Experimental study on microstructural modification of coal by liquid CO2 extraction

Abstract

Liquid CO2 injection into coal seam for CH4 displacement is one of the important means to strengthen gas extraction and carbon sequestration in recent years. After liquid CO2 is injected into coal seam, physical and chemical interactions occur with coal through the process of “liquid phase seepage-phase transformation supercharging-gas–liquid mass transfer”, and microscopic characteristics of coal such as pore distribution, surface chemical properties and adsorption characteristics change. Three kinds of coal samples with different degrees of metamorphism were selected as research objects, theoretical analysis, numerical simulation and experimental testing were combined to carry out research on the modification of coal body by liquid CO2 extraction. The results show that with the increase of extraction pressure, the diffusion rate of small organic molecules in coal increases, which is conducive to the dissolution of small organic molecules; In larger pores, the contact area between liquid CO2 and pore surface increases, and small molecular organic matter is easy to precipitate; The infrared spectra of raw coal and residual coal after extraction are basically the same, and the absorption peak intensity is significantly different, liquid CO2 extraction does not significantly damage the macromolecular structure of coal. The research results have important reference value for liquid CO2 anti-reflection coal seam and geological storage.