Experimental Study On Supercritical Carbon Dioxide Dissolution Of Bituminous Coal

With the depletion of shallow coal seam resources,deep coal seam mining is gradually on the schedule,and the characteristics of coal seam such as the increase of temperature,the increase of in-situ stress and the decrease of permeability become more obvious.The low permeability is not conducive to methane desorption,diffusion and seepage in coal seams,which severely restricts the development of coalbed methane mining technology.Therefore,how to increase the permeability of coal seam has become an important research direction for effective development of coalbed methane.As an important emission reduction technology in carbon capture,utilization and storage technology(CCUS),injecting CO₂ into coal seams to displace and increase coalbed methane production can not only achieve geological storage of CO₂ to a greater extent in a relatively short period of time to achieve the purpose of CO₂ emission reduction,but also improve the recovery rate of coalbed methane,with dual environmental and economic benefits.Especially in deep coal seams,CO₂ usually exists in supercritical state under the influence of temperature and pressure.Supercritical CO₂ displacement to increase coalbed methane production can not only promote desorption of adsorbed gas,but also increase permeability of the coal seam.However,the research on micro-mechanism of improving pore structure of coal seam and enhancing permeability of coal seam is still in the initial stage.Therefore,based on the"supercritical CO₂ isothermal adsorption device"as the experimental platform,the supercritical CO₂dissolution and soaking experiments of bituminous coal under different soaking times have been carried out in this paper.By means of analysis and testing such as gas chromatograph-mass spectrometer,low temperature liquid nitrogen adsorption,field emission scanning electron microscope,etc.,the influence of the interaction between coal matrix and supercritical CO₂ on the organic matter structure and pore and fissure structure of coal has been analyzed from two aspects of the single effect of supercritical CO₂ and the combined effect of supercritical CO₂ and Nano-silica,so as to seek the interaction mechanism among supercritical CO₂,Nano-silica and coal matrix,and provide a certain theoretical basis for improving the recovery rate of coalbed methane.The experimental results show that Supercritical CO₂ has a significant effect on micropores and macropores with a pore diameter larger than 150nm in coal,which promotes effective connectivity of pores in coal matrix.Nano-silica has a prominent effect on mesopores and macropores with pore sizes less than 150nm,which stimulates the penetration of ineffective connectivity pores in the coal matrix.Moreover,under the action of Nano-silica,supercritical CO₂ has a better effect on the pore and fissure structure of coal body,making its influence on pores gradually transition from macropores to mesopores.At the same time,due to the active catalysis of Nano-silica,the chemical reaction activity between supercritical CO₂ and coal matrix is improved,the organic matter conversion efficiency is enhanced,the connectivity of mesopores and fissures in coal matrix is stimulated,and the permeability of coal seam is further enhanced.In particular,comprehensive experimental analysis results show that the interaction between supercritical CO₂ and coal has formed three stages with soaking time:the first week is the dissolution and permeability stage,the second to third weeks is the adsorption and expansion stage,and the fourth week is the expansion and contraction deformation stage.