As the typical unconventional reservoir, shale gas is believed to be the most promising alternative for the conventional resources in future energy patterns, attracting more and more attention throughout the world. Generally, the majority of shale gas is trapped within the tight shale rock with ultralow porosity (<10%) and ultrasmall pore size (as less as several nanometers). Thus, the accurate understanding of gas transport characteristic and its underlying mechanism through these microporous/nanoporous media is critical for the effective exploitation of shale reservoir. In this context, we present a comprehensive review on the current advances of multiscale transport simulations of shale gas in microporous/nanoporous media from molecular to pore-scale. For the gas transport in shale nanopores using molecular dynamics (MD) simulations, the structure and force parameters of various nanopore models, including organic models (graphene, carbon nanotubes, and kerogen) and inorganic models (clays, carbonate, and quartz), and flow simulation strategies (such as nonequilibrium molecular dynamics (NEMD) and Grand Canonical Monte Carlo simulations) are systematically introduced and clarified. The significant MD simulation results about gas transport characteristic in shale nanopores then are elaborated respectively for different factors, including pore size, ambient pressure, nanopore type, atomistic roughness, and pore structure, as well as multicomponent. Besides, the two-phase transport characteristic of gas and water is also discussed, considering the ubiquity of water in shale formation. For the lattice Boltzmann method (LBM) and pore network model (PNM) approaches to conduct pore-scale simulations, we briefly review its origins, modifications, and applications for gas transport simulations in a microporous/nanoporous shale matrix. Particularly, the upscaling methods to incorporate MD simulation into LBM and PNM frameworks are emphatically expounded in the light of recent attempts of MD-based pore-scale simulations. It is hoped that this Review would be helpful for the readers to build a systematical overview on the transport characteristic of shale gas in microporous/nanoporous media and subsequently accelerate the development of the shale industry.
In the past decades, shale gas has been recognized as the promising unconventional resource for global energy storage, and a clear understanding of the gas-transport characteristic within nonporous shale organic matter (i.e., kerogen) is fundamental for the effective development of shale reservoirs. In this regard, previous studies were generally conducted based on the ideally smooth nanochannels (e.g., graphite slit or tube) without considering the atomistic-scale roughness of the walls. Herein, using molecular dynamics (MD) simulations, we perform a systematical investigation on the gas-transport characteristic through amorphous organic nanopores constructed by realistic kerogen molecules. The results show that the gas-transport velocity in amorphous organic nanopores drops dramatically (40, 70, and 90%) only with tiny roughness factors (0.3, 0.6, and 1.2%) when compared with ideally smooth nanochannels. Further analysis of the potential energy surface and the particle trajectory justifies the entirely different gas-transport mechanisms in ideally smooth (surface diffusion) and relative rough (viscosity diffusion) organic nanopores. Besides, based on the insights of numerous MD simulations (pore sizes: 3−9 nm and system pressures: 5−50 MPa), a new analytical model that is able to consider the key effect of roughness factor on gas transport in organic-rich shale is developed, which is well verified with the experimental results. It is particularly found that the gas-transport capacity in organic-rich shale (∼1 nm of slippage length) would be enormously overrated as much as 2 orders of magnitude by the traditional cognition based on ideally smooth nanopores (∼100 nm of slippage length).
We studied the hard x-ray emission and the Kα x-ray conversion efficiency (η K ) produced by 60 fs high contrast frequency doubled Ti: sapphire laser pulse focused on Cu foil target. Cu Kα photon emission obtained with second harmonic laser pulse is more intense than the case of fundamental laser pulse. The Cu η K shows strong dependence on laser nonlinearly skewed pulse shape and reaches the maximum value 4x10 -4 with 100 fs negatively skewed pulse. It shows the electron spectrum shaping contribute to the increase of η K . Particle-in-cell simulations demonstrates that the application of high contrast laser pulses will be an effective method to optimize the x-ray emission, via the enhanced "vacuum heating" mechanism.
Previous attempts to characterize shale gas transport in nanopores are not fully successful due to the fact that the presence of water within shale reservoirs is generally overlooked. In addition, shale is known as a wettability-varying (hydrophilic and hydrophobic) rock depending on various components and maturity grades. Herein, toward this end, we performed a comprehensive study about two-phase transport characteristic of shale gas and water through hydrophilic and hydrophobic nanopores by integrating the molecular dynamics (MD) simulations and analytical models. Using MD simulations, we showed that water molecules prefer to accumulate at the walls (water film) in hydrophilic nanopores while form the water cluster at the center region of hydrophobic nanopores, which significantly alters the shale gas transport behavior. For hydrophilic nanopores, the existence of water film weakens the gas–walls collisions (slip effect), resulting in a viscosity dominant transport mechanism. In contrary, shale gas transport in hydrophobic nanopores is mainly contributed by slip effect where the gas–gas collisions (viscosity) is abated by the water cluster. On this basis, we proposed an analytical model to quantitatively depict the shale gas transport behavior in moist nanopores, which is well verified by MD simulations results. Particularly, according to our flow model, the gas transport capacity decreases to only 15% when mixing with 50% water molecules for both hydrophilic and hydrophobic nanopores, which would be greatly overestimated by traditional models neglecting the presence of water molecules. The deep insights gained in this work will further the exploitation and development of shale reservoirs.
Bioinspired control of ion transport at the subnanoscale has become a major focus in the fields of nanofluidics and membrane separation. It is fundamentally important to achieve rectifying ion-specific transport in artificial ion channels, but it remains a challenge. Here, we report a previously unidentified metal-organic framework nanochannel (MOF NC) nanofluidic system to achieve unidirectional ultrafast counter-directional transport of alkaline metal ions and proton. This highly effective ion-specific rectifying transport behavior is attributed to two distinct mechanisms for metal ions and proton, elucidated by theoretical simulations. Notably, the MOF NC exhibits ultrafast proton conduction stemming from ultrahigh proton mobility, i.e., 11.3 × 10 −7 m 2 /V·s, and low energy barrier of 0.075 eV in MIL-53-COOH subnanochannels. Furthermore, the MOF NC shows excellent osmotic power–harvesting performance in reverse electrodialysis. This work expects to inspire further research into multifunctional biomimetic ion channels for advanced nanofluidics, biomimetics, and separation applications.
Nanovaccines have emerged as promising alternatives or complements to conventional cancer treatments. Despite the progresses, specific co‐delivery of antigen and adjuvant to their corresponding intracellular destinations for maximizing the activation of antitumor immune responses remains a challenge. Herein, a lipid‐coated iron oxide nanoparticle is delivered as nanovaccine (IONP‐C/O@LP) that can co‐deliver peptide antigen and adjuvant (CpG DNA) into cytosol and lysosomes of dendritic cells (DCs) through both membrane fusion and endosome‐mediated endocytosis. Such two‐pronged cellular uptake pattern enables IONP‐C/O@LP to synergistically activate immature DCs. Iron oxide nanoparticle also exhibits adjuvant effects by generating intracellular reactive oxygen species, which further promotes DC maturation. IONP‐C/O@LP accumulated in the DCs of draining lymph nodes effectively increases the antigen‐specific T cells in both tumor and spleen, inhibits tumor growth, and improves animal survival. Moreover, it is demonstrated that this nanovaccine is a general platform of delivering clinically relevant peptide antigens derived from human papilloma virus 16 to trigger antigen‐specific immune responses in vivo.
In recent years, since the fast mass transport emerges from some low-dimensional nanostructures (e.g., graphene and CNTs), the applicability of the continuous model (e.g., hydrodynamics) for describing the nanoscale flow has been intensely challenged, even for shale gas. As the typical tight rock with numerous nanopores, most scholars considered that gas transport capacity (permeability) within a shale organic matrix (i.e., kerogen) would be significantly enhanced as well, due to the nanoscale slip effect. Herein, we perform comprehensive molecular dynamics (MD) simulations to reveal the realistic gas transport behavior through shale kerogen nanopores. The results show that, interestingly, all the velocity profiles for different kerogen (type I, II, and III) nanopores display no-slip parabolic shape, which are quantitatively satisfied with the continuous model under various conditions, including pressure drop (0.25−1 MPa), pore size (2−8 nm), and ambient pressure (5−50 MPa) and temperature (300−390 K). In particular, using potential energy surface (PES) and particle trajectory capture (PTC) technologies, we find that, confined by the rough kerogen walls and ultra-high reservoir pressure, the methane molecules collide with the walls frequently but just go round and round without moving along the walls (confined reflection), and thus, the tangential momentum (slip velocity) is negligible at the walls. Importantly, this work demonstrates that traditional consideration of the slip effect is redundant for methane transport in organic shale (kerogen) nanopores and will overestimate the gas permeability immensely (as much as 100 times). These new insights would be helpful for the precise understanding and accurate modeling of methane transport within nanoporous shale rocks.
Enhanced gas recovery (EGR) is believed to be a promising technology to improve the production of shale gas reservoirs and simultaneously reduce the emissions of greenhouse gas via the injection (sequestration) of carbon dioxide, to which great effort has been devoted by scholars. However, traditional investigations are generally limited to the ideal model of nanochannels and statistic characterization of competitive adsorption, neglecting the nanoporous structure of the kerogen matrix and the complex dynamic behavior during the EGR process. In this work, we present a comprehensive study of the EGR process in a realistic kerogen pore network (matrix) which is obtained from the artificial pyrolysis of bulk kerogen through reactive force field molecular dynamics (ReaxFF MD) simulations. The influence of pore properties (e.g., porosity) of the kerogen matrix under different maturities, and the proportions (i.e., methane and carbon dioxide) of injection gas with various injection pressures are revealed and meticulously discussed. In addition, the underlying mechanisms including diffusion and displacement effects behind the EGR process are analyzed by combining them with with particle trajectory capture technology. In particular, based on the MD simulation results, an analytical model to depict the dynamic recovery process in the kerogen matrix is proposed by coupling consideration of recovery time and capacity, which are examined against the simulation and experimental data. The hybrid recovery strategy is developed by utilizing the advantages of depressurization and gas-injection recoveries to achieve the optimization of both recovery time and capacity. The insights acquired from this work would be helpful for efficient exploitation of shale gas reservoirs and pave the way to capture the realistic EGR processes within the kerogen matrix from molecular and theoretical perspectives.
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