Lattice Boltzmann Simulations of Supercritical CO₂–Water Drainage Displacement in Porous Media: CO₂ Saturation and Displacement Mechanism

CO₂ geosequestration in deep aquifers requires the displacement of water (wetting phase) from the porous media by supercritical CO₂ (nonwetting phase). However, the interfacial instabilities, such as viscous and capillary fingerings, develop during the drainage displacement. Moreover, the burstlike Haines jump often occurs under conditions of low capillary number. To study these interfacial instabilities, we performed lattice Boltzmann simulations of CO₂–water drainage displacement in a 3D synthetic granular rock model at a fixed viscosity ratio and at various capillary numbers. The capillary numbers are varied by changing injection pressure, which induces changes in flow velocity. It was observed that the viscous fingering was dominant at high injection pressures, whereas the crossover of viscous and capillary fingerings was observed, accompanied by Haines jumps, at low injection pressures. The Haines jumps flowing forward caused a significant drop of CO₂ saturation, whereas Haines jumps flowing backward caused an increase of CO₂ saturation (per injection depth). We demonstrated that the pore-scale Haines jumps remarkably influenced the flow path and therefore equilibrium CO₂ saturation in crossover domain, which is in turn related to the storage efficiency in the field-scale geosequestration. The results can improve our understandings of the storage efficiency by the effects of pore-scale displacement phenomena

Molecular Dynamics Simulations of Asphaltenes at the Oil–Water Interface: From Nanoaggregation to Thin-Film Formation

We have investigated the interfacial behavior of asphaltene molecules at the oil–water interface using molecular dynamics simulations. Oil precipitants and solvents are represented by heptane and toluene, respectively. It was found that asphaltenes are preferably distributed in the oil phase in the case of pure toluene, whereas they accumulate at the oil–water interface for pure heptane. Interestingly, the interfacial tension (IFT) of the interfacial system containing a small amount of asphaltene molecules is close to that of a pure heptane–water system, while the IFT of the system containing a large amount of asphaltene molecules is much reduced, ∼12 mN/m. Further, it was shown that the reduced IFT results from a complete asphaltene film formed at the oil–water interface when asphaltenes are abundant. In addition, it was found that a small amount of asphaltene molecules stacked their aromatic planes and formed a nanoscale aggregate, which exhibited an exotic molecular oscillation behavior of asphaltene molecules at the oil–water interface

Phase Behavior of Methane/<i>n</i>‑Butane Binary Mixtures in Organic Nanopores under Bulk Vapor Conditions

Nanopores can change the phase boundary of fluid mixtures. A recent study has reported that the bubble-point pressure of binary mixtures in nanopores can be merged with the dew point, showing there is no phase coexistence region but a line. On the other hand, previous molecular-scale simulations showed the existence of a phase envelope. In experiments, it is difficult to determine the composition of a mixture in nanopores. In this study, we investigated the selectivity and phase behavior of CH4/n-C4H10 binary mixtures in 10, 5, and 2 nm graphite nanopores under bulk vapor conditions by grand canonical Monte Carlo molecular simulations. The selectivity was high at low pressures, and the selectivity isotherms can be classified as classes I–III. We observed the nanopore-induced capillary condensation. When we used the bulk mole fractions to prepare the phase diagram, the dew- and bubble-point pressures were almost the same. Both were below the dew-point line of the corresponding bulk mixture. This is in good agreement with recent experiments. We observed a narrower phase envelope when we used the mole fractions of the mixture in nanopores. In general, the bubble-point pressure decreased compared with that of the bulk system, whereas the corresponding dew-point pressure increased. Furthermore, the critical pressure decreased with decreasing pore size and the supercritical region expanded accordingly. The interplay between the fluids in nanopores and bulk can yield various phase diagrams, providing us with a unified picture of the phase behavior in nanopores. The simulation results comprehensively describe the phase behavior of hydrocarbon mixtures in organic nanopores for shale gas and shale oil development

Ultraviolet Random Laser Based on a Single GaN Microwire

Random lasing (RL) from self-constructed localized cavities based on micropits scatters in a single GaN microwire (MW) was investigated. The spectra and spatial resolution of RL exhibits that the lasing modes originated from different regions in the MW. Temperature-dependent lasing measurement of GaN RL shows an excellent characteristic temperature of about 52 K. In addition, the dependence of spatial localized cavities’ dimension on the pumping intensity profile and temperature was studied by fast Fourier transform spectroscopy. For GaN RL, the optical feedback was supported by localized paths through the scattering effect of micropits in the MW. The scattering feedback mechanism for RL can avoid the enormous difficulty in fabricating artificial cavity structures for GaN. Hence, the results in this paper represent a low-cost technique to realize GaN-based ultraviolet laser diodes without the fabrication difficulty of cavity facets