Semi-Hybrid Models for Determining Gas Solubility in Brines with Salt Mixtures: Application to Carbon Capture and Storage and Gas Processing

Summary Most studies in the literature on estimating gas solubility in brine are restrictive and valid only for specific gases such as CO2 as well as a few salts (NaCl/KCl/CaCl2). Here, we develop a semi-hybrid framework that can estimate the solubility of any gas in a given brine composition that consists of a wide range of salts and salt mixtures, which is the main novelty of the work. The work includes the coupling of a semi-empirical model with the machine learning (ML) approach. In particular, it is an extension of Setschenow’s correlation, where coefficients are evaluated using an ML tool based on the decision tree (DT). The features in the ML models include the ionic properties of cations and anions and thermodynamic properties of gases. This work captures combinations of various salts such as chlorides, carbonates/bicarbonates, and sulfates (as they are seen in real formation brines and water utility applications) and various standard gases (including hydrocarbon, nonhydrocarbon/polar, and acidic gases). A semi-hybrid (physics-augmented) framework is developed to estimate gas solubility in brines for a generic gas-brine system. It is applicable for a wide range of pressures, temperatures, and brine compositions. The prediction from semi-hybrid models was validated against the available experimental data. The main results are as follows: (1) The Setschenow coefficients for any cations, anions, and gases can be generated within 1–3% accuracy. (2) The semi-hybrid models predict the experimental trends of gas solubility in brine solution accurately, within a relative error of 1–6% for complex gas-brine systems. (3) Most importantly, the framework is general, fast, and convenient and can easily be extended for a novel species including greenhouse or hydrocarbon gases, as well as for a variety of salts. In addition, it can fill the gaps in experimental data for the gas-brine systems and can extrapolate to elevated pressure and temperature conditions. In this work, the applicability is demonstrated for many salts that are seen in formation brine and many gases that are used in gas injection/storage and gas processing applications. Most of the studies in the literature on estimating gas solubility in brine are restrictive and valid only for specific gases, such as CO2, and a few salts (NaCl/KCl/CaCl2). Here, we develop a semi-hybrid framework that can estimate the solubility of any gas in a given brine composition that could consist of a wide range of salts and salt mixtures, which is the main novelty of the work.