Molecular Simulation of Shale Gas Adsorption and Diffusion in Clay Nanopores

During the fracking process in shale, an interaction occurs between shale and fracking fluid that contains a cocktail of chemicals. One of the chemicals used in fracking fluid is often surfactant, which is generally used as a viscofier. However, surfactants also have the potential of significantly influencing the wettability and thus gas desorption−key factors affecting ultimate gas recovery from shale reservoirs. Even though a few studies discussed the ability of surfactants to alter wettability in shale, the implication of that change in adsorption/desorption behavior has never been experimentally investigated beyond hypothetical inferences. In this study, the influence of the wettability change by anionic surfactant on gas adsorption/desorption behavior in shale was investigated through a series of experiments. Baseline wettability readings of two shale samples were established by measuring the contact angles (BG-1 = 22.7°, KH-1 = 35°) between a drop of pure water placed on their polished surfaces, indicating that the affinity of pure water for the BG-1 surface was greater than that for KH-1. This difference can be attributed to the higher clay content and lower total organic carbon found in BG-1 as compared to KH-1. To investigate the impact of the interaction between shale and surfactants on wettability during the fracking process, we measured the contact angles again, this time with 1 wt % solution of internal olefin sulfate surfactant. The surfactant-induced wettability changes of the two shale samples were investigated by measuring the contact angles again (BG-1 = 3.5°, KH-1 = 19.2°) between a drop of surfactant solution and their polished surfaces. The effect of wettability changes on gas adsorption/desorption was then evaluated utilizing the United States bureau of mines' modified method. Experiments were conducted on the two shale samples in two ways: after pure water treatment, and after surfactant treatment. The results suggest that due to the wettability alteration of the two shale samples by IOS surfactant toward more water-wet during the treatment, the methane adsorption/desorption characteristics were influenced. In BG-1 sample, IOS solution dramatically changed its wettability to become completely waterwet. Therefore, the volume of desorbed methane dropped by nearly 54%. A similar but less pronounced influence was found in the KH-1 sample, where its desorbed methane dropped by 10% because of wettability alteration toward more water-wet. These reductions in the amount of desorbed gas suggest that prior to selecting a surfactant for addition to fracking fluid, its effect on wettability and gas desorption should be investigated to optimize shale gas recovery potential.

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“…A similar finding was reported in a simulation study where water was found to reduce methane adsorption in clay pores. 12…”

Section: Energy and Fuelsmentioning

confidence: 99%

Effect of Anionic Surfactant on Wettability of Shale and Its Implication on Gas Adsorption/Desorption Behavior

2018

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“…In addition to experimental studies, molecular simulation is considered as an effective tool to gain microscopic insights into complex physical phenomena or processes in many research areas [20][21][22][23][24]. The adsorption behaviors in nanopores of shale reservoirs were investigated via molecular simulation by many researchers [25][26][27][28]. The effects of temperature, pressure, and pore size on the adsorption performance of pure methane and a CO 2 /CH 4 mixture in the slit pore of shale formations were examined.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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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.

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“…Using the GCMC method, Sun et al (2015) performed research on the methane adsorption behaviours of different types of clay minerals (montmorillonite, illite and kaolinite) and also studied the effects of different temperatures on the methane adsorption behaviours. Sui et al (2015) studied the microscopic structural properties and diffusion behaviours of methane in slit-like montmorillonite pores using the GCMC and MD methods. Xiong et al (2016) studied the microscopic adsorption mechanism of methane in slit-like montmorillonite pores using the GCMC method.…”

Section: Introductionmentioning

confidence: 99%

Investigation of methane adsorption on chlorite by grand canonical Monte Carlo simulations

et al. 2017

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In this paper, the methane adsorption behaviours in slit-like chlorite nanopores were investigated using the grand canonical Monte Carlo simulation method, and the influences of the pore sizes, temperatures, water, and compositions on methane adsorption on chlorite were discussed. Our investigation revealed that the isosteric heat of adsorption of methane in slit-like chlorite nanopores decreased with an increase in pore size and was less than 42 kJ/mol, suggesting that methane adsorbed on chlorite through physical adsorption. The methane excess adsorption capacity increased with the increase in the pore size in micropores and decreased with the increase in the pore size in mesopores. The methane excess adsorption capacity in chlorite pores increased with an increase in pressure or decrease in pore size. With an increase in temperature, the isosteric heats of adsorption of methane decreased and the methane adsorption sites on chlorite changed from lowerenergy adsorption sites to higher-energy sites, leading to the reduction in the methane excess adsorption capacity. Water molecules in chlorite pores occupied the pore wall in a directional manner, which may be related to the van der Waals and Coulomb force interactions and the hydrogen bonding interaction. It was also found that water molecules existed as aggregates. With increasing water content, the water molecules occupied the adsorption sites and adsorption space of the methane, leading to a reduction in the methane excess adsorption capacity. The excess adsorption capacity of gas on chlorite decreased in the following order: carbon dioxide [ methane [ nitrogen. If the mole fraction of nitrogen or carbon dioxide in the binary gas mixture increased, the mole fraction of methane decreased, methane adsorption sites changed, and methane adsorption space was reduced, resulting in the decrease in the methane excess adsorption capacity.

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Molecular Simulation of Shale Gas Adsorption and Diffusion in Clay Nanopores

Cited by 50 publications

References 41 publications

Effect of Anionic Surfactant on Wettability of Shale and Its Implication on Gas Adsorption/Desorption Behavior

Effect of Anionic Surfactant on Wettability of Shale and Its Implication on Gas Adsorption/Desorption Behavior

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

Investigation of methane adsorption on chlorite by grand canonical Monte Carlo simulations

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