Characteristics of CO2/supercritical CO2 adsorption-induced swelling to anthracite: An experimental study

Coal reservoirs especially deep unminable coal reservoirs, are viable geological target formations for CO2 storage to mitigate greenhouse gas emissions. However, CO2 storage in deep coal seam faces many difficulties, such as low initial injectivity and further permeability loss during injection, induced by the volumetric swelling effects of coal. This paper presents a set of measurements on anthracite cores exposed to CO2/supercritical CO2 (ScCO2) under temperature and pressure condition. Testing includes the measurements of CO2 adsorption amount and volumetric swelling ratio, and these measurements are conducted by a novel experimental apparatus, which design base on the methodology of coal swelling-induced confining pressure liquid flowing. Noteworthy, we have measured the CO2 adsorption-induced swelling to anthracite cores at temperatures between 35 °C and 55 °C and adsorption equilibrium pressures up to 9.3 MPa. The results indicate that the anthracite in Qinshui basin capable of adsorbing a large amount of CO2 then inducing a large swelling ratio, the absolute adsorption amount is between 54.4 mL/g and 69.5 mL/g, and swelling ratio is between 2.32% and 3.45%. The Langmuir-like equation is not suit to the data fitting between excess adsorption amount and adsorption equilibrium pressure. The growth of micropores and ultramicro-fracture increase the specific surface area of anthracite for adsorbing CO2. The apparatus, by the liquid-displacement method, capable of directly observing CO2/ScCO2 adsorption-induced swelling to the whole anthracite core, which reduces the effect of heterogeneous minerals distribution on the swelling measurement.