Experimental study the influences of geochemical reaction on coal structure during the CO2 geological storage in deep coal seam

Deep unminable coal seams are suitable geological bodies for carbon dioxide capture and storage (CCS). In order to discuss the geochemical reaction between the injected carbon dioxide (CO2) and deep coal seam, and study its influence on the structure of reservoir, the high-ash anthracite, high-volatile anthracite, low-volatile bituminous coal obtained from Qinshui basin, China, and high-volatile bituminous coal obtained from Bohaiwan basin, China, were processed to 4-8 mm. Then the simulation experiments for the CO2 injection were conducted for 10 days at the burial depth of 1500 m under in-situ reservoir conditions. The conclusions were obtained based on the changes of minerals, elemental concentrations and pore structure parameters of coal before and after the reaction. Different minerals underwent various geochemical reactions and mechanisms after CO2 injection. These geochemical reactions can transform the reservoir structure, which was mainly reflected in the connection of pores and fissures, and the blockage on the pore throat. The reactions between carbonate minerals and the CO2 acid fluid were violent and quick, which can increase the volume of macropores and fractures. While the weak reactions of silicate minerals may affects the micropores in coal. The chlorite and feldspar could continuously react with the CO2 acid fluid, with the slow dissolution of potassium, sodium and silicon, which makes it possible to use these characteristics to monitor the geochemical reaction. Besides, the volumes of the micropores (1-2 nm) in the anthracite were significantly increased, making it a potential geological body for CCS.