Variation And Mechanism Of Coal Physical Property In The Fluid-solid Coupling Between Supercritical CO₂ And Coal

CO₂ enhanced coalbed methane recovery（CO₂-ECBM） is an important mean of CO₂ geological storage. Five different rank coals were collected to simulate the fluid-solid coupling between supercritical CO₂ and coal. The X-ray fluorescence spectrometry（XRF）, the inductively coupled plasma mass spectrometry（ICP-MS）, and the inductively coupled plasma optical emission spectrometer（ICP-OES） were used to detect the content characteristics of the constant and trace elements in coals before and after the reaction. The low-temperature liquid nitrogen adsorption instrument was used to test pore structure of samples. The commercial isothermal adsorption instrument was used to test adsorption characteristics of samples. The minerals migration due to the fluid-solid coupling between supercritical CO₂ and coal and its influence on the coal petrography were analyzed.The characteristics of elements migration and its regularity caused by fluid-solid coupling between supercritical CO₂ and coal were discussed. Found that the migrations of the constant elements such as Ca, Mg, Na, K and the trace element such as Sb, Tl, V, Cu, Zn, Rb, Sr, Zr, Cs were obvious; the migrations of the constant elements such as Si, Al, P and trace elements such as Li, Sc, Ba, Hf, Th were weak, the migration of the trace elements such as Co, Ga, Pb, Nb, Mo, Cd almost have no migration. The results indicate that the migrations of elements are related to occurrence state of elements in the coals, the elements interrelated with carbonates, sulfates and sulfides have strong migration,the elements interrelated with silicate and oxide have few migration.Fluid-solid coupling between supercritical CO₂ and coal has remarkable reconstructive effect on nanoscale pore structure characteristics of coal were founded, and the reconstructive effect includes increasing pore number and enlarging pore volume. But the connectivity of pore structure was changed slightly. Besides, the paper holds that the fluid-solid coupling effect mainly occurred in the fractured surface and inside the opening pores.The characteristics and basic regulation of coal adsorbing methane before and after the reaction of fluid-solid coupling were described. The adsorption characteristic curve of methane on the coal samples were fitted. The relation between pore structure and isothermal adsorption constant during the fluid-solid coupling, and coal adsorption variation mechanism were discussed. Geological model of element migration- pore structure variation- adsorption variation effected by fluid-solid coupling between supercritical CO₂ and coal was established.