Microscopical and macroscopic structural damage evolution of low-rank coal by supercritical CO2 pulsation treatment

LIU Jiajia; LI Yuanlong; GAO Jianliang; WANG Dan; ZHANG Yunlong

doi:10.12438/cst.2024-0758

Abstract：In order to clarify the effect and mechanism of supercritical CO2 pulsation on low-rank coal,deep low-rank coal from the Aiwi-ergou Mine in Xinjiang was collected as the experimental object.Experimental research was carried out through an independently built su-percritical CO2 pulsation fracturing experimental system.Before and after the pulsation,the coal samples were respectively measured by XRD,FTIR,low-temperature liquid nitrogen adsorption test,low-field nuclear magnetic resonance,and uniaxial compression test.The changes in the microscopic structure,pore structure,and mechanical properties of the coal before and after the supercritical CO2 pulsation were quantitatively analyzed.The results show that after the supercritical CO2 pulsation,the mass fractions of calcite and clay minerals in the coal increase.The change in the mass fraction of clay minerals is mainly affected by the increase of ammoniated illite.The peak values of various organic functional groups in the coal show different degrees of decrease,indicating that the extraction effect of supercritical CO2 on organic matter is obvious.After the pulsation,the microcrystalline structure of the coal is damaged,the crystal lamellar spacing in-creases by 0.002 9 nm,and the coal structure tends to be loose.Compared with before the supercritical CO2 pulsation,the effective porosit-ies of coal samples XJ-A and XJ-B increase by 66.7%and 128.6%respectively after the pulsation.The connectivity of pores inside the coal is enhanced,and the pulsation has a more obvious effect on the modification of micropores and mesopores.After the supercritical CO2 pulsation,the mechanical properties of the coal deteriorate significantly.The uniaxial compressive strength and elastic modulus decrease by 54.73%and 59.82%respectively.The acoustic emission ring-down count and cumulative energy are both greatly reduced.The peak value of the ring-down count drops to 51%of that before the pulsation,the cohesion of internal particles in the coal sample decreases,and the cumulative energy decreases by 62%.In conclusion,the supercritical CO2 pulsation can significantly damage the micro-macro struc-ture of low-rank coal,which is of great significance for the fracturing and permeability enhancement of deep low-rank coal reservoirs and the extraction of coalbed methane.