Impact of Nanoporosity on Hydrocarbon Transport in Shales' Organic Matter (original) (raw)
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Subcontinuum mass transport of condensed hydrocarbons in nanoporous media
Nature Communications, 2015
Although hydrocarbon production from unconventional reservoirs, the so-called shale gas, has exploded recently, reliable predictions of resource availability and extraction are missing because conventional tools fail to account for their ultra-low permeability and complexity. Here, we use molecular simulation and statistical mechanics to show that continuum description-Darcy's law-fails to predict transport in shales nanoporous matrix (kerogen). The non-Darcy behaviour arises from strong adsorption in kerogen and the breakdown of hydrodynamics at the nanoscale, which contradict the assumption of viscous flow. Despite this complexity, all permeances collapse on a master curve with an unexpected dependence on alkane length. We rationalize this non-hydrodynamic behaviour using a molecular description capturing the scaling of permeance with alkane length and density. These results, which stress the need for a change of paradigm from classical descriptions to nanofluidic transport, have implications for shale gas but more generally for transport in nanoporous media.
Molecular simulation of shale gas adsorption and diffusion in inorganic nanopores
Molecular Simulation, 2014
We studied the structural and dynamical properties of methane and ethane in montmorillonite (MMT) slit pore of sizes 10, 20 and 30 Å using grand canonical Monte Carlo and classical molecular dynamics (MD) simulations. The isotherm, at 298.15 K, is generated for pressures up to 60 bar. The molecules preferentially adsorb at the surface as indicated by the density profile. In case of methane, we observe only a single layer, at the pore wall, whose density increases with increasing pressure. However, ethane also displays a second layer, though of low density in case of pore widths 20 and 30 Å . In-plane self-diffusion coefficient, D k , of methane and ethane is of the order of 10 26 m 2 /s. At low pressure, D k increases significantly with the pore size. However, D k decreases rapidly with increasing pressure. Furthermore, the effect of pore size on D k diminishes at high pressure. Ideal adsorbed solution theory is used to understand the adsorption behaviour of the binary mixture of methane (80%) and ethane (20%) at 298.15 K. Furthermore, we calculate the selectivity of the gases at various pressures of the mixture, and found high selectivity for ethane in MMT pores. However, selectivity of ethane decreases with increase in pressure or pore size.
Surface Diffusion in Nanopores and Its Effects on Total Mass Transport in Shale Gas Reservoirs
International Journal of Energy and Environmental Science, 2023
In the 21st century, shale gas reservoirs have emerged as a significant and valuable source of natural gas. However, their distinct characteristics, particularly the nanoscale pore throat and pore-size distribution, set them apart from conventional reservoirs. These unique features have a profound impact on the storage and flow behavior of hydrocarbons within the shale, making them challenging to exploit using conventional methods. One of the primary challenges associated with shale gas reservoirs is the confined space phase behavior, which alters the fluid properties compared to what is typically observed in a standard PVT (Pressure-Volume-Temperature) cell. In particular, the increased surface adsorption of gas molecules in the shale leads to deviations in fluid properties. This means that the properties of gas within the shale differ from those predicted by conventional models, making it crucial to understand and account for these differences to efficiently extract gas from these reservoirs. Surface diffusion is a critical parameter in assessing the transport ability of adsorbed gas in shale organic matter. Surface diffusion refers to the movement of gas molecules along the surfaces of organic matter in the shale. It is a complex process influenced by various factors. Recent research has provided some insights, indicating that the shale-methane surface diffusion coefficient has a value of around 10-16 cm 2 /g. However, accurately measuring this coefficient remains a challenge, and there is a need for a definitive and reliable method to do so. Despite the importance of surface diffusion, it has been found that its contribution to total mass transport in shale gas reservoirs is not as significant as previously anticipated. Other mechanisms, such as desorption and matrix diffusion, also play essential roles in the overall transport of gas within shale. To improve our understanding of shale gas reservoirs and optimize gas extraction, this paper proposes an interdisciplinary approach. It suggests combining insights and advances from different industries and fields of research to gain a comprehensive understanding of these complex reservoirs. By bringing together knowledge from geology, engineering, chemistry, and other relevant disciplines, researchers can develop more accurate models and strategies to unlock the full potential of shale gas reservoirs. In summary, shale gas reservoirs have revolutionized the natural gas industry in the 21st century, but their unique characteristics require a specialized approach. Surface diffusion is an important factor affecting gas transport in shale, but its contribution is not as significant as initially thought. Through interdisciplinary research, we can enhance our understanding of these reservoirs and develop more efficient methods for gas extraction.
Molecular Simulation of CH4 Nanoscale Behavior and Enhanced Gas Recovery in Organic-Rich Shale
Geofluids, 2022
Occurrence and transport are two important nanoscale behaviors in the exploitation of shale gas. Nanopores in a realistic shale organic matrix are composed of kerogen molecules, which will have a great impact on surface-gas interactions and gas nanoconfined behavior. Although there are previous studies, the physics of gas transport through shale systems remains ambiguous. In this work, cylindrical nanopore models representing different pore sizes and organic-rich shale were constructed. By applying the molecular dynamics simulation method, the occurrence characteristics and transport characteristics of CH4 in the nanopores of organic-rich shale were studied. At last, the process of the adsorbed CH4 displaced by CO2 and N2 in shale nanopores at the subsurface condition was explored. This work can provide a better understanding of gas nanoscale behavior in shale systems and assist the future design of the CO2 sequestration and enhanced gas recovery technique.
Molecular Dynamics Study of Transport and Storage of Methane in Kerogen
SPE Eastern Regional Meeting, 2016
Studying kerogen structure and its interactions with fluids is important for understanding the mechanisms involved in storage and production of hydrocarbons from shale. In this study, adsorption and transport of methane in a three dimensional type II kerogen model are studied using molecular dynamics simulations. Grand Canonical Monte Carlo (GCMC) simulations are used to simulate the adsorption of methane and NonEquilibrium Molecular Dynamics (NEMD) simulations are employed to simulate transport of methane in the kerogen model. The kerogen model prepared by Ungerer et al. (2014) is used in this study. In order to build a representative solid state model of kerogen, eight kerogen molecules are placed in a periodic cubic cell. Once the initial configuration of kerogen molecules is prepared, constant-temperature constant-volume (NVT) simulations and then constant-temperature constant-pressure (NPT) simulations are performed to obtain the final structure. For the final structure, densit...
Energy & Fuels, 2014
In this work, we use molecular simulations to determine the structural and physical properties of the organic matter present in type II shales in the middle of the oil generation window. The construction of molecular models of organic matter constrained by experimental data is discussed. Using a realistic molecular model of organic matter, we generate, by molecular dynamics simulations, structures that mimic bulk organic matter under typical reservoir conditions. Consistent results on density, diffusion, and specific adsorption are found between simulated and experimental data. These structures enable us to provide information on the fluid distribution within the organic matter, the pore size distributions, the isothermal compressibility, and the dynamic of the fluids within the kerogen matrix. This study shows that a consistent description at the molecular level combined with molecular simulations can be useful, in complement of experiments, to investigate the organic matter present in shales.
Energy Reports
Gas transport in ultra-tight rock is non-Darcian. In addition to continuum flow, there are multiple other flow mechanisms such as slip flow and pore and surface diffusion. Various multi-physics models have been put forth in the literature to forecast the apparent permeability of gas in shales and ultratight formations. However, a means of accurately describing the relative contributions of physics in multiscale pore systems remains a challenge. Moreover, it is important to explain pore size, pressure dependency, and the relationships among adsorption, diffusion, and permeability in porous media. For these reasons, a semi-analytical model is proposed to predict gas permeability according to the viscous flux, pore diffusion and surface diffusion and establish control of the adsorbed gas layer. The reliability of the equations developed was checked by validation using experimental and molecular simulation data obtained from macropore-and micropore-sized nanotubes systems respectively. Furthermore, the equations' performance for micropores was compared to existing theoretical shale permeability models. The subsequent sensitivity analysis showed that permeability is sensitive to the nanoscale geometry factor and adsorption mechanisms. Moreover, the relevance of the surface diffusion was found to increase as the pore size decreased. For instance, surface diffusion constituted over 50% of the apparent permeability below the 10 MPa and 5.0 MPa conditions in micro-and mesopore systems, respectively, while the Darcy scale phenomenon controlled the transport of gas in macropores. Across all diffusion regimes, the microstructure geometry and sorption dynamics significantly influenced the total diffusion of methane, particularly at low pressures and decreased pore sizes. The decline in reservoir pressure during production shifted the relative importance of the adsorption and diffusion mechanisms, consequently altering the apparent gas permeability. Therefore, reservoir management teams should take into account the dynamics of gas permeability at different pressures and representative pore sizes throughout the life cycle of the asset.
Lattice Boltzmann simulation of shale gas transport in organic nano-pores
Scientific reports, 2014
Permeability is a key parameter for investigating the flow ability of sedimentary rocks. The conventional model for calculating permeability is derived from Darcy's law, which is valid only for continuum flow in porous rocks. We discussed the feasibility of simulating methane transport characteristics in the organic nano-pores of shale through the Lattice Boltzmann method (LBM). As a first attempt, the effects of high Knudsen number and the associated slip flow are considered, whereas the effect of adsorption in the capillary tube is left for future work. Simulation results show that at small Knudsen number, LBM results agree well with Poiseuille's law, and flow rate (flow capacity) is proportional to the square of the pore scale. At higher Knudsen numbers, the relaxation time needs to be corrected. In addition, velocity increases as the slip effect causes non negligible velocities on the pore wall, thereby enhancing the flow rate inside the pore, i.e., the permeability. The...