Multicomponent Shale Oil Flow in Real Kerogen Structures via Molecular Dynamic Simulation

Anisotropic fluid flows in black phosphorus nanochannels

With the development of advanced micro/nanoscale technologies, two-dimensional materials have emerged from laboratories and have been applied in practice. To investigate the mechanisms of solid-liquid interactions in potential applications, molecular dynamics simulations are employed to study the flow behavior of n-dodecane (C12) molecules confined in black phosphorus (BP) nanochannels. Under the same external conditions, a significant difference in the velocity profiles of fluid molecules is observed when flowing along the armchair and zigzag directions of the BP walls. The average velocity of C12 molecules flowing along the zigzag direction is 9-fold higher than that along the armchair direction. The friction factor at the interface between C12 molecules and BP nanochannels and the orientations of C12 molecules near the BP walls are analyzed to explain the differences in velocity profiles under various flow directions, external driving forces, and nanochannel widths. The result shows that most C12 molecules are oriented parallel to the flow direction along the zigzag direction, leading to a relatively smaller friction factor hence a higher average velocity. In contrast, along the armchair direction, most C12 molecules are oriented perpendicular to the flow direction, leading to a relatively larger friction factor and thus a lower average velocity. This work provides important insights into understanding the anisotropic liquid flows in nanochannels.

Molecular Dynamics Simulation Study on the Occurrence of Shale Oil in Hybrid Nanopores

The molecular dynamics simulation was used to simulate the influence of the composite wall stacking effect on shale oil occurrence. The kerogen-illite heterogeneous wall pore model was established to study the effects of temperature, pore size, and wall component ratio on the adsorption ratio and diffusion capacity of shale oil. The calculation results show that the fluid density distribution in the hybrid nanopore is not uniform. When the pore size increases, the proportion of the first adsorption layer to the total adsorption amount decreases rapidly, and the phenomenon of the "solid-like layer" of shale oil in small pores is more obvious. In addition, increases in temperature have little effect on the density peak of the first adsorption layer. With increases in organic matter content in the shale pore model, the diffusion coefficient of fluid decreases gradually, along with adsorption capacity. The influence of the irregular arrangement of kerogen molecules on the adsorption of shale oil is greater than the influence of surface roughness caused by illite on the adsorption.

Molecular Alignment-Mediated Stick-Slip Poiseuille Flow of Oil in Graphene Nanochannels

The flow behavior of oil in nanochannels has attracted extensive attention for oil transport applications. In most, if not all, of the prior theoretical simulations, oil molecules were observed to flow steadily in nanochannels under pressure gradients. In this study, non-equilibrium molecular dynamics simulations are conducted to simulate the Poiseuille flow of oil with three different hydrocarbon chain lengths in graphene nanochannels. Contrary to the conventional perception of steady flows of oil in nanochannels, we find that oil molecules with the longest hydrocarbon chain (i.e., n-dodecane) exhibit notable stick-slip flow behavior. An alternation between the high average velocity of n-dodecane in the slip motion and the low average velocity in the stick motion is observed, with a drastic, abrupt velocity jolt of up to 40 times occurring at the transition in a stick-slip motion. Further statistical analyses show that the stick-slip flow behavior of n-dodecane molecules originates from the molecular alignment change of oil near the graphene wall. The molecular alignment of n-dodecane shows different statistical distributions under stick and slip motion states, leading to significant changes of friction forces and thus notable velocity fluctuations. This work provides new insights into the Poiseuille flow behavior of oil in graphene nanochannels and may offer useful guidelines for other mass transport applications.

Molecular Dynamics Simulations of Ion Transport through Protein Nanochannels in Peritoneal Dialysis

In recent decades, the development of dialysis techniques has greatly improved the survival rate of renal failure patients, and peritoneal dialysis is gradually showing dominance over hemodialysis. This method relies on the abundant membrane proteins in the peritoneum, avoiding the use of artificial semipermeable membranes, and the ion fluid transport is partly controlled by the protein nanochannels. Hence, this study investigated ion transport in these nanochannels by using molecular dynamics (MD) simulations and an MD Monte Carlo (MDMC) algorithm for a generalized protein nanochannel model and a saline fluid environment. The spatial distribution of ions was determined via MD simulations, and it agreed with that modeled via the MDMC method; the effects of simulation duration and external electronic fields were also explored to validate the MDMC algorithm. The specific atomic sequence within a nanochannel was visualized, which was the rare transport state during the ion transport process. The residence time was assessed through both methods to represent the involved dynamic process, and its values showed the temporal sequential order of different components in the nanochannel as follows: H₂O > Na⁺ > Cl^-. The accurate prediction using the MDMC method of the spatial and temporal properties proves its suitability to handle ion transport problems in protein nanochannels.

Study of the Imbibition Phenomenon in Porous Media by the Smoothed Particle Hydrodynamic (SPH) Method

Over recent decades, studies in porous media have focused on many fields, typically in the development of oil and gas reservoirs. The imbibition phenomenon, a common mechanism affecting multi-phase flows in porous media, has shown more significant impacts on unconventional reservoir development, where the effect of the pore space increases with decreased pore sizes. In this paper, a comprehensive SPH method is applied, considering the binary interactions among the particles to study the imbibition phenomenon in porous media. The model is validated with physically meaningful results showing the effects of surface tension, contact angle, and pore structures. A heterogeneous porous medium is also constructed to study the effect of heterogeneity on the imbibition phenomenon; it can be referred from the results that the smaller pore throats and wetting surfaces are more preferred for the imbibition. The results show that the SPH method can be applied to solve the imbibition problems, but the unstable problem is still a sore point for the SPH method.

Effects of Membrane Structure on Oil–Water Separation by Smoothed Particle Hydrodynamics

Membrane has been considered an effective tool for oil–water separation. By using the smoothed particle hydrodynamics (SPH) method, the effects of membrane structure on fluid separation were studied thoroughly in this paper. The oil–water two-phase fluid was generated as particles, while the membrane was built with solid particles, which was able to select the fluid particles. In general, the developed SPH method in this paper can evaluate separation performance with different membrane shapes, pore size distributions, membrane thickness and fluid properties. We suggest to the industry a potential approach to promote separation based on our simulation results, including adding the external force in the selected direction and demulsification for the bulk phase liquid particles. The triangular membrane performs well with the conditions for various parameters, as a result of its insensitivity to inhibiting factors. The effectiveness and robustness of the proposed SPH scheme was validated by a number of numerical experiments, and we assessed the optimized membrane structure and operation manners in order to improve separation efficiency and long-term safety.