Research on the characteristics of CO₂-water interface and the law of dissolution and mass transfer under the condition of carbon sequestration in goaf

As an important negative carbon technology to solve the carbon emission problem in the coal industry, the CO₂ sequestration in the mine goaf has a wide application prospect in the secondary utilization of waste resources in the goaf and the capture and storage of CO₂. In this study, the influence of different temperatures and pressures, formation water salinity and cationic solution type on the interfacial tension (I_FT) of CO₂-formation water system was investigated by using in-situ interfacial tension meter. The gas-liquid interface diffusion effect of CO₂ injection into water-bearing coal rock mass was clarified. The equation of state (SAFT-LJ equation of state) based on statistical association theory combined with the Lanner-Jones potential energy model and density gradient theory (DGT) were combined to predict the theoretical value of I_FT. Using a self-developed geological sequestration and geochemical reaction simulation experimental platform, various exploratory experiments were conducted to investigate the solubility of CO₂ under the same conditions. The characteristics of CO₂ solubility variation in the reservoir environment of the goaf were obtained, and the corresponding theoretical values of CO₂ solubility were calculated using the D-S model. The experimental results show that when the ambient temperature is constant, the reservoir pressure in the goaf is linearly negatively correlated with the I_FT value. As the reservoir temperature increases, the I_FT value increases correspondingly, but the change range is small. Under constant temperature and pressure conditions, there is a positive correlation between salinity and I_FT value. Within the scope of this experiment, low pressure, high temperature, and high salinity promote an increase in the I_FT value. The I_FT values between CO₂-salt solutions show an increasing trend with the increasing valence of cations (K⁺ < Na⁺ < Ca²⁺ < Mg²⁺). The pressure of the depleted reservoir is positively correlated with the CO₂ solubility. When the temperature is 25 °C and under conditions of pure water, as the pressure increases from 0.5 MPa to 2.5 MPa, the corresponding CO₂ solubility increases from 0.1627 mol/kg to 0.7141 mol/kg. The CO₂ solubility decreases with the increases of temperature and salinity. Under the same concentration, monovalent cation solutions (NaCl, KCl) can dissolve more CO₂ than divalent cation solutions (CaCl₂, MgCl₂). The free phase CO₂ injected into the goaf overcomes the interfacial tension and breaks the geochemical balance of the goaf strata through diffusion and dissolution mass transfer. By clarifying the influence of reservoir temperature and pressure conditions and goaf water environment on I_FT value and CO₂ solubility, the gas-liquid interface effect and dissolution mass transfer mechanism of CO₂-formation water are clarified, so as to provide a theoretical basis for the safety and evaluation of CO₂ sequestration in the closed mine goaf.