Stress-strain response of pre-compacted granular coal samples exposed to CO2, CH4, He and Ar

The interaction between supercritical carbon dioxide and coal has been the subject of many studies in recent years. This paper reports two different types of swelling experiments, one on confined and one on unconfined samples of pre-pressed, crushed bituminous coal, performed at gas or fluid pressures up to 10 MPa and temperatures of around 40 °C. These experiments confirm earlier observations of increased swelling of coal due to carbon dioxide adsorption compared with methane and argon. However, the experiments reveal a previously unrecognised swelling phenomenon. The swelling observed in the present samples is up to 3 times larger than that seen in block-shaped coal matrix fragments that have not been pre-compacted and is largely irreversible. We propose that pre-pressing led to permanent compaction of the internal pore structure of the coal matrix grains, decreasing the mean pore aperture and increasing the proportion of micropores. Introduction of CO2 and CH4 resulted in increased micropore adsorption which, through the associated surface forces developed within the micropores, led to a disjoining pressure effect that caused recovery of the pre-pressing-induced permanent strain. The higher affinity of micropores for CO2 than for CH4 adsorption, explains the markedly bigger effect of CO2. After the recovery of the permanent strain was achieved, the samples seem to have exhibited a normal, reversible response to adsorption. We thus attribute the irreversible swelling effect seen in our experiments to the stress-history effect of pre-pressing. The findings of this study may be relevant for coals that have undergone deep burial after formation, or for naturally stressed coals in tectonic zones, which may exhibit more swelling upon gas adsorption than expected from their reversible adsorption capacity