Direct Observation of the Vapor-Liquid Phase Transition and Hysteresis in 2 nm Nanochannels.

The characterization of fluid phase transitions in nanoscale pores remains a challenging problem that can significantly affect various applications, such as drug delivery, carbon dioxide storage, and enhanced oil recovery. Previous theoretical and experimental studies have shown that the fluid phase transition changes drastically when the fluid is confined within nanocapillaries with dimensions of <10 nm, potentially due to the dominance of fluid-surface interactions compared to bulk effects. However, due to challenges in performing experiments at the nanoscale, there have been limited experimental observations of the phase transition at this scale. Recent advances in lab-on-a-chip (LOC) technology have enabled the observation of many nanoscale phenomena. In this study, for the first time, we present the direct observation and visualization of n-butane vapor-liquid phase transitions in a 2 nm slit pore using LOC technology. Our experiments, for the first time, measured and directly visualized the deviation of the vapor-liquid phase transition pressure in a 2 nm slit pore compared to the associated unconfined or bulk value. We also measured the liquid-vapor phase transition pressure and observed a significant difference from the vapor-liquid phase transition pressure. We complemented our experimental observations with grand canonical ensemble Monte Carlo molecular simulations to understand the underlying molecular-level mechanisms.

Confinement-Induced Supercriticality and Phase Equilibria of Hydrocarbons in Nanopores.

For over a century, the phase behavior of bulk fluids has been described as PVT (pressure-volume-temperature) three-dimensional properties, but it has become increasingly clear that the liquid-vapor phase behavior in confined geometries is significantly altered from the bulk. Efforts have been devoted to accessing confined phase transitions using sorption, molecular simulations, and theoretical methods. However, a comprehensive picture of PVT relationships for confined hydrocarbons remains uncertain. Herein, we introduce d (confining pore diameter) as a fourth dimension, and we present PVT-d behavior of confined fluids in nanopores. For the first time, a T-d phase diagram is presented for n-hexane, n-octane, and n-decane under multiple confinement scales (37.9, 14.8, 9.8, 6.0, 4.1, 3.3, and 2.2 nm cylindrical pore diameter) using experimental differential scanning calorimetry and PVT-d equation of state theory at atmospheric pressure. As pore diameter decreases from 37.9 to 4.1 nm, the bubble point increases by as much as 15 K above bulk, until we observe behavior consistent with a supercritical state, pointing to confinement-induced supercriticality. Remarkably, experimental and theoretical findings overlap very well, showing that this approach effectively captures the phase boundaries between the liquid, vapor, and supercritical fluid regions. The model and completed EOS are additionally extended to calculation of isothermal capillary adsorption, and its validity is discussed.

Case study on combined CO2 sequestration and low-salinity water production potential in a shallow saline aquifer in Qatar.

CO2 is one of the byproducts of natural gas production in Qatar. The high rate of natural gas production from Qatar's North Field (world's largest non-associated gas field) has led to the production of significant amounts of CO2. The release of CO2 into the atmosphere may be harmful from the perspective of global warming. In this work, we study the CO2 sequestration potential in Qatar's Aruma aquifer. The Aruma aquifer is a saline aquifer in the southwest of Qatar. It occupies an area of approximately 1985 km2 on land (16% of Qatar's total area). We have developed a compositional model for CO2 sequestration in the Aruma aquifer on the basis of available log and flow test data. We suggest water production at some distance from the CO2 injection wells as a possible way to control the pore pressure. This method increases the potential for safe sequestration of CO2 in the aquifer without losing integrity of the caprock and without any CO2 leakage. The water produced from this aquifer is considerably less saline than seawater and could be a good water source for the desalination process, which is currently the main source of water in Qatar. The outcome of the desalination process is water with higher salinity than the seawater that is currently discharged into the sea. This discharge can have negative long-term environmental effects. The water produced from the Aruma aquifer is considerably less saline than seawater and can be a partial solution to this problem.

Molecular Dynamics Simulation of the Thermal Diffusion Effect in n-Alkane Binary Mixtures.

In oil and gas reservoirs, the thermal diffusion effect leads to compositional variations of hydrocarbon fluids in both horizontal and vertical directions. Compared with experimental methods, molecular dynamics (MD) simulations can cover a broader range of pressures and temperatures for the investigation of the thermal diffusion effect. However, previous MD simulations of the thermal diffusion effect for n-alkane binary mixtures have been limited to only nC5-nC10, nC6-nC10, and nC6-nC12 mixtures. In this work, for the first time, we perform a series of MD simulations on n-alkane binary mixtures, C1-C3, C1-nC4, nC7-nC12, nC7-nC16, and nC10-nCi (i = 5, 7, 8, 12, 14, 16), with different mole fractions and temperature and pressure conditions. The boundary-driven nonequilibrium molecular dynamics (BD-NEMD) with the enhanced heat exchange (eHEX) algorithm is used to generate the temperature gradient and measure the thermal diffusion effect. Additionally, a workflow for molecular simulations of thermal diffusion of n-alkane binary mixtures is proposed to ensure their repeatability and reliability. The errors for our MD simulation results are generally less than 10% compared with experimental data. Our results show that in the binary mixture, the heavy component tends to move to the cold region, while the lighter component tends to aggregate near the hot region, which is consistent with experimental observations.