Space Evolution Of Shale And Molecular Dynamics Mechanism During CO₂ Geological Sequestration

Elucidating the interaction mechanism between CO₂ and shale under storage conditions is the basis for realizing efficient CO₂ storage.In this thesis,the marine shale of the Lower Silurian Longmaxi Formation in the southern margin of the Sichuan Basin is collected.On the basis of the high-pressure hydrothermal reaction device,a portfolio of analytical tests and integrated studies,effects of mineral dissolution and secondary precipitation on material composition and microscopic pore structure are revealed,and the storage space evolution of the shale gas reservoir is clarified.By combining the ReaxFF molecular dynamics simulation software and pure/mixed scCO₂ interaction experiments,diffusion,adsorption,protonation,element migration,mineralization and associated physiochemical events before and after interactions were investigated,and the molecular-level insights of CO₂ geo-sequestration in shale gas reservoirs were revealed.In accordance with typical storage events and stages,a quantitative evaluation model was preliminarily established to clarified the storage potential of shale gas reservoirs in different geological conditions.The main results are as follows:（1）The storage space variation of shale is controlled by mineral dissolution and secondary carbonate precipitation.Calcite is generally corroded,and clay minerals are geochemically influenced as well,resulting in release of Mg²⁺,K⁺,Ca²⁺,Na⁺ions,and precipitation of soluble and insoluble minerals that block or occupy pore space in shales.The expansion of the storage space caused by dissolution dominates the entire interaction process,and the precipitation usually begins to appear gradually in the middle and late stages,which jointly lead to an increasing trend of storage capacity.In comparison,the effect of dissolution on the crystal structure of illite is weak,which is featured by mineralization-associated storage space reduction induced by release of interlayer ions,and the corresponding mineralization degree is about 15.22%～33.12%.（2）The early stage of scCO₂-illite interaction is mainly characterized by protonation,HCO₃^-ion generation,surface Si–O and Al–O stretching and distortion.At middle-late stages,with neutralization of the interlayer charges,the K⁺ions gradually diffuse into the bulk fluids,bonding with HCO₃^-ions to form K₂CO₃molecules and thus to trigger mineralization.The mineralization process will cause storage space to be blocked.The precipitated minerals are often generated in the area with sufficient K⁺ion supply,which has a typical layered structure and a large fluid-solid contact area.Therefore,the interactions between HCO₃^-and interlayer K⁺ions in these areas can be enhanced,and the mineralization effect is remarkable.（3）As revealed by scCO₂-mixed mineral simulation results,the early stage is mainly characterized by the rapid diffusion,adsorption and protonation of CO₂ and H₂O molecules,and the later stage is characterized by the migration of interlayer ions in illite and montmorillonite and calcite corrosion.The dissolution of clay mineral ions in the whole process will promote the mineralization reaction process,which will eventually lead to the gradual consumption of CO₂ molecules in the system.Calcite corrosion generally has a weak effect on the CO₂ consumption process,while the enlargement of storage space caused by corrosion is the main pathway to dominate the CO₂ storage efficiency.（4）The scCO₂-shale interaction is accompanied by two stages of rapid decline and slow decline of total pressure.The former is controlled by CO₂ dissolution and adsorption processes,and the latter,in addition to the weakened dissolution and adsorption,is dominated by mineralization,which progressively exerts an influence.The dissolution,adsorption and mineralization capacities of shale were respectively determined by means of the solubility equation,isothermal adsorption experiment and mineralization contribution coefficient.The results show that non-water-bearing and weak water-bearing reservoirs have great adsorption potential,water-bearing reservoirs have high dissolution and mineralization potential,and the adsorption and mineralization can be facilitated as burial depth increasing.The short-term sequestration is represented by dissolution and adsorption,and the long-term sequestration mode is mineralization-dependent process.Note that the clay-rich depleted shale gas reservoirs show great storage potentials due to their abundant storage space and activity.