Fluid Density Distribution Research Articles

Inhomogeneous Fluid Transport Modeling in Dual-Scale Porous Media Considering Fluid-Solid Interactions.

Dynamics of fluid transport in ultratight reservoirs such as organic-rich shales differ from those in high-permeable reservoirs due to the complex nature of fluid transport and fluid-solid interaction in nanopores. We present a multiphase multicomponent transport model for primary production and gas injection in shale, considering the dual-scale porosity and intricate fluid-solid interactions. The pore space in the shale matrix is divided into macropores and nanopores based on pore size distribution. We employ density functional theory (DFT) to account for fluid-solid interactions and to compute the inhomogeneous fluid density distribution and phase behavior within a dual-scale matrix. The calculated fluid thermodynamic properties and transmissibility values are then integrated into the multiphase multicomponent transport model grounded in Maxwell-Stefan theory to simulate primary oil production from and gas injection into organic-rich shales. Our findings highlight DFT's adeptness in detailing the complex fluid inhomogeneities within nanopores─a critical concept that a cubic equation of state does not capture. Fluids within pores are categorized into confined and bulk states, restricted by a threshold pore width of 30 nm. Different compositions of fluid mixtures are observed in macropores and nanopores: heavier hydrocarbon components preferentially accumulate in nanopores due to their strong fluid-solid interactions. We utilize the developed model to simulate hydrocarbon production from an organic-rich shale matrix as well as CO2 injection into the matrix. During primary hydrocarbon production, strong fluid-solid interactions in nanopores impede the mobility of heavy components in the near-wall region, leading to their confinement. Consequently, heavy components mostly remain within the nanopores in the shale matrix during primary hydrocarbon production. During the CO2 injection process, the injected CO2 alters fluid composition within macropores and nanopores, promoting fluid redistribution. Injected CO2 engages in competitive fluid-solid interactions against intermediate hydrocarbons, successfully displacing a considerable number of these hydrocarbons from the nanopores.

Studies into confined methane adsorption in shale nanopores using a simplified local density model

The large number of nanopores in shale reservoirs provide an effective storage space for gases, which can assist in accurately characterizing the confined adsorption characteristics, including the adsorption amount of fluid in nanopores that is important for a detailed understanding of shale gas storage mechanisms and production prediction. Although the simplified local density (SLD) model has been widely used to describe the behavior of confined adsorption in slit-like pores, its application in relation to cylindrical pores has not been studied in detail. In this study, the importance of fluid–fluid interaction and fluid–solid interaction has been analyzed, with the expressions for the distribution of confined attractive parameter in cylindrical pores are derived by combining with the (3/8)σff assumption to establish a new SLD model. The reliability of the new model was verified by the publicly available experimental methane excess adsorption data for shale at different temperatures and the simulation results from the Grand Canonical Monte Carlo (GCMC). Moreover, by comparing with the SLD model with the slit pore structure, it was found that the adsorption isotherms and the confined fluid density distribution pattern characterized by the SLD model with the cylindrical pores were closer to the actual reservoir conditions of shale. In addition, the densities characterized by SLD models with a (3/8)σff assumption and with a (1/2)σff assumption at different Λb have been discussed and analyzed, and it is found that the selection of a suitable Λb would help to improve the accuracy of SLD model. The results might provide new theoretical insights into confined methane adsorption, with the new SLD model providing better engineering applications in shale gas development, gas storage, and pore structure characterization.

The heat transfer mechanism coupling pyrolysis and flow of hydrocarbon fuel in heated tube under supercritical pressure

The coupling mechanism between pyrolysis and fluid flow of endothermic hydrocarbon fuels plays a crucial role in the design of regenerative cooling systems for advanced aircraft. In this work, the convective heat transfer for HF-1 with pyrolysis including 18 species and 24 reactions is numerically investigated in a horizontal circle tube under supercritical pressure (5 MPa). The results show that the mechanism of small molecule products on different heat transfer phenomena varies. In the inlet heat transfer deterioration region, small molecule products exacerbate the non-uniform radial distribution of fluid density and dynamic viscosity. At the outlet position, it significantly enhances the physical heat sink of the mixture with its high thermal conductivity and specific heat capacity. Additionally, the aromatic hydrocarbons significantly contribute to non-uniformities in thermal conductivity and specific heat capacity due to the low specific heat capacity and thermal conductivity in inlet HTD region. When the radial non-uniformity coefficient of fuel conversion keeps between −0.1 and 0, the pyrolysis of the fuel enhances heat transfer, remarkably.

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 dynamics investigation of shale oil occurrence and adsorption in nanopores: Unveiling wettability and influencing factors

Shale oil, an unconventional petroleum resource, has gained significant attention in the industry. Understanding how shale oils exist in nanopores is crucial for unraveling their flow mechanisms and production prediction. The molecular dynamics simulations are conducted to explore oil occurrence and adsorption in various mineral nanopores, and the wettability and influencing factors. This study reveals a non-uniform fluid density distribution within nanopores, decreasing from the solid wall to the center. Distinct differences are observed between adsorbed and free-state oil phases. The effects of temperature, pressure, mineral type, and pore size effects on wettability, adsorption layer thickness, and molecule proportion are studied. Adsorption energy is calculated using physicochemical methods. Findings show temperature and mineral properties primarily influence shale oil wettability, with less impact from pressure. Adsorption capacity and layer thickness are affected by temperature, mineral type, and size. Octane distribution in nanopores is non-uniform, with a 1.5–1.75 nm layer on non-polar mineral walls and 0.67–1.35 nm on polar minerals. A positive correlation exists between shale oil adsorption on mineral nanopores, layer thickness, and adsorption energy. This study enhances our understanding of oil behavior in nanopores of shale oil reservoirs.

Adsorption behavior of shale oil and water in the kerogen-kaolinite pore by molecular simulations

It is very meaningful to study the adsorption behavior of oil and water in shale nanopores in the context of enhanced oil recovery. Whereas, the investigation of shale and water adsorption in organic–inorganic pores is scarce. In this work, molecular dynamics (MD) simulations were used to study the adsorption behavior of multi-component shale oil and water in the novel kerogen-kaolinite pores. The adsorption characteristics of shale oil were studied through fluid density distribution, and the interaction energies between the fluid and the walls were calculated to describe the adsorption strength. The results show that different roughness, wettability, and polarity of the organic and inorganic walls caused the adsorption characteristics and strengths of shale oil and water to be asymmetrical. In the kerogen-gibbsite pore, water can form a film and adsorb near the hydrophilic gibbsite surface, while the shale oil exhibits heterogeneous adsorption on the other side of the pore. The presence of water will shield the influence of hydrophilic surfaces on the adsorption behavior of shale oil. Moreover, the active components of shale oil will form hydrogen bonds with water and regularly arrange near the water film, which makes it easy to capture the asphaltene clusters and block the pores. In the kerogen-siloxane pore, the adsorption peak of shale oil near the hydrophobic siloxane wall is much higher than that near the kerogen wall, while the adsorption strength is lower than that of the kerogen wall. Furthermore, we investigated the effects of water content, saline, and deprotonation (protonation) of active components on fluid adsorption behavior. Our study provides an in-depth understanding of shale oil and water adsorption in complex nanopores, especially the influence of wall wettability on the adsorption behavior.

Effects of Numerical Schemes of Contact Angle on Simulating Condensation Heat Transfer in a Subcooled Microcavity by Pseudopotential Lattice Boltzmann Model

Various numerical schemes of contact angle are widely used in pseudopotential lattice Boltzmann model to simulate substrate contact angle in condensation. In this study, effects of numerical schemes of contact angle on condensation nucleation and heat transfer simulation are clarified for the first time. The three numerical schemes are pseudopotential-based contact angle scheme, pseudopotential-based contact angle scheme with a ghost fluid layer constructed on the substrate with weighted average density of surrounding fluid nodes, and the geometric formulation scheme. It is found that the subcooling condition destabilizes algorithm of pseudopotential-based contact angle scheme. However, with a ghost fluid layer constructed on the substrate or using geometric formulation scheme, the algorithm becomes stable. The subcooling condition also decreases the simulated contact angle magnitude compared with that under an isothermal condition. The fluid density variation near a microcavity wall simulated by pseudopotential-based contact angle scheme plays the role of the condensation nucleus and triggers “condensation nucleation”. However, with a ghost fluid layer constructed on the substrate or using geometric formulation scheme, the simulated fluid density distribution near the wall is uniform so that no condensation nucleus appears in the microcavity. Thus, “condensation nucleation” cannot occur spontaneously in the microcavity unless a thin liquid film is initialized as a nucleus in the microcavity. The heat flux at the microcavity wall is unphysical during the “condensation nucleation” process, but it becomes reasonable with a liquid film formed in the microcavity. As a whole, it is recommended to use pseudopotential-based contact angle scheme with a ghost fluid layer constructed on the substrate or use the geometric formulation scheme to simulate condensation under subcooling conditions. This study provides guidelines for choosing the desirable numerical schemes of contact angle in condensation simulation by pseudopotential lattice Boltzmann model so that more efficient strategies for condensation heat transfer enhancement can be obtained from numerical simulations.

Thermoacoustic waves and Piston Effect inside a microchannel with Carbon Dioxide near critical conditions

• Adiabatic thermalization inside microchannels. • Thermoacoustic waves inside microchannels. • Computational Fluid Dynamics study of Piston Effect ( PE ) under terrestrial and Zero-gravity conditions. • Existence and Quantification of PE in forced convection. • PE in near-critical carbon dioxide. • Comparison of acoustic time scales and diffusion time scales. Heat transfer near the critical condition of Carbon Dioxide due to thermo-acoustic waves in a 100-µm high microchannel was numerically studied. The fluid temperature outside the thermal boundary layer was compared between computational fluid dynamics (CFD) models and a pure conduction model. The comparison revealed that the CFD model predicted a temperature increase in the bulk fluid much faster than the time constant required for such increase purely by conduction. It is believed that another heat transfer process, termed the piston effect ( PE ), which is associated with pressure waves in the fluid, was responsible for this increase. Explicit unsteady fluid model indicated that propagation of pressure waves due to the rapid expansion of the boundary layer and the associate change in the fluid density distribution resulted in this temperature rise. It was confirmed that natural convection wasn’t responsible for the temperature increase under quiescent conditions and it was revealed that the PE time scales are 5 to 6400 times faster than diffusion time scales. In addition, it was discovered that the PE is significant for certain forced convection conditions that transfer 5-7% of the total power input to the bulk fluid. A new correlation was proposed to correlate the temperature rise in the bulk fluid in terms of Fourier and Reynolds numbers.

A novel DEM-based pore-scale thermal-hydro-mechanical model for fractured non-saturated porous materials

For fracture propagation, a novel DEM-based pore-scale thermal-hydro-mechanical model of two-phase fluid flow with heat transfer in non-saturated porous materials with low porosity was developed. Numerical computations were performed for bonded granular specimens, using a DEM fully coupled with CFD (based on a fluid flow network) and heat transfer, which integrated discrete mechanics with fluid mechanics and heat transfer at the meso-scale. Both the fluid (diffusion and advection) and bonded particles (conduction) were involved in heat transfer. The numerical findings of the coupled thermal-hydraulic-mechanical (THM) model were first compared to the analytical solution of the classic 1D heat transport problem. The numerical and analytical outcomes were in perfect agreement. Advection's impacts on the cooling of a bonded particle assembly were next numerically demonstrated for low and high Peclet numbers. Finally, the THM model's utility was proved in a thermal contraction test employing a bonded particle assembly during cooling, which resulted in the creation of a macro-crack. The effects of a macro-crack on the distribution of fluid pressure, density, velocity, and temperature were studied.

Thermodynamic modeling for confined fluids in nanopores using an external potential-advanced equation of state

Quantitatively understanding thermodynamic properties of confined fluids in nanoporous media is of great significance to the development of shale gas. Due to the complex intermolecular forces in the nanopore, it is difficult to accurately predict the thermodynamic properties of fluid molecules. Focusing on the fluid-fluid and fluid-wall molecules interactions in shale reservoir system, the thermodynamic model of fluid molecules in pores is constructed, and a modified equation of state is proposed by systematically coupling the original Soave-Redlich-Kwong equation of state with Tjatjopoulos-Feke-Mann potential model in this work. The advanced EoS could facilitate a good prediction on thermodynamic properties of confined fluids without any introduction of new empirical parameters. For verification, fluid density as the important thermodynamic property was targeted and pure methane at a wide pressure range was employed to represent the fluid. The results indicated that the calculated densities accord well with the reported ones in the free gas zone. The deviation of discrete density ranges from 0.239% to 1.7329%. The fluid density distribution in the nanopores is found to be nonuniform, exhibiting a greater value near the wall than that in the pore center, which would be ascribed to the more dominant fluid wall molecule interaction. For example, the local density is 16.90 kg/m3 in the pore center, while it increases to 26.67 kg/m3 in the region which is 0.76 nm to the wall at 350 K, 3 MPa, and 5 nm (radius). Moreover, effects of other critical factors on fluid density distribution were also conducted, and it was indicated that higher pressure, lower temperature, and smaller pore size could be favorable for the occurrence of confined fluid. In general, the novel EoS could provide a quantitative and simple method in predicting the thermodynamic properties of confined fluids relating to applications of shale gas storage and exploitation.

Linking theoretical and simulation approaches to study fluids in nanoporous media: Molecular dynamics and classical density functional theory

We propose an approach that combines classical density functional theory (DFT) and molecular dynamics (MD) simulation to study fluid behavior in nanopores in contact with bulk fluid (macropores). The approach consists of two principal steps: a DFT calculation of fluid composition and density distribution in a nanopore under specified thermodynamic conditions and an MD simulation of the confined system with obtained characteristics. Thus, we investigate an open system in a grand canonical ensemble. This method allows us to identify both structural and dynamic properties of confined fluid at given bulk conditions. Our approach does not require a computationally expensive simulation of a bulk reservoir. This work presents equilibrium density profiles of pure methane, ethane and carbon dioxide and their binary mixtures in slit-like nanopores with carbon walls. Good agreement between the structures obtained by theory and simulation confirms the applicability of the proposed method.

Development of unsteady natural convection in a square cavity under large temperature difference

To investigate how the nonuniform fluid density distribution caused by large temperature variations affects the development of unsteady natural convection, we perform a series of direct numerical simulations of two-dimensional compressible natural convection in an air-filled square cavity. The cavity has a hot wall on the left and a cold wall on the right, and two horizontal walls are adiabatic. The simulations are done using a kinetic approach based on a modeled Boltzmann equation, from which the fully compressible Navier–Stokes–Fourier equations are recovered. No Boussinesq approximation or low Mach number approximation is made. An extra source term is introduced to adjust the fluid Prandtl number. Simulations are performed for a range of Rayleigh numbers (107−109) with a fixed dimensionless temperature difference of ε=0.6 to determine the critical Rayleigh number and study the development of unsteady flow. To illustrate the instability mechanism, instantaneous fluctuation field, time trace of temperature, and velocity at selected monitoring points, the spectrum and other statistics are presented and discussed. As expected, significant differences are observed between the instability of compressible natural convection and the Boussinesq-type natural convection. With a large temperature difference, the transition to unsteady flow is asymmetric for the flows near the hot wall and cold wall. For the Rayleigh number range we studied, the cold wall region is dominated by low-frequency impact instability of the boundary thermal jet at the bottom corner. For the hot wall region, besides the upper corner impact instability, a boundary layer instability featuring high-frequency oscillations is observed.

Numerical investigation on the cooling performance of a novel jet cooler design for a supercritical CO2 turbine rotor shaft cooling

Numerical investigation was carried out to study the heat transfer performance for a high-speed rotating cylindrical surface subjected to single row array round jets impingement, under a very small gap spacing. Various parameters that affect heat transfer, such as the fluid density, flow velocity and Nusselt number distributions of the radius clearance were studied based on varied nozzle to target surface spacing H and mass flow rate. It has been found that the fluid density was a dominant factor and the velocity was the secondary factor for the gas jet heat transfer performances. The overall heat transfer was improved with a reduction in the number of nozzles, for given inlet mass flow rate boundary conditions. The decrease of H/di (di, nozzle diameter) may have positive or negative effects on the heat transfer performance from the impingement surface. Reducing the radius gap H, for a certainty, increases the average density of the fluid in the clearance, which is desirable in applications that enhance heat transfer performance. But when the radius gap (H) is small enough, increasing di may have a negative impact on heat transfer.

Influence of gas aggregation on water-solid interface: molecular simulation

ABSTRACT In this study, we conducted a series of molecular dynamics simulations of gas accumulation on homogeneous graphite substrates with a focus on the structure of fluid in the region near the substrate. The fluid density distribution revealed that epitaxial layers and gas enrichment layer arise from the adsorption of fluid particles on solids. Our analysis suggests that interfacial gas molecules transform into a liquid-like state and water molecules tend to cluster within the thin region adjacent to the substrate as the strength of gas–solid interaction increases. We then calculated the argon-water interfacial tension using three different methods: the Young-Laplace equation, direct integration method, and force balance method. Our analysis suggests that higher pressure within the gaseous domain tends to reduce tension at the curved argon-water interface. Although widely adopted to describe tension at gas–liquid interfaces, our results indicate that Bakker's equation is not necessarily suitable for the calculation of tension at fluid-solid interfaces. Our results indicate that fluid-solid interfacial properties and the morphology of the gaseous domain are influenced by the liquid-like epitaxial gas layer and gas enrichment layer, which could be attributed to the highly energetic nature of gas molecules and the tendency of gasses to adsorb on solids.

Density distribution characteristics of fluid inside vortex in supersonic mixing layer

Based on the large eddy simulation, the boundary of a vortex and the coordinates of its core are both obtained by using the Lagrangian coherent structure method and the location extraction method of the vortex core, and thus the method of representing fluid density inside a vortex is proposed. The density distribution characteristics of fluid inside the vortex in a supersonic mixing layer are revealed by analyzing the changes in density of the fluid inside a vortex under different conditions (e.g. spatial size of the vortex, compressibility of the supersonic mixing layer, and merging process of the two paired vortices) as follows. For the weak and medium compressive supersonic mixing layers, the density distribution of the fluid inside a vortex is symmetrical about both the flow direction (<i>x</i>-axis) and longitudinal direction (<i>y</i>-axis), the fluid density at the vortex core is lowest while it is highest at the vortex boundary, and fluid density increases monotonically and nearly uniformly along the ray connecting the vortex core and the vortex boundary. For the strongly compressible supersonic mixing layer, however, the density distribution of the fluid inside the vortex is no longer symmetrical about any flow direction and moreover it shows the fluctuation characteristics of fluid density distribution. With the increase of the spatial size of a vortex and the compressibility of a supersonic mixing layer, the fluid density at the vortex core decreases (the maximum reduction is about 31%–56%) while it changes about 6%–27% at the vortex boundary. In the merging process of two adjacent vortices, the variation of fluid density in the two vortices is slight, which shows that the merging process is probably of a peer-to-peer combination of fluid inside the two adjacent vortices. Considering the practical engineering applications, the density distribution characteristics of fluid inside the vortex in the supersonic mixing layer with different inflow densities of its upper and lower layers are also investigated, and the results show that the density distribution of the fluid inside a vortex is symmetrical about the longitudinal direction (<i>y</i>-axis), but not the flow direction (<i>x</i>-axis). It is also found that the density distribution near the vortex boundary is determined by the inflow density there, so a good strategy of reducing the aero-optical effects caused by the supersonic mixing layer is that the difference in density between the upper and lower layers should be as small as possible.

Viscous heating and temperature profiles of liquid water flows in copper nanochannel

Understanding nanoscale fluidic transport becomes increasingly important due to the rapid development of nanotechnology and nanofabrication. By using molecular dynamics (MD) simulations, we investigated the viscous heating of water flows in copper nanochannels. The two scenarios that were studied are Couette flows and Poiseuille flows. We observed the scale effects on the distribution of fluid density, streaming velocity, fluid viscosity, and temperature across the channel. The results revealed the significant effects of surface forces on causing a large deviation between simulation results and classical hypothesis. We found that the energy equation coupled with the thermal-slip boundary conditions still fails to predict the temperature distributions. Hereby, further scale effects are taken into account, which leads to better predictions. The model that we developed in this study shows the relative deviation to the simulation data within 5 %, which is small compared to the conventional continuum approach (i.e., up to 51 %).

Reducing the effects of compressibility in DPD-based blood flow simulations through severe stenotic microchannels

Viscous fluid flow simulations based on dissipative particle dynamics (DPD) may bear compressible flow effects when flowing through severe stenotic geometries. This is caused by the soft repulsive potential employed in the DPD force field, which limits the particle-based fluid system ability to sustain a large degree of compression. To mitigate this problem, a Morse potential was added to the DPD force field. We studied the fluid properties of the modified fluid model (DPD–Morse) and compared it with a previously published conventional DPD based fluid model. Our DPD–Morse model demonstrated reduced compressibility while preserving other fluid properties such as the fluid density and viscosity. We further investigated the fluid flow properties for a severe 3D stenotic microchannel with a 67% stenosis, using the two models. The DPD fluid model presented a significant density gradient along the flow direction, where the fluid density increased upstream towards the stenosis and decreased downstream from the stenosis before regaining its initial value. In contrast, the DPD–Morse model demonstrated a far better uniform fluid density distribution along the flow direction. We compared both solutions with CFD simulations. The DPD–Morse fluid resembled the behavior of the continuum fluid model whereas DPD fluid deviated from it. To estimate the effect that the difference between the two DPD formulations may have on the platelet activation potential, we have further embedded a platelet model within the flow field and investigated the shear stress accumulation along the platelet transport trajectory. In the stenotic section, the DPD fluid demonstrated a larger stress gradient than the DPD–Morse fluid. The platelet transport period was shorter for the DPD fluid as it generated a larger fluid density gradient that overestimated the acceleration of the platelet through the stenosis. With reduced fluid compressibility, our modified DPD–Morse fluid model was more accurate than the DPD fluid model when computing the platelet activation potential in a severe stenosis.

Identifying two regimes of slip of simple fluids over smooth surfaces with weak and strong wall-fluid interaction energies.

The slip behavior of simple fluids over atomically smooth surfaces was investigated in a wide range of wall-fluid interaction (WFI) energies at low shear rates using non-equilibrium molecular dynamics simulations. The relationship between slip and WFI shows two regimes (the strong-WFI and weak-WFI regimes): as WFI decreases, the slip length increases in the strong-WFI regime and decreases in the weak-WFI regime. The critical value of WFI energy that separates these regimes increases with temperature, but it remains unaffected by the driving force. The mechanism of slip was analyzed by examining the density-weighted average energy barrier (ΔE¯) encountered by fluid atoms in the first fluid layer (FFL) during their hopping between minima of the surface potential. We demonstrated that the relationship between slip and WFI can be rationalized by considering the effect of the fluid density distribution in the FFL on ΔE¯ as a function of the WFI energy. Moreover, the dependence of the slip length on WFI and temperature is well correlated with the exponential factor exp(-ΔE¯/(kBT)), which also determines the critical value of WFI between the strong-WFI and weak-WFI regimes.

Modeling the density profiles and adsorption of pure and mixture hydrocarbons in shales

The production from shale resources in the US has shifted from the gas window to the condensate and oil windows recently due to the low natural gas price. Liquid-rich shales, such as Barnett, Woodford and Eagle Ford Shales, etc., are brought more attentions than ever before. Therefore, it is critical to understand the fluid phase behavior and their impacts on production in the condensate systems.Fluid phase behavior in porous media is governed by not only fluid molecule–fluid molecule interactions but also fluid molecule-porous media wall interactions. In the shale formations, a large amount of hydrocarbons are stored within the organic matters where the pore sizes are in the order of nanometers. Inside these nanopores, the interactions between the fluid molecules and porous walls play such an important role that can change the fluid properties of the stored fluids. Our work focused on the predictions of fluid density distributions of both dry gas and liquid rich systems inside nanoporous media. Simplified Local-Density theory coupled with Modified Peng–Robinson Equation of State was used to predict the density profiles of pure and mixture hydrocarbons in confined pores. Adsorption isotherms were generated based on the density profiles calculated. The adsorption isotherms of pure methane and the methane/ethane binary mixture were calculated and compared to experimental data and molecular simulation results in the literature with excellent accuracy.Our results showed that due to the fluid-wall interactions, the fluid density is not uniformly distributed across the pore width. In general, the fluid density is higher near the porous media wall than that in the center of the pore. It also showed the fluid density profiles are temperature, pressure, pore size and fluid composition dependent. In general, the adsorbed amount increased by increasing pressure and decreased by increasing temperature. The pore size range of interest is from 2nm to 20nm. In order to present the condensate system, a binary mixture of 80% methane and 20% n-butane was used. It was found that fluid composition for the fluid mixture was not uniformly distributed across the pore. Heavier component (n-butane) tended to accumulate near the wall while lighter component (methane) would like to stay in the center region of the pore. For the methane/ethane binary mixture, the composition of methane in confined space was found much smaller than the bulk methane composition. For example, the methane composition in confined silicalite is 7% when the bulk methane composition is 50% at 355kPa and 300K.

Flow of methane in shale nanopores at low and high pressure by molecular dynamics simulations.

Flow in shale nanopores may be vastly different from that in the conventional permeable media. In large pores and fractures, flow is governed by viscosity and pressure-driven. Convection describes the process. Pores in some shale media are in nanometer range. At this scale, continuum flow mechanism may not apply. Knudsen diffusion and hydrodynamic expressions such as the Hagen-Poiseuille equation and their modifications have been used to compute flow in nanopores. Both approaches may have drawbacks and can significantly underestimate molecular flux in nanopores. In this work, we use the dual control volume-grand canonical molecular dynamics simulations to investigate methane flow in carbon nanopores at low and high pressure conditions. Our simulations reveal that methane flow in a slit pore width of 1-4 nm can be more than one order of magnitude greater than that from Knudsen diffusion at low pressure and the Hagen-Poiseuille equation at high pressure. Knudsen diffusion and Hagen-Poiseuille equations do not account for surface adsorption and mobility of the adsorbed molecules, and inhomogeneous fluid density distributions. Mobility of molecules in the adsorbed layers significantly increases molecular flux. Molecular velocity profiles in nanopores deviate significantly from the Navier-Stokes hydrodynamic predictions. Our molecular simulation results are in agreement with the enhanced flow measurements in carbon nanotubes.