Accurate Determination of the CO₂–Brine Interfacial Tension Using Graphical Alternating Conditional Expectation

A newly developed CO₂–brine interfacial tension (IFT) correlation based on the alternating condition expectation (ACE) algorithm has been successfully proposed to more accurately estimate the CO₂–brine IFT for a wide range of reservoir pressure, temperature, formation water salinity and injected gas composition. The new CO₂–brine correlation is expressed as a function of reservoir pressure, temperature, monovalent cation molalities (Na⁺ and K⁺), bivalent cation molalities (Ca²⁺ and Mg²⁺), N₂ mole fraction and CH₄ mole fraction in injected gas. This prediction model is originated from a CO₂–brine IFT database from the literature that covers 1609 CO₂–brine IFT data for pure and impure CO₂ streams. To test the validity and accuracy of the developed CO₂–brine IFT model, the entire dataset was divided into two groups: a training database consisting of 805 points and a testing dataset consisting of 804 points, which was arbitrarily selected from the total database. To further examine its predicted capacity, the new CO₂–brine IFT correlation is validated with four commonly used pure CO₂–pure water IFT correlations in the literature, it is found that the new CO₂–brine IFT correlation provides the comprehensive and accurate reproduction of the literature pure CO₂–pure water IFT data with an average absolute relative error (% AARE) of 12.45% and standard deviation (% SD) of 18.57%, respectively. In addition, the newly developed CO₂–brine IFT correlation results in the accurate prediction of the CO₂–brine IFT with a % AARE of 10.19% and % SD of 13.16%, respectively, compared to two CO₂–brine IFT correlations. Furthermore, sensitivity analysis was performed based on the Spearman correlation coefficients (rank correlation coefficients). The major factor influenced on the CO₂–brine IFT is reservoir pressure, which has a major negative impact on the CO₂–brine IFT. In contrast, the effects of CO₂ impurities and salt components in the water on the CO₂–brine IFT are in the following order in terms of their positive impact: bivalent cation molalities (Ca²⁺ and Mg²⁺), CH₄, N₂, and monovalent cation molalities (Na⁺ and K⁺).