Equilibrium Modelling of CO₂-Brine Systems for Geological Carbon Storage Applications; Experimental Measurements and Thermodynamic Modelling

The thermodynamic properties of CO2–brine fluid system play a critical role in optimalgeological carbon storage (GCS), particularly in saline aquifers. Dissolving CO2 in brine increases storagecapacity, but it also causes brine acidity, which could compromise the containment integrity of the storagesite. Precise modelling of mutual solubility parameter in CO2-brine system under in-situ reservoir conditionsis required to estimate storage efficiency and potential risks associated with CO2 storage in saline aquifers.Traditional cubic equations of state (EoS) struggle to accurately predict water-CO2 interactions due to theirinability to account for dipole-dipole and quadrupole-quadrupole forces, which are fundamental in polarfluid systems. To address this limitation, this study employs an extended SRK-Polar equation of state (EoS)integrated with Pitzer-Debye-Hückel method (e-SRK-P) to incorporate electrolyte interactions . Bycapturing the role of polar interactions between water and CO2, this approach enhances physical consistencyof solubility predictions, phase equilibria, and brine acidification under subsurface storage conditions.Unlike conventional models, this framework simplifies electrolyte contributions while ensuring greateraccuracy in predicting CO2 behaviour in saline environments. This research presents application of anadvanced thermodynamic model to predict CO2 equilibrium in NaCl solution over a range of temperatures(353 -433 k), pressures (up to 124 bar), and salinity (1 - 4 M ). A systematic procedure was developed inthis study to reliably generate equilibrated CO2-saline water solutions and consistently measure solubilitydata, which was then used to generate an in-house dataset for model validation. Experimental solubilitymeasurements were conducted using both the direct weighing of the fluid sample and water columndisplacement techniques to ensure robustness and accuracy. In addition, some CO2-in-brine solubility dataover a wider range of conditions were borrowed from the literature. Another renowned model, known asDuan’s EoS, was also used for solubility computations, and the results were compared against e-SRK-P EoSpredictions for further validation. Compared to the existing EoS modelling techniques, the proposed EoSmodel demonstrates improved accuracy in predicting CO2 equilibrium concentrations in saline solutions,when compared with the experimental data, with lower Absolute Average Deviations (AADs) of 2.5%. Thisstudy contributes to the development of a more reliable predictive tool for CO2-brine interactions, enhancingthe effectiveness of GCS strategies, particularly CO2 entrapment through “dissolution in brine” mechanism.By improving the accuracy of CO2 solubility predictions, this work supports better risk assessment andoptimization of the GCS operations.