Methane and Carbon Dioxide Sorption and Transport Rates in Coal at In-situ Conditions

Geologic sequestration of carbon dioxide is an option for the mitigation of industrial emissions. However, considerable effort remains to shift this technology from its current status as potential solution to a safe, effective and trusted foundation to the global energy system. Characterization of gas movement and sorption capacity of coal at in-situ conditions is required. Using the volumetric method, measurements of CH₄ and CO₂ sorption and diffusion in coal have been made on powder and non-powder confined coal. Results obtained, emphasized that the sorption capacity and the kinetics of gas in coal are both influenced by the stress state of the sample. The application of 6.9 MPa confining stress contributed to about 30% and 80% of sorption capacity reduction for CO₂ and CH₄ respectively. The sorption and diffusion of CO₂ in confined coal follow two distinct rates described with diffusion coefficients of 2.3×10^-6 m2/s and 9.4×10^-12 m2/s respectively. In contrast, the flow of methane is characterized by a continuous process with a diffusion coefficient of 3.8×10^-7 m²/s. These observations confirms the complex interaction of CO₂ with the coal structure and stressed that CH₄ and CO₂ sorption and transport in coal should be characterized differently, specifically when dealing with non-powder confined samples. Consequently, the use of information collected on pulverized coal samples for the simulation and prediction of long term underground sequestration and enhanced coalbed methane is not justified.