Adsorption characteristics and storage models of subcritical/supercritical CO2 in coal seams
Large-scale efficient CO2 geological storage is one of the key means to realize CO2 emission reduction rapidly in a short term.CO2 storage in deep unminable coal seams with adsorption as the main storage mechanism has the potential of large-scale implementation.In order to study the mechanisms of CO2 storage in deep coal seams,this paper carries out a high-temperature and high-pressure CO2 adsorption experiment on the anthracite coal taken from the Qinshui Basin.Then,the SLD-PR model is used for fitting,the adsorption density distribution characteristics of different pores with the variation of temperature and pressure are analyzed,and the CO2 storage mechanisms in coal seams with the collaborative change of adsorbed phase density and maximum adsorption layer thickness as the core are revealed.Finally,the volumes of CO2 storage in coal seams at different burial depths are predicted by classifying adsorption space and free space in pores accurately.And the following research results are obtained.First,the adsorption curves of supercritical CO2 at different temperatures all present the typical adsorption characteristics of supercritical gas,with the excessive adsorption capacity(the maximum value ranging from 1.25 to 1.75 mmol/g)decreasing after reaching the maximum value.Second,the CO2 adsorption layers in pores are divided into contact layers,inner layers and transition layers.In pores,subcritical CO2 is mainly in the mode of monomolecular layer adsorption,while supercritical CO2 is in the mode of multi-molecular layer adsorption.Third,the maximum adsorption layer thickness decreases with the increase of burial depth,and it is in positive correlation with temperature and negative correlation with pressure.Fourth,the average CO2 adsorbed phase density increases firstly and then decreases with burial depth,and the total storage and the absolute adsorption curves are different when CO2 is in the supercritical state.In conclusion,under the collaborative change of adsorption space and adsorbed phase density,the CO2 microscopic storage models in coal seams can be classified into three types with the variation of burial depth,including subcritical monomolecular layer adsorption,supercritical gas-like multi-molecular layer adsorption,and supercritical liquid-like multi-molecular layer adsorption.What's more,the contribution of free phase storage to the total storage increases with burial depth,but adsorption storage is still the main mode of CO2 storage in coal seams.The research results reveal the storage mechanisms of supercritical CO2 in in-situ coal seams,and can provide new understandings for the evaluation of CO2 storage capacity in deep coal seams.
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