Thermodynamic Phase Behavior and Miscibility Studies of Confined Fluids in Tight Formations
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In this study, the nanoscale-extended theoretical models and experimental nanofluidic system are developed to calculate and measure the thermodynamic phase behavior and miscibility of confined pure and mixing fluids in tight formations. First, a new nanoscale-extended equation of state (EOS) is developed to calculate the phase behavior of confined fluids in nanopores, based on which two correlations are modified to predict the shifts of critical properties. The nanoscale-extended EOS model has been proven to accurately calculate the phase behaviour of confined fluids. The thermodynamic phase behavior of confined fluids in nanopores are substantially different from those in bulk phase. The confined critical temperature and pressure always decrease with the reducing pore radius. The shifts of critical properties are dominant factors for the phase changes of confined fluids from bulk phase to nanopores. Second, two new nanoscale-extended alpha functions in Soave and exponential types are proposed for calculating the thermodynamic and phase properties. A novel method is proposed to determine the nanoscale acentric factors. The new alpha functions are validated for the bulk and nanoscale calculations. Moreover, the acentric factors and intermolecular attractivities are increased with the pore radius reductions at most temperatures. It should be noted that the alpha functions decrease with the pore radius reduction at the critical temperature. Furthermore, the first and second derivatives of the Soave and exponential alpha functions to the temperatures are continuous at T 4000 K. Third, the equilibrium two-phase compositions are analyzed to elucidate the pressure dependence of the interfacial tensions (IFTs), and the confined fluid IFTs in nanopores are calculated. The phase density difference is found to be a key factor in the parachor model for the IFT predictions, which results in three distinct pressure ranges of the IFT vs. pressure curve. The IFTs in bulk phase of the hydrocarbon systems are always higher than those in nanopores. The feed gas to liquid ratio (FGLR), temperature, pore radius, and walleffect distance are found to have different effects on the IFTs in bulk phase and nanopores. Fourth, a new interfacial thickness-based diminishing interface method (DIM) and a nanoscale-extended correlation are developed to determine the minimum miscibility pressures (MMPs) in bulk phase and nanopores. Using DIM, the MMP is determined by extrapolating ( / P)T to zero. Physically, the interface between fluids diminishes and the two-phase compositional change completes at the determined MMP from the DIM. The developed correlation is proposed as a function of the reservoir temperature, molecular weight of 5 C , mole fraction ratios of volatile to intermediate components in oil and gas samples, and pore radius. The new correlation provides the accurate MMPs with overall percentage average absolute deviations (AADs%) of 5.21% in bulk phase and 6.91% in nanopores. Fifth, thermodynamic miscibility of confined fluids in nanopores are studied. The thermodynamic free energy of mixing and solubility parameter are quantitatively determined to evaluate the fluid miscibility in nanopores. The liquid‒gas miscibility is beneficial from the pore radius reduction and the intermediate hydrocarbons perform better with the liquid C8 in comparison with the lean gas (e.g., N2 and CH4). Moreover, the molecular diameter of single liquid molecule is determined to be the bottom limit, the pore radius above which is concluded as a necessary condition for the liquid‒gas miscibility. Last, a series of nanofluidic experiments were conducted to measure the static phase behavior of confined fluids and verify the calculated data from some theoretical models.