Understanding the thermodynamic effects of multiphase fluids in subsurface porous media is essential for industrial applications in water resources hydrology, energy resources extraction, and carbon geological storage. This dissertation employs molecular simulations and pore-network modeling to delve into these effects across scales. Molecular simulations are used to offer deep insights into fluid-solid and fluid-fluid interactions, highlighting the impact of single vapors on clay swelling and the role of multiphase fluid mixtures on interfacial properties. As an experiment-driven tool, molecular simulation strategies including molecular Monte Carlo and molecular dynamics in this work are all validated against experimental data. Then, those simulated systems are extended to elucidate molecular-level mechanisms of thermodynamic effects, potentially reducing the demands for extensive experiments. This work systematically explores the influence of ions, temperature, pressure, and fluid compositions under geological conditions. Pore-network models complement this investigation by characterizing constitutive properties of porous media and elucidating pore-scale phenomena influenced by fluid thermodynamics. By bridging molecular insights with macroscopic behavior, this work aims to enhance our understanding of multiphase fluid dynamics in complex geological environments.
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