequently, for evaluating the potential sequestration capacity in coals. Adsorption isotherms performed on samples from the Seelyville Coal in Indiana indicate CO₂ holding capacities at 16ºC and 2.7MPa (60ºF; 400 psi) between 14 and 20.3 ml/g (450 - 650 scf/ton) and CH₄ capacities between 3 and 7 ml/g (100 - 220 scf /ton) on dry ash-free basis.

Maps of coal thickness and depth indicate that Gibson, Knox, Posey, and Sullivan Counties in Indiana are the best targets for CO₂ sequestration and enhanced coalbed methane recovery. Drilling operations will be conducted in Gibson and Knox counties to help select target pilot areas for both in-situ measurements of CBM and CO₂ sequestration potential of these unminable units.