Adsorption Behavior of Methane on Kaolinite

Study on the Adsorption Law of n-Pentane in Silica Slit Nanopores

As the main components of shale, inorganic minerals are important carriers for oil and gas adsorption, whose pore structures and surface properties have significant effects on the fluid adsorption capacity. In this study, slit nanopores (SNPs) were constructed by silica. To investigate the microscopic adsorption law of n-pentane in silica, the grand canonical Monte Carlo (GCMC) method was used to simulate the adsorption behaviors of n-pentane in silica nanoparticles. The effects of different surface wettability, pore size, temperature, and pressure values on the adsorption behavior of pentane were discussed, revealing the micro adsorption mechanism of pentane in silica with different pore sizes and wettability and evaluating the degree of oil and gas utilization. The research results indicate that the adsorption capacity of pentane is greatly affected by the temperature under low-pressure conditions. With the increase of the pore size, the adsorption capacity of pentane increases linearly, and the number of adsorbed pentane molecules gradually decreases. The availability of oil and gas increases, and the oil and gas are more easily extracted. As the surface hydrophobicity of minerals increases, the van der Waals force between minerals and pentane also increases, leading to an increase in the number of adsorbed states of pentane. The stronger the hydrophilicity of the wall, the fewer the pentane molecules adsorbed on the surface, which would improve the efficiency of oil and gas extraction. This study provides potential for the development of novel surfactants based on adsorption selectivity.

Giant Effect of CO2 Injection on Multiphase Fluid Adsorption and Shale Gas Production: Evidence from Molecular Dynamics

Carbon dioxide (CO2) injection in unconventional gas-bearing shale reservoirs is a promising method for enhancing methane recovery efficiency and mitigating greenhouse gas emissions. The majority of methane is adsorbed within the micropores and nanopores (≤50 nm) of shale, which possess extensive surface areas and abundant adsorption sites for the sequestration system. To comprehensively discover the underlying mechanism of enhanced gas recovery (EGR) through CO2 injection, molecular dynamics (MD) provides a promising way for establishing the shale models to address the multiphase, multicomponent fluid flow behaviors in shale nanopores. This study proposes an innovative method for building a more practical shale matrix model that approaches natural underground environments. The grand canonical Monte Carlo (GCMC) method elucidates gas adsorption and sequestration processes in shale gas reservoirs under various subsurface conditions. The findings reveal that previously overlooked pore slits have a significant impact on both gas adsorption and recovery efficiency. Based on the simulation comparisons of absolute and excess uptakes inside the kerogen matrix and the shale slits, it demonstrates that nanopores within the kerogen matrix dominate the gas adsorption while slits dominate the gas storage. Regarding multiphase, multicomponent fluid flow in shale nanopores, moisture negatively influences gas adsorption and carbon storage while promoting methane recovery efficiency by CO2 injection. Additionally, saline solution and ethane further impede gas adsorption while facilitating displacement. Overall, this work elucidates the substantial effect of CO2 injection on fluid transport in shale formations and advances the comprehensive understanding of microscopic gas flow and recovery mechanisms with atomic precision for low-carbon energy development.

Occurrence Characteristics of Water in Nano-Slit Pores under Different Solution Conditions: A Case Study on Kaolinite

The presence of water in narrow pore spaces affects the occurrence and flow of methane, which in turn affects shale gas production. Therefore, studying the occurrence and distribution characteristics of water is of great significance to predict gas production. Based on molecular dynamics simulations, this study investigated the occurrence characteristics and influencing variables of liquid water in kaolinite nanopores in situ. Owing to its widespread distribution, kaolinite is the most prevalent clay mineral with two surfaces with different characteristics. Three systems of pure water, a CaCl₂ solution, and a H₂O/CH₄ mixed phase were created at varied temperatures (80-120 °C) and pressures (70-120 MPa). The presence of gas and water in the nanopores was investigated thoroughly. The results showed that the adsorption of water on the Al-O octahedral surface of kaolinite was not affected by external conditions under in situ conditions, whereas the adsorption of water on the Si-O tetrahedral surface decreased with increasing temperature, but the change was small. When ions were present in the system, the water capacity decreased. Based on the aforementioned results, external conditions, such as temperature and pressure do not affect the basic state of water. However, if there are more than two fluid types in the system, the adsorption of water on the mineral surface is reduced owing to competitive adsorption. In addition, a CH₄-H₂O mixed system was simulated, in which methane molecules were distributed in clusters. There are two types of adsorptions in pores: gas-solid interactions and solid-liquid-gas interactions. CH₄ molecules are thought to be clustered in water molecules because of the strong hydrogen bonding interactions among the water.

Experimental studies on combined production of CH4 and safe long-term storage of CO2 in the form of solid hydrate in sediment

The production of the confirmed enormous resources of CH₄ trapped in permafrost and deep ocean sediments in the form of hydrates has been hampered by the lack of an extraction procedure that is both effective and environmentally sensitive. This research explores experimentally the dynamic rate limiting steps in the dissociation of methane hydrates and the formation of CO₂ hydrates in a sediment matrix. The use of CO₂ injection and substitution for hydrate extraction takes advantage of novel thermodynamics and also provides a safe storage option for greenhouse gas. This experimental work incorporates a high-pressure facility dedicated for CH₄ hydrates exchange with CO₂ that replicates creation of natural gas hydrate from incoming gas below water in the pore space. The hydrate formation/exchange chamber follows the state-of-art hydrate science and is equipped with sensors distributed in several sections: the top section for gas release, a CH₄ hydrate section, and a subsequent injection of CO₂ from the bottom section, which also mimics hydrate dissociation towards incoming seawater through fracture systems connected from the seafloor. Four experimental conditions were examined. They comprise pure CO₂ injection, and 10, 20, and 30 mole% N₂ added to the CO₂. We observed an increase in CH₄ release from pure CO₂ injection to 10 mole% N₂ addition. A significant extra release of CH₄ occurred by stepping up to 20 mole% N₂ addition but no significant change was observed from 20 to 30 mole% N₂ addition. Maximum conversion in this study is 34 mole% of CO₂, and 2 mole% N₂ taking the place of methane hydrate in large and small cavities. The results also show that effective substitution for hydrate production cannot rely on pure carbon dioxide injection.

Molecular Origin of Wettability Alteration of Subsurface Porous Media upon Gas Pressure Variations

Upon extraction/injection of a large quantity of gas from/into a subsurface system in shale gas production or carbon sequestration, the gas pressure varies remarkably, which may significantly change the wettability of porous media involved. Mechanistic understanding of such changes is critical for designing and optimizing a related subsurface engineering process. Using molecular dynamics simulations, we have calculated the contact angle of a water droplet on various solid surfaces (kerogen, pyrophyllite, calcite, gibbsite, and montmorillonite) as a function of CO₂ or CH₄ gas pressure up to 200 atm at a temperature of 300 K. The calculation reveals a complex behavior of surface wettability alteration by gas pressure variation depending on surface chemistry and structure, and molecular interactions of fluid molecules with surfaces. As the CO₂ gas pressure increases, a partially hydrophilic kerogen surface becomes highly hydrophobic, while a calcite surface becomes more hydrophilic. Considering kerogen and calcite being the major components of a shale formation, we postulate that the wettability alteration of a solid surface induced by a gas pressure change may play an important role in fluid flows in shale gas production and geological carbon sequestration.

CH₄ and CO₂ Adsorption Mechanism in Kaolinite Slit Nanopores as Studied by the Grand Canonical Monte Carlo Method

To confirm the rules and transformation conditions of shale gas adsorption and establish a model for evaluating the adsorption capacity of shale gas quantitatively, it is necessary to reveal the shale gas adsorption mechanism. The adsorption mechanism of CH₄ and CO₂ in Kaolinite slit nanopores has been studied under the simulated conditions of 90 °C and 30 or 50 MPa by the grand canonical Monte Carlo (GCMC) method. The results indicate that CH₄ is controlled only by the Van der Waals forces on the mineral surface because CH₄ is nonpolar, while CO₂ is controlled by both Van der Waals forces and Coulomb forces due to a certain electric quadrupole moment, which makes the adsorption capacity of CO₂ on kaolinite greater than that of CH₄. Due to the overlapping adsorption potential on the kaolinite surface of micropores (1 nm), the peak of the density profile is higher in the micropores than the peak in the mesopores (4 nm), resulting in the filling effect in the micropores. On the surface of the silicon-oxygen octahedron, the adsorption site for CH₄ and CO₂ is in the center of the silicone hexagon-ring, and CO₂ with a quadrupole moment shifts near the polar oxygen atoms. In contrast, the adsorption sites of CH₄ are relatively dispersed on the surface of the aluminum-oxygen octahedron with a hydroxyl group, while the adsorption sites of CO₂ are concentrated in the location of the aggregated oxygen atoms. When CH₄ and CO₂ coexist, CO₂ tends to be adsorbed prior to CH₄. With the proportion of CO₂ increasing, the competitive adsorption effect is gradually aggravated, which suggests the rationality of injecting CO₂ to improve the recovery efficiency of shale gas. These findings can provide theoretical support for shale gas exploration and development.

Effect of Pressure and Temperature on CO2/CH4 Competitive Adsorption on Kaolinite by Monte Carlo Simulations

The adsorption of CO₂ and CO₂/CH₄ mixtures on kaolinite was calculated by grand canonical Monte Carlo (GCMC) simulations with different temperatures (283.15, 293.15, and 313.15 K) up to 40 MPa. The simulation results show that the adsorption amount of CO₂ followed the Langmuir model and decreased with an increasing temperature. The excess adsorption of CO₂ increased with an increasing pressure until the pressure reached 3 MPa and then decreased at different temperatures. The SCO2/CH4 decreased logarithmically with increasing pressure, and the SCO2/CH4 was lower with a higher temperature at the same pressure. The interaction energy between CO₂ and kaolinite was much higher than that between CH₄ and kaolinite at the same pressure. The interaction energy between the adsorbent and adsorbate was dominant, and that between CO₂ and CO₂ and between CH₄ and CH₄ accounted for less than 20% of the total interaction energy. The isothermal adsorption heat of CO₂ was higher than that of CH₄, indicating that the affinity of kaolinite to CO₂ was higher than that of CH₄. The strong adsorption sites of carbon dioxide on kaolinite were hydrogen, oxygen, and silicon atoms, respectively. CO₂ was not only physically adsorbed on kaolinite, but also exhibited chemical adsorption. In gas-bearing reservoirs, a CO₂ injection to displace CH₄ and enhance CO₂ sequestration and enhanced gas recovery (CS-EGR) should be implemented at a low temperature.

Effect of external pressure on the release of methane through MFI zeolite nanochannels

In this work, the effects of external pressure on the release of methane through zeolite nanochannels are studied through molecular dynamics (MD) simulations. The release percentage of methane under three types of pressure loadings with various strengths and frequencies are obtained. Specifically, constant, sawtooth-shaped, and sinusoidal pressures are examined. As the pressure strength is increased, it is found that the release percentage first decreases and then increases significantly before finally approaching a constant. At sufficiently high pressures, the release percentage of methane under constant external pressure is about 65%, while it reaches over 90% for sawtooth-shaped and sinusoidal pressures. The loading frequency for periodic external pressures appears to be unimportant compared with the effect of the pressure strength. Theoretical predictions of the release percentage are made on the basis of the kinetic energy of methane molecules and the energy barrier inside the nanochannels, which are in good agreement with MD simulations.

In this work, the effects of external pressure on the release of methane through zeolite nanochannels are studied through molecular dynamics (MD) simulations.

In situ study on interactions between hydroxyl groups in kaolinite and re-adsorption water

The interactions between O–H groups in kaolinite and re-adsorption water is an important aspect that should be considered in the hydraulic fracturing method for the production of shale gas, because the external water adsorbed by kaolinite in shale would significantly affect the desorption of methane. In this study, the interactions were investigated via changing the amount of O–H groups and re-adsorption water in kaolinite by heating treatment and water re-adsorption. To overcome the overlap of IR vibration bands of the O–H functional groups in H₂O and those in parent kaolinite, kaolinite samples with D₂O re-adsorption were prepared by drying the H₂O from raw kaolinite and soaking the dried kaolinite in D₂O. The interactions between O–H groups in kaolinite and D₂O molecules were investigated by in situ DRIFT and TG-MS. The results demonstrated that the vibration at 3670 ± 4 cm⁻¹ in the DRIFT spectra could be due to the outer O–H groups of the octahedral sheet on the upper surface of the kaolinite microcrystal structure, rather than a type of inner-surface O–H group. All types of O–H groups, including the inner O–H groups in kaolinite, could be transformed into O–D groups after D₂O re-adsorption at room temperature. The inner-surface O–H groups in kaolinite are the most preferred sites for D₂O re-adsorption; thus, they would be the key factor for studying the effect of re-adsorption water on methane desorption. When the temperature increased from 100 °C to 300 °C, two layers of kaolinite slipped away from each other, resulting in the transformation of inner-surface O–H groups into outer O–H groups. Thus, the temperature range of 100 to 300 °C was suggested for the heat treatment of kaolinite to decrease the content of inner-surface O–H groups; thereby, the amount of re-adsorption water was reduced. However, to thoroughly remove the re-adsorption water, a temperature higher than 650 °C should be used.

Because two layers slipped away from each other, inner-surface O–H was transformed into outer O–H during heating from 100–300 °C. Re-adsorption water could be thoroughly removed at 650 °C.

A new method to determine varying adsorbed density based on Gibbs isotherm of supercritical gas adsorption

In the calculation of the absolute adsorption of supercritical gas adsorbed on the microporous materials, most existing methods regard the adsorbed density as a constant, which is very unreasonable. In this study, an extended pressure point method combined with Langmuir adsorption model is proposed in which the varying adsorbed density under different pressures is considered at the same time. The utility of the proposed method to correlate accurately the experimental data for supercritical gas adsorption system is demonstrated by high-pressure methane adsorption measurements on two groups of shale samples. Taking advantage of the proposed method, we can obtain the adsorbed density and the adsorbed volume corresponding to different pressures. Compared with the conventional methods under the assumption of fixed and parameterized adsorbed density, the proposed method yields better fitting results with the experimental data. Our work should provide important fundamental understandings and insights into the supercritical gas adsorption system.

Investigation of dynamical properties of methane in slit-like quartz pores using molecular simulation

The dynamical properties of adsorption media confined in micropores play an important role in the adsorptive separation of fluids. However, a problem is that it is difficult to directly use approaches based on experimental measurements. Molecular simulation has been an effective tool for investigating the diffusion of fluids on the microscale in recent years. In this work, the diffusion properties of methane in quartz were mainly investigated from a microscale viewpoint using MD (molecular dynamics) methods, and this paper primarily discusses the influence of parameters such as pressure, temperature, pore size and water content on the diffusion and thermodynamic parameters of methane in slit-like quartz pores. The results demonstrate that the transport ability of quartz pores decreases with an increase in pressure in pores of a fixed size at a certain temperature and increases with an increase in pore size or temperature at a fixed pressure, which is related to changes in the interaction between methane molecules and quartz. In the pressure range used in the simulation, the average isosteric heat of adsorption of methane increases with an increase in pressure and is in the range of 6.52–10.794 kJ mol⁻¹. Therefore, the gas adsorption behavior is classed as physical adsorption because the heat of adsorption is significantly lower than the minimum heat of gas adsorption for chemisorption. The increase in the total adsorption entropy is caused by an increase in temperature due to an increase in internal energy, which brings about a reduction in the interactions between gas molecules and walls of quartz. However, with an increase in pore size the total adsorption entropy increases, for which an explanation may be that in pores of a larger size methane molecules are adsorbed at higher-energy sites and generate a higher isosteric heat, which causes a reduction in interactions between the adsorbate and adsorbent. Regarding the influence of different water contents on the diffusion of methane, it was demonstrated that with an increase in moisture the mobility of methane molecules initially increases and then decreases, which is related to the distance between gas molecules.

The dynamical properties of adsorption media confined in micropores play an important role in the adsorptive separation of fluids.