Meitan xuebao (Mar 2024)
Research on the characteristics of CO2-water interface and the law of dissolution and mass transfer under the condition of carbon sequestration in goaf
Abstract
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 (IFT) 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 IFT. 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 IFT value. As the reservoir temperature increases, the IFT value increases correspondingly, but the change range is small. Under constant temperature and pressure conditions, there is a positive correlation between salinity and IFT value. Within the scope of this experiment, low pressure, high temperature, and high salinity promote an increase in the IFT value. The IFT 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 IFT 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.
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