Supercritical CO2 adsorption and desorption characteristics and pore structure controlling mechanism of tectonically deformed coals

Characteristics of supercritical CO2 adsorption/desorption on coals are the key scientific problems for CO2 geological sequestration in deep coal seams. The differences in pore structures of tectonically deformed coals will affect their adsorption/desorption characteristics. Isothermal adsorption/desorption experiments of CO2 on five medium rank coal samples with different tectonic deformations were carried out at 45℃ and 0–16 MPa. Characteristics of adsorption/desorption, including isotherm, adsorption extremes, desorption deviation and negative adsorption, were investigated. Based on mercury intrusion porosimetry, low temperature N2 and CO2 adsorption experiments, pore structure and fractal characteristics were analyzed. Finally, effects of pore structure on characteristics of adsorption/desorption and adsorption mechanism were investigated. Results show that CO2 adsorption and desorption isotherms shows the “two-stage” variation with multiple extremes and negative adsorption. The adsorption peak values at low pressure stage are mainly affected by pore specific surface area of transition pores, while the adsorption capacity at high pressure stage is mainly controlled by the development of ultra-micropores, especially for ductile deformed coals. As the coal deforms from weak to strong, the desorption deviation shows the change trend of “premature desorption - reversible desorption - premature desorption”, which is controlled by fractal characteristics of pores (D1 and D3). Extraction of small organic molecules by supercritical CO2 has been considered as a possible cause for increase in density of bulk free phase and negative adsorption.