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Pashin, Jack C.1, Richard E. Carroll1
(1) Geological Survey of Alabama, Tuscaloosa, AL

ABSTRACT: Sorption Capacity of Coal in the Black Warrior Basin of Alabama: Implications for Carbon Sequestration and Enhanced Coalbed Methane Recovery

Sorption isotherms for CO2 and CH4 from 30 coal samples in the Black Warrior basin establish that the sorption capacity of bituminous coal can vary greatly and is influenced by coal quality, reservoir temperature, and reservoir pressure. Ash content and coal rank are major controls on sorption capacity, whereas maceral content is a subordinate control. Sorption capacity generally decreases as ash content increases, although samples containing less than 5% ash have diminished sorption capacity. Sorption capacity increases with coal rank, but CO2 capacity is less strongly correlated with rank than is CH4 capacity. Ash content is highly variable and is difficult to interpolate between control points, whereas rank parameters can be interpolated with higher confidence. Therefore, coal rank is a critical parameter for regional assessment of sequestration potential. 

Sorption capacity increases exponentially as reservoir temperature decreases, and reservoir temperature in the Black Warrior basin generally ranges from 75 to 125°F. Across this temperature range, gas capacity can change by more than 25 percent, and mapping reservoir temperature increases certainty when assessing the potential for sequestration and enhanced recovery. Reservoir pressure in mature coalbed methane reservoirs of the Black Warrior basin ranges from about 50 to 350 psi. At 100 psi, the coal samples stored an average of 361 scf/t of CO2 and 128 scf/t of CH4. These results indicate that a large quantity of CO2 can be sequestered at modest pressure in mature coalbed methane reservoirs and that a significant coalbed methane resource remains untapped. 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.