Reconstructions of supercritical CO2 adsorption isotherms and absolute adsorption estimation in nanoporous coals considering volumetric effects and varying adsorbed phase densities

Understanding supercritical CO2 (ScCO2) adsorption behavior in nanoporous coals is crucial for long-term CO2 geological storage. Four coal samples with a broad spectrum in rank were selected to obtain CO2 excess adsorption isotherms at 35 celcius and pressure up to 14 MPa using the volumetric method. As conversion from excess adsorption to absolute adsorption requires accurate capture of adsorbed phase density (APD), the simplified local density (SLD) theory and Peng-Robinson (PR) equation of state (EOS) were tailored to calculate the APD considering both pore size distribution (PSD) and pressure. Besides, volumetric effects, including void volume overestimation and coal matrix shrinkage/swelling, were quantificationally calibrated to reconstruct excess adsorption isotherms. Then, we proposed a novel conversion method based on real adsorption space distribution and adsorption mechanism. Results show that micropores in the range of 0.30-1.50 nm dominate the total pore space. APD and fugacity profiles reveal distinct CO2 adsorption behaviors in micropores and mesopores. Fitted expansion coefficients indicate that coal matrix undergoes a superimposed effect of shrinkage and swelling during adsorption and the reconstructed isotherms could amend the large deviations around 8 MPa caused by the drastic changes of CO2 bulk density and compressibility factor. Compared with conventional fixed APDs, the converted absolute adsorption isotherms generated by the varied ones exhibit slight fluctuations in the supercritical state and could plateau at high pressures. Continual iterations suggest that "averaged " APD in the range of 33-36 mol/L could be utilized for conversion in engineering practice only when the original excess adsorption isotherms are correctly reconstructed.