Molecular insights into competitive adsorption of CO2/CH4 mixture in shale nanopores

Multiscale modeling of gas flow behaviors in nanoporous shale matrix considering multiple transport mechanisms

This study proposes a multiscale model combining molecular simulation and the lattice Boltzmann method (LBM) to explore gas flow behaviors with multiple transport mechanisms in nanoporous media of shale matrix. The gas adsorption characteristics in shale nanopores are first investigated by molecular simulations, which are then integrated and upscaled into the LBM model through a local adsorption density parameter. In order to adapt to high Knudsen number and nanoporous shale matrix, a multiple-relaxation-time pore-scale LBM model with a regularization procedure is developed. The combination of bounce-back and full diffusive boundary condition is adopted to take account of gas slippage and surface diffusion induced by gas adsorption. Molecular simulation results at the atomic scale show that gas adsorption behaviors are greatly affected by the pressure and pore size of the shale organic nanopore. At the pore scale, the gas transport behaviors with multiple transport mechanisms in nanoporous shale matrix are explored by the developed multiscale model. Simulation results indicate that pressure exhibits more significant influences on the transport behaviors of shale gas than temperature does. Compared with porosity, the average pore size of nanoporous shale matrix plays a more significant role in determining the apparent permeability of gas transport. The roles of the gas adsorption layer and surface diffusion in shale gas transport are discussed. It is observed that under low pressure, the gas adsorption layer has a positive influence on gas transport in shale matrix due to the strong surface diffusion effect. The nanoporous structure with the anisotropy characteristic parallel to the flow direction can enhance gas transport in shale matrix. The obtained results may provide underlying and comprehensive understanding of gas flow behaviors considering multiple transport mechanisms in shale matrix. Also, the proposed multiscale model can be considered as a powerful tool to invesigate the multiscale and multiphysical flow behaviors in porous media.

Predicting Adsorption of Methane and Carbon Dioxide Mixture in Shale Using Simplified Local-Density Model: Implications for Enhanced Gas Recovery and Carbon Dioxide Sequestration

Carbon dioxide (CO2) capture and storage have attracted global focus because CO2 emissions are responsible for global warming. Recently, injecting CO2 into shale gas reservoirs is regarded as a promising technique to enhance shale gas (i.e., methane (CH4)) production while permanently storing CO2 underground. This study aims to develop a calculation workflow, which is built on the simplified local-density (SLD) model, to predict excess and absolute adsorption isotherms of gas mixture based on single-component adsorption data. Such a calculation workflow was validated by comparing the measured adsorption of CH4, CO2, and binary CH4/CO2 mixture in shale reported previously in the literature with the predicted results using the calculation workflow. The crucial steps of the calculation workflow are applying the multicomponent SLD model to conduct regression analysis on the measured adsorption isotherm of each component in the gas mixture simultaneously and using the determined key regression parameters to predict the adsorption isotherms of gas mixtures with various feed-gas mole fractions. Through the calculation workflow, the density profiles and mole fractions of the adsorbed gases can be determined, from which the absolute adsorption of the gas mixture is estimated. In addition, the CO2/CH4 adsorption selectivity larger than one is observed, illustrating the preferential adsorption of CO2 over CH4 on shale, which implies that CO2 has enormous potential to enhance CH4 production while sequestering itself in shale. Our findings demonstrate that the proposed calculation workflow depending on the multicomponent SLD model enables us to accurately predict the adsorption of gas mixtures in nanopores based on single-component adsorption results. Following the innovative calculation flow path, we could bypass the experimental difficulties of measuring the multicomponent mole fractions in the gas phase at the equilibrium during the adsorption experiments. This study also provides insight into the CO2/CH4 competitive adsorption behavior in nanopores and gives guidance to CO2-enhanced gas recovery (CO2-EGR) and CO2 sequestration in shale formations.

Evaluation of Gas Adsorption in Nanoporous Shale by Simplified Local Density Model Integrated with Pore Structure and Pore Size Distribution

Simplified local density (SLD) model has been widely used to describe the gas adsorption behaviors in porous media. However, the slit pore geometry and constant pore width associated with the SLD model may fail to represent the heterogeneous pore network structure in shale. In this study, a new method to integrate the SLD model with the slit and cylindrical pore structures as well as the pore size distribution (PSD) is proposed and validated by the grand canonical Monte Carlo (GCMC) simulations and the experimentally measured adsorption of methane on shale with complex pore network. Comparison results show that reasonably good agreement is achieved between the SLD model and GCMC simulations for both the gas adsorption isotherms and discrete-density profiles in multiwalled carbon nanoslit and nanotube. The corresponding average absolute percentage deviations (% AADs) are below 0.3 and 9.3 for gas adsorption isotherm and discrete-density profile, respectively. In addition, the SLD model coupled with the PSD of slit and cylindrical pores ranging from micro- to macropores properly characterizes the measured excess adsorption of methane on Wolfcamp shale core sample with % AADs between 1.7 and 3.6. It is found that when the pore volume is fixed, the gas adsorption isotherm and gas density profile are heavily dependent on the pore geometry and pore size. Furthermore, integrating the PSD into the SLD model can guarantee the valid identification of the adsorbed- and free-gas regions in flow channels with different sizes based on the gas density profiles. The findings of this study shed light on the effects of pore structure on gas adsorption in nanopores and enable us to precisely evaluate and predict the gas adsorption behaviors in slit and cylindrical pores over a wide range of pore sizes.

Tuning the Adsorption Properties of Metal-Organic Frameworks through Coadsorbed Ammonia

In this work, we report a novel strategy to increase the gas adsorption selectivity of metal organic framework materials by coadsorbing another molecular species. Specifically, we find that addition of tightly bound NH₃ molecules in the well-known metal-organic framework MOF-74 dramatically alters its adsorption behavior of C₂H₂ and C₂H₄. Combining in situ infrared spectroscopy and ab initio calculations, we find that-as a result of coadsorbed NH₃ molecules attaching to the open metal sites-C₂H₂ binds more strongly and diffuses much faster than C₂H₄, occupying the available space adjacent to metal-bound NH₃ molecules. Most remarkably, C₂H₄ is now almost completely excluded from entering the MOF once C₂H₂ has been loaded. This finding dispels the widespread belief that strongly coadsorbed species in nanoporous materials always undermine their performance in adsorbing or separating weakly bound target molecules. Furthermore, it suggests a new route to tune the adsorption behavior of MOF materials through harnessing the interactions among coadsorbed guests.

Molecular Investigation on the Displacement Characteristics of CH4 by CO2, N2 and Their Mixture in a Composite Shale Model

The rapid growth in energy consumption and environmental pollution have greatly stimulated the exploration and utilization of shale gas. The injection of gases such as CO2, N2, and their mixture is currently regarded as one of the most effective ways to enhance gas recovery from shale reservoirs. In this study, molecular simulations were conducted on a kaolinite–kerogen IID composite shale matrix to explore the displacement characteristics of CH4 using different injection gases, including CO2, N2, and their mixture. The results show that when the injection pressure was lower than 10 MPa, increasing the injection pressure improved the displacement capacity of CH4 by CO2. Correspondingly, an increase of formation temperature also increased the displacement efficiency of CH4, but an increase of pore size slightly increased this displacement efficiency. Moreover, it was found that when the proportion of CO2 and N2 was 1:1, the displacement efficiency of CH4 was the highest, which proved that the simultaneous injection of CO2 and N2 had a synergistic effect on shale gas production. The results of this paper will provide guidance and reference for the displacement exploitation of shale gas by injection gases.

Molecular Investigation of CO2/CH4 Competitive Adsorption and Confinement in Realistic Shale Kerogen

The adsorption behavior and the mechanism of a CO2/CH4 mixture in shale organic matter play significant roles to predict the carbon dioxide sequestration with enhanced gas recovery (CS-EGR) in shale reservoirs. In the present work, the adsorption performance and the mechanism of a CO2/CH4 binary mixture in realistic shale kerogen were explored by employing grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. Specifically, the effects of shale organic type and maturity, temperature, pressure, and moisture content on pure CH4 and the competitive adsorption performance of a CO2/CH4 mixture were investigated. It was found that pressure and temperature have a significant influence on both the adsorption capacity and the selectivity of CO2/CH4. The simulated results also show that the adsorption capacities of CO2/CH4 increase with the maturity level of kerogen. Type II-D kerogen exhibits an obvious superiority in the adsorption capacity of CH4 and CO2 compared with other type II kerogen. In addition, the adsorption capacities of CO2 and CH4 are significantly suppressed in moist kerogen due to the strong adsorption strength of H2O molecules on the kerogen surface. Furthermore, to characterize realistic kerogen pore structure, a slit-like kerogen nanopore was constructed. It was observed that the kerogen nanopore plays an important role in determining the potential of CO2 subsurface sequestration in shale reservoirs. With the increase in nanopore size, a transition of the dominated gas adsorption mechanism from micropore filling to monolayer adsorption on the surface due to confinement effects was found. The results obtained in this study could be helpful to estimate original gas-in-place and evaluate carbon dioxide sequestration capacity in a shale matrix.

Effects of temperature and pore structure on the release of methane in zeolite nanochannels

In this work, we investigate the effects of temperature and pore size on the release of methane in zeolite nanochannels through molecular dynamics (MD) simulations. The methane release percentage at different temperatures and for different zeolite structures is calculated. In all-silica MFI (silicalite-1) zeolite, it is found that the release percentage increases with increasing temperature roughly at a constant rate when the temperature is below 598 K. For higher temperatures, the release percentage reaches about 90% and remains almost constant. For other structures, the release percentage is greatly affected by the average pore size. The release percentage is determined by the temperature and energy barrier inside the pores. Based on the energy barriers obtained in MD simulations, theoretical predictions of the release percentage are made, which are in good agreement with numerical results.

The effects of temperature and pore size on release of methane in zeolite nanochannels is investigated by molecular dynamics simulations.

Molecular simulation of CO2/CH4/H2O competitive adsorption and diffusion in brown coal

Carbon dioxide enhanced coalbed methane recovery (CO₂-ECBM) has been proposed as a promising technology for the natural gas recovery enhancement as well as mitigation of CO₂ emissions into the atmosphere. Adsorption and diffusion of CO₂/CH₄ mixture play key roles in predicting the performance of CO₂-ECBM project, i.e., the production of coalbed methane as well as the geological sequestration potential of carbon dioxide. In the present work, the mechanism of competitive adsorption and diffusion of CO₂/CH₄/H₂O mixture in brown coal were investigated by employing grand canonical Monte Carlo and molecular dynamics simulation. The effects of temperature and pressure on competitive adsorption and diffusion behaviours were explored. It is found that CO₂ has much stronger adsorption ability on brown coal than CH₄. The adsorption amounts of CO₂/CH₄ increase with pressure but have a decreasing trend with temperature. High adsorption selectivity of CO₂/CH₄ is observed with pressure lower than 0.1 MPa. In addition, the effects of moisture content in brown coal on the adsorption characteristics have been examined. Simulation results show that the adsorption capacities of CO₂/CH₄ are significantly suppressed in moist brown coal. The competitive adsorption of CO₂/CH₄/H₂O follows the trend of H₂O ≫ CO₂ > CH₄. Moreover, the results reveal that moisture content has great effects on the self-coefficients of CO₂/CH₄. Compared with dry coal, the self-diffusion coefficients of CO₂ and CH₄ reduce by 78.7% and 75.4% in brown coal with moisture content of 7.59 wt%, respectively. The microscopic insights provided in this study will be helpful to understand the competitive adsorption and diffusion mechanism of CO₂/CH₄/H₂O in brown coal and offer some fundamental data for CO₂-ECBM project.

Competitive adsorption and diffusion behaviours of CO₂/CH₄/H₂O in brown coal were explored by GCMC and MD simulations.