Effects of subcritical and supercritical CO2 sorption on deformation and failure of high-rank coals

This paper presents the results of an extensive experimental analysis aimed at establishing the effects of sub critical and supercritical CO2 sorption on deformation and failure of coals. Two high-rank anthracitic coals from the South Wales coalfield, obtained from different locations and depths of 150 m and 550 m, are employed for that purpose. The investigations include i) determination of unconfined compressive strengths and elastic moduli of the cores both non-saturated and saturated with CO2 at 2.1 MPa, 4.3 MPa and 8.5 MPa, ii) assessing the dependence of the parameters obtained on CO2 pressure, iii) analysing the effect of CO2 saturation on failure patterns of the samples tested and iv) determination of the particle size distribution after the failure of the samples. Based on the results of twenty coal specimens tested, it is demonstrated that CO2 sorption reduces the uniaxial compressive strengths and elastic moduli by between 29% and 83% for the range of pressures studied. The reductions observed increase gradually up to 4.3 MPa and then reach a plateau. By accommodating the effect of effective stress on compressive strength values, it is shown that chemical weakening of high rank coals is mostly associated with sorption of subcritical CO2, with negligible impact of supercritical CO2 on further parameter reduction. Inspection of failure patterns during uniaxial compression suggests that non-saturated coal specimens fail through axial splitting with rapid crack propagation and high outburst of coal pieces while the failure of cores subjected to CO2 injection occurs through multiple fractures with negligible material outburst. The post-failure analysis demonstrates that CO2 treated samples disintegrate on smaller particles than non saturated specimens, as up to 5.6 more CO2 saturated coal pieces passed through the sieves considered in this study than non-saturated pieces. It is claimed that this study presents novel insights into the geomechanical response of high rank anthracitic coals to high pressure CO2 injection.