The Law And Mechanism Of Water Effect On Carbon Dioxide Sequestration And Methane Displacement In Coal

Injecting carbon dioxide（CO₂）into coal seam with enhanced coalbed methane（CH₄）recovery can efficiently mitigate CO₂ emission.Hence,the focus on the CO₂-ECBM technology can not only alleviate the climate change issue caused by CO₂,but also strengthen the CO₂ emission reduction capability of our country.Storing CO₂into deep unmineable coal seam is a complicated multi-coupled process typically involving adsorption,desorption,diffusion and flow of CO₂ and CH₄.Generally,the practical coal seams always contain water（H₂O）.Investigation regarding the influence of H₂O on CO₂ adsorption and displacement of CH ₄ by injecting CO₂ is of great significance to the establishment of CO ₂-ECBM theoretical system and its further implement.Hereby,four kinds of dry and moisture-equilibrated coals with different coal ranks were selected to investigate the H₂O occurrence characteristics and its influence on CO₂ adsorption on coals.Moreover,the displacement of CH₄ by injecting CO₂ with different pressures was also conducted to clarify the opti mum CO₂injection pressure and the influence of H ₂O during the displacement process.The main research contents include the H₂O occurrence characteristics within coals,the influence of H₂O on the CO₂ adsorption on coals,the influence of H₂O during the displacement process and the optimum CO₂ injection pressure.The main conclusions derived from study are as follows:（1）The H₂O adsorption on the above-mentioned four coal samples has similar changing trends,which can be divided into the following four stages,i.e.single H₂O molecule attaches to the oxygen-containing functional groups on coal surface,the previously adsorbed H₂O molecule acts as new adsorption site for other H₂O molecules,H₂O molecules form clusters,and H₂O clusters fill in micropore and subsequently condense in mesopore.Additionally,the H₂O adsorption on coals is mainly dominant by the mesopore structure of coals.Since SCG coal and SH coal have developed mesopore structure while those of DQ coal and XY coal are underdeveloped,the H₂O adsorption amounts of SCG coal and SH coal are much higher than those of DQ coal and XY coal.（2）The modified Brunauar-Emmett-Teller（BET）model can well describe the H₂O adsorption behavior on coals.Its fitting results indicate that both the adsorption energies of H₂O at primary sites（E₁）and secondary sites（E₁）are above 40 k J/mol,which are much higher than the adsorption energy of CO ₂ on coal surface.Therefore,the coal surface presents higher affinity for H₂O rather than CH₄ and CO₂.（3）The CO₂ adsorption study indicates that the Ono-Kondo lattice model and simplified bidisperse kinetics model can well predict the CO₂ adsorption equilibrium and kinetics behaviors of all coals,respectively.Therein,H₂O impairs the CO₂adsorption capacity of coals and hinders the CO ₂ diffusion in the micro-and macropores.The aforementioned results are mainly attributed to that H₂O occupies the main CO₂ adsorption sites and occurrence space,thereby leading to part of CO ₂hardly enter pore interior.This weakening influence of H₂O on CO₂ adsorption gradually becomes weakness with the increasing equilibrium press ure and coal rank.（4）According to the experimental results regarding the CH ₄ displaced by injecting CO₂,some conclusions can be drawn as follows.Injecting CO₂ into coals can discharge the situ CH₄,thereby demonstrating the feasibility of CO₂-ECBM technology.With the increasing CO₂ injection pressure,the mixed gas adsorption capacity and CO₂ adsorption capacity of coal samples increase while the CH₄adsorption capacity gradually decreases,indicating that the elevated CO₂ injection pressure contributes to CO₂ adsorption and CH₄ desorption.Within the scope of CO₂injection pressures in this study,the maximum CO₂ adsorption capacity and the maximum CH₄ desorption capacity can be obtained when the CO ₂ injection pressure is approximately 8.5 MPa.Therefore,the aforementioned pressure is the optimum injection pressure in this study.