Experimental Study On The Reaction Of ScCO₂ And Organic Matter In Coal And Its Pore Structure Response

CO₂ geological sequestration and enhanced coalbed methane recovery technology combines win-win effect of greenhouse gas emission reduction and new energy development,which has attracted worldwide attention.Deep unmineable coal seams are the potential geological body for CO₂ geological sequestration,however,under the influence of formation water,temperature and pressure in deep coal reservoir,CO₂,especially for supercritical CO₂?ScCO₂?,can produce geochemical reaction with the organic matter of coals and rocks,resulting in the changes of coal physical and chemical structure.Based on the“ScCO₂/H₂O simulation system and coal/rock geochemical reaction device”,five coal samples with different ranks were chosen to conduct simulated experiments of coal/rock-ScCO₂-H₂O geochemical reaction under three buried depths.Based on various analytical approaches of X-ray diffraction,Raman spectroscopy,Fourier transform infrared spectroscopy,mercury injection porosimetry,low temperature liquid nitrogen adsorption,CO₂ adsorption and field emission scanning electron microscope,the effects of coal/rock-ScCO₂-H₂O interaction on the organic matter and pore-fracture structure of coal reservoir were investigated.The influences of coal rank,temperature and pressure conditions on the geochemical reaction of coal/rock were also studied.Further,the mechanism of ScCO₂ and organic matter in coal and its control actions on coal pore structure were discussed.The main research achievements are concluded as follows:?1?The coal/rock-ScCO₂-H₂O reaction significantly influences the crystal structure,carbon order degree and aromatic ring condensation of organic matter in coalsThe coal/rock-ScCO₂-H₂O reaction destroys the chemical bonds and crystal structural integrity and changes the macromolecular structure of coals,and which effect mainly depends on the coal rank,i.e.,for low rank coals,the lamellae spacing decreases,the degree of parallel orientation increases,the aromaticity and aromatic ring condensation increases;while for medium-high rank coals,the disordered structure and lamellae spacing of the coal microcrystalline structure increases,the crystal size decreases and reduces the carbon order degree of coals,resulting in a reduction in the aromaticity and aromatic ring condensation.In addition,the influence of temperature and pressure on coal macromolecular structure is not significant in when simulate actual coal seam depth in this paper.?2?The effect of coal/rock-ScCO₂-H₂O reaction on the pore structure in coals is significant,and the reaction mainly affects the pore structure of coals via changing that of mineral and organic matter structureAfter reaction,the brittleness,porosity and macropore volume of coals increases,the micropore volume and specific surface area of low rank coals decreases,which are contrary for medium-high rank coals increase.Despite coal/rock-ScCO₂-H₂O reaction does not change the pore size distribution of coals,it produces a large number of<100nm and>30?m pores.Specifically,the pore volume with diameter<3 nm and>30?m in medium-high rank coals considerably increases.The ScCO₂-H₂O reaction increases the non-effectively interconnected pores in coals,indicating that ScCO₂ is dispensable in improving pore connectivity.The change of pore structure in coals is mainly controlled by coal rank,the influence of temperature and pressure on pore structure change is relatively weak,and porosity increases with the increase of temperature and pressure.After ScCO₂-H₂O reaction,the variation characteristics of pore structure are influenced by the mineral content,type and organic structure of coals.Mineral mainly affects macropore and mesopore with larger pore diameter,while the effect of organic matter on pores is concentrated in the<10 nm pore size range.?3?Simplified models of low rank and medium-high rank coal structure were constructed,the mechanism of ScCO₂-H₂O acting on organic matter in coals was ascertained,and coal rank mainly controlling the changes of organic matter structure was proposedThe ScCO₂-H₂O system mainly affects the organic matter structure through the chemical reaction:swelling effect,addition reaction,substitution reaction,bond dissociation reaction,etc.The structure changes of different rank coals are influenced by various chemical reactions,and can be related to the characteristics of the aromatic structures,categories of main organic groups and essences of cross-links.Low rank coals possess a low condensation degree of its aromatic structure,and its aromatic nuclei can facilitate addition reactions.Swelling effect primarily break cross-links between aromatic nuclei in the same aromatic layer.These characteristics favor the polymerization addition of aliphatic side chains of aromatic nuclei and promote the orientation rearrangement of molecular structure at the same layer,which makes the coal structure in a lower energy and more stable state,causing an increase in the aromaticity and aromatic ring condensation.Medium-high rank coals have a high condensation degree of their aromatic structures,and the aromatic favor substitution reactions.Swelling primarily break cross-links connecting different aromatic layer,and bond dissociation reactions are more significant for high rank coals,the force between aromatic layer is abate,crosslinking degree is reduced.These characteristic cause an increase in the lamellae spacing and decreases in the aromaticity and aromatic ring condensation in medium-high rank coal.?4?The mode that changes of organic structure influence small pore structure of coals was established,and the control mechanism of the organic structure changes for intermolecular pores and lamellae spacing pores in coals are clarifiedCoal/rock-ScCO₂-H₂O reaction has“increasing pore”and“shrinkage/expansion pore”effects on small pores in coals,and its influence depends on the organic structure of coals.The“increasing pore”effect is mainly lamellae spacing level pores,formed from aromatic layers stacked.Their pore diameter is about 0.4 nm,which is affected by the swelling effect,addition reactions and polymerization reactions in aromatic layer,and only a small part increase in the low rank coals with loose structure.The“shrinkage/expansion pore”effect is exhibited in the intermolecular pores and lamellae spacing pores,which affects the aperture<10 nm.This effect is caused by the changes of lamellae spacing and aromatic ring condensation,affected by swelling between layers,substitution reactions and bond dissociation reactions.The effect of temperature and pressure conditions on small pores is weak in this paper,indicating that the temperature and pressure range studied in this paper is insufficient to induce substantial changes on the organic structure of coals.