Swelling and acoustic emission behaviour of unconfined and confined coal during sorption of CO2

• CO2 sorption induced swelling, stress and acoustic emission in coal were measured.
• Applied stress results in internal stress in the coal sample and reduced swelling.
• Self-stressing should be taken into account in ECBM reservoir modelling.
• Sorption of CO2 weakens the mechanical strength of coal.

This paper presents the results of simultaneous measurements of strains, stress and acoustic emission (AE) induced in cylindrical coal samples by CO2 sorption. The experiment comprised two sorption tests. In the first one, an unconfined sample was used, while in the second, before sorption, the sample was axially compressed under high vacuum up to 25–30% of its compressive strength (previously determined on a parallel sample in a uniaxial compression test under vacuum). The experimental set-up consisted of a pressure vessel mounted in a loading frame and the associated pressurisation and monitoring systems. Measurements were made keeping the gas pressure inside the test vessel constant.The experiments have shown that an applied axial stress results in the development of internal stress within the coal and also in the reduction of both swelling and acoustic emission by about 60% and 80%, respectively. This may indicate that the effect of self-stressing as a result of CO2 sorption by coal under constrained conditions should be taken into account in ECBM reservoir modelling.