To support drilling gas influx control, saline aquifer CO₂ sequestration and CCUS development under the dual carbon goals, this study proposes a high-precision calculation method for the solubility of high-temperature, high-pressure, CO₂-rich natural gas in formation water. An activity–fugacity coupling model is established: fugacity coefficients of gas components are solved via the dimensionless Helmholtz free energy equation of state, and liquid-phase activity coefficients are characterized by the Pitzer electrolyte model. Comparative experiments with three natural gas and three formation water samples are carried out at 393.15–453.15 K and 5–100 MPa to analyze the influences of temperature, pressure, salinity and CO₂ content on solubility for model verification. The overall relative error between calculated and experimental data is below 10% (max 4.5%). Solubility rises rapidly with pressure then plateaus, declines with salinity, and grows with CO₂ content; CO₂ solubility far exceeds that of alkanes. This efficient, widely applicable model cuts engineering costs and guides safe oil-gas exploitation and CCUS deployment.

The methods for determining total organic carbon (TOC) in carbonate rocks are mainly the conventional TOC method and the montmorillonite thickening method. However, both methods suffer from organic matter loss during the measurement process and inaccuracy in determining low TOC concentrations (TOC ≤ 0.5%). In this study, carbonate rock samples from the Ordovician of the Ordos Basin and synthetic standard samples were investigated using the conventional TOC method, the montmorillonite thickening method, and a newly developed elemental analysis method based on sodium polyacrylate thickening after acid digestion. The results show that gypsum-facies carbonate rocks contain organic acid salts, whereas shoal-facies and argillaceous carbonate rocks do not develop organic acid salts. For synthetic samples spiked with small-molecule organic acid salts, the relative error of the conventional TOC method exceeds 98%; for large-molecule organic acid salts, the relative error is relatively small, below 16%. The TOC measurement error of the sodium polyacrylate thickening method is lower than that of the montmorillonite thickening method. Compared with the conventional and montmorillonite methods, when carbonate rock TOC ≤ 0.5%, the sodium polyacrylate thickening method yields TOC errors mostly ≤10%, with better stability and lower error rates. The new TOC method for carbonate rocks exhibits higher accuracy and stability in determining organic carbon content, and can more objectively reflect the total organic carbon (TOC) of samples.