Phase behavior of hydrocarbons in nano-pores

Abstract

Inside nanopores, solid-fluid interactions are of the same order of magnitude as intermolecular interactions of fluid molecules. This fact strongly modifies the thermodynamic properties of confined fluids with respect to bulk phases. Tight oil and shale gas reservoirs, where the proportion of micro (below 2 nm) and mesopores (between 2 and 50 nm) can reach more than 20% of the volume distribution, represent an environment with such problems and industrial challenges to hydrocarbon fluid pressure/volume/temperature (PVT) modeling. This study provides a detailed understanding of the thermodynamic behavior of confined fluid and reference data of the thermodynamic properties of pure components (methane, ethane, n-pentane, n-decane) and mixtures (methane/ethane, ethane/n-pentane) confined in graphite slit pores. Furthermore, a detailed explanation of the different pressures considered in a porous medium with nano-pores is given. The Gibbs Ensemble Monte Carlo (GEMC) NVT simulation is used for pure components instead of the more traditional Grand Canonical Monte Carlo ensemble (GCMC) simulation to get more precise results of liquid and vapor confined pressure avoiding the phase change location determination problem. The evolution of critical temperature and pressure versus pore radius is compared to literature correlations and confined vapor and liquid densities are calculated. A new and robust method in the GEMC ensemble called GEMC NPT Bubble point Monte Carlo (BPMC) completed with GEMC NVT simulations has been developed to get thermodynamic properties including pressures at equilibrium of confined mixtures. Pressure versus density diagrams, pressure versus molar fraction isotherms and examples of pressure versus temperature diagram for a specific composition are built. The phase envelope of the confined fluid is shifted and closes with respect to phase envelope of bulk fluid. The critical temperature and pressure are shifted from the bulk value to a lower value and the bubble point pressure is decreased as the dew point pressure is increased. With regards to the selectivity of the confined system compared to the bulk fluid, for the methane/ethane and ethane/n-pentane mixtures, the heavier component is preferentially adsorbed in the vapor phase and the lighter component is preferentially adsorbed in the liquid phase. All these results for pure components and mixtures provide relevant information concerning the understanding of the phase behavior in confined systems such as shale gas and tight oil reservoirs, emphasizing the difference from the bulk fluid. Furthermore all these data may be used as references for pore radius dependent equation of state (EOS) calibration.

Advances and challenges in shale oil development: A critical review

“…Thirdly, fluids in the shale reservoir are prone to condensate due to the high proportion of light components in the fluids. Previous studies (Teklu et al, 2014;Sobecki et al, 2019) have confirmed that the physical and chemical properties of fluids confined in shale nanopores, such as interfacial tension, contact angle, critical temperature, and pressure, etc., are tremendously different from those of bulk fluids. This will lead to the fluid transport behavior in the nanoscale pore is significantly different from those predicted using classical theory, such as enhanced flow in carbon nanotube (Majumder et al, 2005;Holt et al, 2006) and strong hydrophobicity of the materials (Lafuma and Quéré, 2003).…”

“…They suggested that the variation of the critical temperature is tremendously influenced by the nature of the solid surface. Sobecki et al (2019) developed a robust NPT bubble point Monte Carlo method in the Gibbs ensemble to estimate the thermodynamic properties of confined mixtures. They reported that, in comparison to the bulk fluid, the critical temperature and pressure under confinement tend to be lower and the bubble point pressure decreases while the dew point pressure increases.…”

CO2-Fluid-Rock Interactions and the Coupled Geomechanical Response during CCUS Processes in Unconventional Reservoirs

“…In this way, a more accurate equilibrium pressure of liquid and gas can be obtained. GEMC-NPT-BPM (Bubble point Monte Carlo) was developed in conjunction with GEMC NVT simulations to obtain thermodynamic properties [39], including confined equilibrium pressure for the mixture. The simulation results show the shifts and closure of the confined fluid phase envelope relative to the bulk fluid.…”

Statistical physics and applied geosciences: some results and perspectives

“…To conclude that section, SP tools such as MD or concepts (fluctuation theory, Kubo relations, density functional), coupled with continuous improvements of the knowledge of molecular fluid and rock properties are leading to rich applications that permit quantitative and more and more predictive descriptions [32]. This fine understanding of flow in nanopores and the associated changes in thermodynamical properties due to confinement have major consequences in applied geosciences, from nuclear waste disposal, CO 2 [33][34][35], to osmotic energy conversion using fresh and sea-water [36].…”