Experimental study on liquid products and pore structure characteristics of anthracite saturated by supercritical CO2

The coal reservoir modification technology that utilizes supercritical CO₂ as a fracturing medium has demonstrated remarkable results through its unique physical, mechanical and chemical effects. However, existing research has not yet systematically determined the specific organic chemical reactions or established correlations between these reactions, pore structure evolution, and permeability enhancement. This study employs GC-MS to characterize liquid product composition, characteristics, and conversion mechanism of coal samples with different saturating times by supercritical CO₂. Additionally, it investigates variation patterns of pore volume and specific surface area of coal pore structure through the liquid nitrogen adsorption tests and elucidates the permeability and seepage characteristics after supercritical CO₂ saturation. The results show that hydrocarbons and oxygen-containing organics are almost equally divided in the saturating products of 1-day and 3-day. However, after 5-day and 7-day saturations, hydrocarbons constitute only about 10 % of the saturating products, with the majority being oxygen-containing organics. Furthermore, the physical extraction dominates the increment in the pore volume of macropore after 1-day saturation, the chemical reactions of the organics weaken the construction of macropore, and increase the mesopore space after 3-day saturation. Moreover, the pore volume evolution of mesopore dominates the overall pore volume tendency. The continuous effects of chemical reactions enlarge the micropore space with increasing saturation time, whereas, the evolutions of pore size and pore volume are mainly accomplished through the mutual transformation between mesopores and micropores. The experimental results indicate that supercritical CO₂ tends to penetrate and connect the channels of seepage and migration in the early stage of saturation, and it tends to open the action point of adsorption-displacement-desorption of CO₂ and CH₄ in the later stage of saturation. These research results provide a reliable theoretical basis for the technical micro mechanism of permeability enhancement and reservoir modification using supercritical CO₂ as a fracturing medium.