STRUCTURE EVOLUTION AND ELEMENT MIGRATION OF COAL RESERVOIR WITH CO2 GEOLOGICAL STORAGE EXPERIMENTAL SIMULATION

High pressure supercitical CO₂ geochemical ractor was employed to simulate CO₂ geological storage into coal seam reservoir process. Different coal samples with different coal rank and grain sizes were choosen in the experiments. True density, total pore volume, specific surface area, pore distribution and element geochemical migration were studied before and after the ScCO₂-H₂O treatment with different analysing methods like mercury porosimetry, scanning electron microscope, inductively coupled plasma source mass spectrometer, inductively coupled plasma optical emission spectrometer and X-ray fluorescence spectrometer. New method system of simulating CO₂ geological storage into coal seam reservoir and unveiling structure evolution and element migration has been established. The authors draw the following conclusions:

(1) Coal structures are largely influenced by the CO₂ geological storage process. True densities are all increased because of the increased pore voume in coal after the CO₂ geological storage, while bulk densities are all decreased because of the dissolution of coal minerals. Total pore volume, specific surface area and porosity are all largley increased in all the samples especially the anthracite coal. Changes of pore structure are also influenced by the intrinsic property of coal seam reservoir.

(2) Regularity of elements migration is observed during the CO₂ geological storage. The ability of element migration before and after the CO₂ geological storage is due to the difference of their activity which mainly decided by their occurrence in inorganic mineral matter in coal. The primary way of elements migration is dissolution migration. Meanwhile, some ecological elements such as As, Mo, Zn also show strong migration ability.

(3) Coupling relationship is found between reservoir structure evolution and element migration and it decides on the geochemical model of coal reservoir structure evolution. The evolution of pore-fracture structure are mainly represented in 3 aspects. The geochemical model of pore-fracture structure during the CO₂ geological storage shows four 4 stages. Changes of stress distribution in coal reservoir and dissolution characteristics of minerals in coal are the key factors which control the pore-fracture structure evolution.