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+), N-2 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 CO, 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 newly 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, N-2, and monovalent cation molalities (Na+ and K+).