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⁺)

Interaction between Surfactants and SiO₂ Nanoparticles in Multiphase Foam and Its Plugging Ability

To improve the stability of foam fluids, SiO₂ nanoparticles and trace amount of Gemini cationic surfactant were combined with the main foaming agent, nonionic surfactant, to form a tricomponent multiphase foam. The stability of the multiphase foam was assessed through two parameters of half-life time and dilational modulus. The interaction between surfactants and nanoparticles were studied though surface tension, adsorption amount, and ζ potential measurement. The effects of saline ions and temperature on foam stability were also investigated. The plugging ability of the tricomponent multiphase foam was assessed using a sandpack model. The optimized tricomponent multiphase foam was 10 times more stable than corresponding foam without nanoparticles in terms of half-life time and also resisted to saline and temperature to a certain degree because the adsorption of nanoparticles at the interface improved the mechanic strength of foam film. The tricomponent multiphase foam showed more excellent plugging ability in porous media than foam without nanoparticles during flooding. The adsorption of cationic surfactant not only changed the surface hydrophobicity of the SiO₂ nanoparticles, but also promoted the adsorption of APG molecules. Combined the results of Gemini C₁₂C₃C₁₂Br₂ replaced by CTAB or SDS, and C₁₂C₃C₁₂Br₂/SiO₂ replaced by pretreated partially hydrophobic SiO₂ nanoparticle (H15), it is deduced that the in situ surface modification by cationic adsorption to a suitable hydrophobicity was a key step in multiphase stability. Compared with the pretreated partially hydrophobic SiO₂ nanoparticle, more SiO₂ nanoparticles were distributed at the air/liquid interface and utilized effectively in the tricomponent multiphase foam