Effects Of Long-term Supercritical CO₂ Fluid On Physicochemical Properties Of Water-bearing Coal And Surrounding Rock And CO₂ Adsorption/desorption Performance

The main anthropogenic greenhouse gas i.e.carbon dioxide（CO₂）emitted from the traditional fossil energy including coal,oil and natural gas,has caused global climate change and a series of environmental issues.Hereby,it is urgent to mitigate CO₂emissions.There have been many options to reduce CO₂ emissions in recent years.Among these,the CO₂ sequestration in deep coal seams with enhanced coalbed methane（CH₄）recovery（CO₂-ECBM）is a viable option to mitigate CO₂ emissions.The optimum depth for CO₂ sequestration in coal seams is recommended as 800-1000m,where CO₂ exits in form of supercritical fluid（scCO₂）.Given the characteristics of scCO₂ and coal matrix,multiple fluid-solid interactions do exist between them.The aforementioned interactions affect the pore structure and surface chemistry property of coals,thereby affecting validity regarding CO₂ storage.In addition,the scCO₂ can change the surrounded rocks of coal reservoir including roof and floor,thereby influencing the stability and safety regarding CO₂ storge in target coal seams.Therefore,the interactions of scCO₂-H₂O and coal,roof（standstone）and floor（mudstone）were conducted in a self-designed high-pressure vessel for 8 months.The opereting temperature and pressure are 318.15 K and 12 MPa,respectively.Multiple characracting methods including CO₂ adsorption,N₂ adsorption and desorption,mercury instrusion,X-Ray diffractometer（XRD）and X-ray photoelectron spectroscopy（XPS）were adopted to determine the change of pore structure,mineral compositions and main functional groups of coal matrix,roof and floor before and after long-term scCO₂-H₂O exposure.Moreover,the volumetric method was used to measure the CO₂ adsorption and desorption capability of coal samples before and after long-term scCO₂-H₂O exposure.The main conclusions derived from this study are as follows:（1）The long-term scCO₂-H₂O exposure changes the physicochemical property o f varios rank coals.On the one hand,the exposu re changes coal pore structure.Particularly,it decreases the micropore surface area of ZT coal,while i t increases that of DT and BLG coals.However,the long-term scCO₂-H₂O exposure shows minor effect on micropores of FY coal.Furthermore,the meso-macropore volume of all coals decreases after the long-term scCO₂-H₂O exposure.The exposre dependence of coal pore structure mainly arises from coal matrix swelling and mineral dissolution.Particularly,the exposure increases quartz content of ZT coal,while decreases the content of kaolinite and calcite of DT coal.When it comes to the other two coals,the exposre decreases the content of boehmite,kaolinite and calcite of BLG coal and the content of quartz and kaolinite of FY coal.On the other hand,the exposure alters surface chemistry prope y of coals.Results indicate that the content of oxygenic functional groups（C-O,C=O and COOH）and nitric functional groups（pyridine and oxid-N）reduces.These changes are mainly associated with CO₂ chemisorption and small organic compouds extraction due to scCO₂.（2）The long-term scCO₂-H₂O exposure shows different effects on CO₂ adsorption equilibrium and kinetics on various rank coals.With repect to adsorption equilibrium,the Ono-Kondo（OK）lattice model can well describe CO₂ adsorption equilibrium on coals before and after long-term scCO₂ exposure.The model fitting results show that the maximum CO₂ adsorption capacity of ZT and DT coals increases after exposure,while that of BLG and FY coals decreases.In addition,the long-term scCO₂-H₂O exposure decreases the isosteric heat of adsorption of CO ₂ by 1.52-7.77%for all the coals.With respect to CO₂ adsorption kinetics on coals,the process can be devided into two stages before and after long-term scCO₂-H₂O exposure,i.e.the initial fast stage and subsequent low stage.Compared with the pseudo-first-order model and pseudo-second-order model,the bidisperse model is of higher predictive accuracy for CO₂ adsorption kinetics on coals before and after the long-term scCO₂-H₂O exposure.The model fitting results indicate that the exposure reduces the value of D_macro/R²_macroof coals,implying that scCO₂ weakens the CO₂ adsorption and diffusion rate within coal macropores.（3）The long-term scCO₂-H₂O exposure changes CO₂ adsorption and desorption hysteresis loop on all coals.In comparison with raw coals,the hysteresis loop area of all the coals after exposure increase by 0.18-377.35%.Such change indicates that the long-term scCO₂-H₂O exposure is beneficial to CO₂ storage in coal seams.（4）The long-term scCO₂-H₂O exposure changes the mineral composition and pore structure characteristics of sandstone and mudstone.Particularly,it reduces the content of dolomite and kaolinite in sandstone and the content of kaolinite and quartz in mudstone.The long-term scCO₂-H₂O exposure increases micropore surface area of sandstone and mudstone,the increasing amplitude of which is approximatedly 23.77%and 15.01%,respectively.Moreover,the long-term scCO₂-H₂O exposure increases the meso-and macropore volume of sandstone and mudstone by 5.74%and 4.60%,respectively.Next,the exposure increases the amount of open macropore of mudstone.Finally,the muil-fractal analysis show that the long-term scCO₂-H₂O exposure reduces the pore heterogeneity,while enhances the macroporous connectivity of sandstone and mudstone.Such results imply that the diffusion rate of CO₂ in sandstone and mudstone are expected increased,which do not favor the stable storage of CO₂ within coal seams.To sum up,the long-term scCO₂-H₂O exposure is capable of changing pore structure and function groups,therby affecting CO₂ adsorption and desorption capability of coals.Moreover,the long-term scCO₂-H₂O exposure also alters mineral composition and pore structure of sandstone and mudstone,thereby affecting safety and stability of CO₂ storage.The main results and conclusions devived from this study not only gain further insight into CO ₂-ECBM,but also provide important basis to evaluate the validity,stability and safety regarding CO₂ storage in target coal seams.