Acoustic emission and volumetric strain induced in coal by the displacement sorption of methane and carbone dioxide

The aim of this study was to assess whether acoustic emission (AE) could carry information on preferential sorption/desorption of CH₄ or CO₂ in coal. AE and expansion/contraction of two nearly identical cylindrical coal samples were continuously monitored during displacement sorption experiments. One sample was subjected to presorption of CH₄, followed by sorption of CH₄/CO₂ mixture. With the other one, presorption of CO₂ preceded sorption of the mixture. The results obtained are the following: first, AE and stain kinetics show that the affinity of the coal tested is higher for CO₂ than for CH₄; second, methane is preferentially desorbed after presorption of CH₄ — sorption of mixture of CH₄ and CO₂; third, during displacement sorption, kinetics of AE and sample swelling/shrinkage bring out the importance of presorption and the sorbate used. It matters whether the coal is first exposed to CH₄ or to CO₂. The present study has demonstrated that injection of CO₂ into the coal previously exposed to CH₄ causes considerable swelling of the coal. On desorption after CH₄/CO₂ exchange sorption, initial shrinkage is followed by swelling of the coal. These results could have implications for the sequestration of CO₂ in coal seams and CH₄ recovery from coalbeds (ECBM). Swelling/shrinkage of the coal matrix should be included in models used to predict coal permeability and gas flow rates. They also show that the AE technique can give more insights into coal matrix-gas interactions.