Abstract
• Adsorbed phase density and fugacity facilitate identifying the adsorption mechanisms. • SLD-PR model is efficient in obtaining adsorbed phase density at various pore sizes. • Excess adsorption isotherms are effectively rebuilt considering volumetric effects. • Absolute adsorption amounts are calculated accurately and reasonably considering PSD. • Empirical adsorbed densities for engineering practice are proposed through iteration. Understanding supercritical CO 2 (ScCO 2 ) adsorption behavior in nanoporous coals is crucial for long-term CO 2 geological storage. Four coal samples with a broad spectrum in rank were selected to obtain CO 2 excess adsorption isotherms at 35 ℃ 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 CO 2 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 CO 2 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.
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