Experimental Research On Structure Evolution And Element Migration Of Coal Reservoir Associated With CO₂ Geological Storage

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. Coal compressibility, swelling and evolution of pore-fracture structure were also discussed in this dissertation. The main research achievements are concluded as follows:(1) New method system of simulating CO₂ geological storage into coal seam reservoir has been established and the feasibility is demonstrated.The high pressure supercritical CO₂ geochemical reactor was used to simulate CO₂ geological storage into coal reservoir with different coal rank and grain sizes samples. The simulated experiments were carried out based on the different dissolution characteristics of different minerals in coal at different stage. Coal samples before and after the ScCO₂-H2O treatment and water samples after the treatment were analyzed. The CO₂ geological storage was simulated in labratory as expected. The feasibility was demonstrated from the stability of the simulation apparatus, rationality of the selected environmental parameters and normativity of sample preparation.(2) Coal structures are largely changed after 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. Micropores are highly developed in anthracite which accordingly resulted in that the proportion of micropores after the ScCO₂-H2O treatment increased from the untreated sample of 76.95% to the treated sample of 88.98% while the macropores are decreased slightly . On the contrary, macropores are very developed in lignite which accordingly resulted in the proportion of macropores increased from the untreated sample of 14.40% to the treated sample of 24.25% while the micropores are not changed largely.The soften phenomenon of coal reservoir was also observed after the CO₂ geological storage. According to the calculation of fractal dimensions, the compressibility of coal is directly related to coal rank i.e. higher rank coal is harder to be compressed than the lower rank one. With the dissolution of coal minerals the treated samples are easier to be compressed than the untreated one.(3) 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. Elements such as Ca, Mg, Mn, Sr, Zn, Co, Ba, As, Cr and Cu which interrelated with carbonate mineral and sulfide minerals have the strongest migration ability during the whole geological storage process but elements such as Si, Zr, Be, Sc, Ga and Rb which interrelated with silicate minerals only show their migration ability in the later stage of geological storage. The primary way of elements migration is dissolution migration. Meanwhile, some ecological elements such as As, Mo, Zn also show strong migration ability with migration rate of 31.15%,23.87% and 48.87% respectively. As a result, attention should be paid to the monitor to the elements with high contamination effects such as Pb, As, Cu and Cr.(4) Coupling relationship is found between reservoir structure evolution and element migration and also the geochemical model of pore-fracture evolution has been established.The evolution of pore-fracture structure are mainly represented in 3 aspects, i.e. enlarge and connectivity of pores in the coal matrix, extend of the original fractures and new-forming fractures. Changes of coal structure can be reflected by elements migration or elements combination migration especially those with high migration rate, for instance, elements combination of Ca, Mg, Mn, Sr, Zn, Co and Ba can be a reflectance of reforming of carbonate minerals.Based on the changes of stress distribution in the coal reservoir, dissolution and migration of minerals and migration characteristics of elements combination, the geochemical model of pore-fracture structure during the CO₂ geological storage has been established. The behaviors of pore-fracture evolution at different stage are shown as different characteristics, i.e. decrease of permeability in the initial stage, briefly stability of permeability, increase of permeability in the middle stage and stable permeability in the later stage. 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. When pores and fractures are changed during the CO₂ geological storage process, their opening and connecting characteristics are greatly improved. Gas components, gas concentrations and partial pressure of different gases are all changed because of the injection of CO₂ which resulted in the desorption of CH4. Desorption and diffusion of CH4 are largely improved because the permeability of coal reservoir are increased. These processes all contribute to the enhanced coalbed methane recovery and the geological storage of CO₂ into coal seam reservoir.