Kinetics of Pure and Mixed CO₂-CH₄ Gas Adsorption on Crushed and Sieved Coal from the Black Warrior Basin, West-central Alabama

Blencoe, James G.¹, Michael T. Naney², David R. Cole³, Jack Pashin⁴, Richard E. Carroll⁴ (1) Oak Ridge National Laboratory, Oak Ridge, TN (2) Electrochemical Systems, Inc, Oak Ridge, TN (3) Oakridge National Lab, Oakridge, TN (4) Geological Survey of Alabama, Tuscaloosa, AL

A 60-gram sample of lightly crushed and seived Black Warrior Basin, Alabama coal (1.2-2.0 mm size fraction) was contacted with pure and mixed gases—CO2, CH4, a CO2-CH4 mixture (~50 mole % CO2), H2O, and He—at 35 and 40°C, 324 < P(psi) < 497, to determine the rates and magnitudes of gas adsorption. The sample was placed in a 1.2 liter autoclave suspended in a 0.5 m3, cylindrical infrared oven. Temperature and pressure were measured with ultra-high precision (T, ± 0.01°C; P, ± 0.05 psi) and accuracy (T, ± 0.05°C; P, ± 0.1 psi) using high-quality thermistors and pressure transducers.

Results of the experiments are very systematic, indicating significantly different CO2, CH4, and CO2-CH4 gas adsorption behavior for “dry” and water-saturated conditions. Key conclusions that can be drawn from the work are: (1) Differential CO2/CH4 adsorption kinetics will play a key role in CO2 sequestration/ECBM production in subterranean coalbeds. (2) CO2 adsorption on dry and water-saturated coal is much more rapid than CH4 adsorption. (3) Water saturation decreases the rates of CO2/CH4 adsorption on coal surfaces, but it appears to have minimal effects on the magnitudes of CO2/CH4 adsorption. (4) CO2/CH4 adsorption on coal surfaces is not strongly dependent on formation temperature. (5) Experiments with He suggest negligible adsorption of the gas on coal surfaces.