Super-critical CO2 saturation-induced mechanical property alterations in low rank coal: An experimental study

• Effect of supercritical CO2 on strength of Australian brown coal was explored.
• Higher coal strength reduction with supercritical CO2 compared to gaseous CO2.
• Supercritical CO2 causes 20% higher strength decrease of high rank coal.
• Greater influence on coal strength by supercritical CO2 irrespective to coal rank.

The adsorption of carbon dioxide (CO2) into the coal matrix during CO2-enhanced methane recovery causes significant alterations to the coal mass chemical and physical structures, causing modifications to coal's mechanical properties. Hence, the main objective of this study is to investigate the effects of sequestrated CO2 phase condition on coal strength. A series of unconfined compressive strength tests was conducted on Australian brown coal samples, saturated under various CO2 pressures (2–10 MPa) at 35 °C using an advanced acoustic emission (AE) system and optical 3D deformation analysis. According to the results, super-critical CO2 has the ability to cause a greater reduction of strength (by 46%) and enhancement of elasticity properties (by 20%) in brown coal compared to sub-critical CO2, because super-critical CO2 has greater adsorptive potential, which eventually creates greater coal matrix swelling. According to the AE and deformation analysis, the coal mass natural cleat system contributes significantly to the observed CO2 adsorption-induced changes in mechanical properties.

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