Characterization of Methane Excess and Absolute Adsorption in Various Clay Nanopores from Molecular Simulation.
ABSTRACT: In this work, we use grand canonical Monte Carlo (GCMC) simulation to study methane adsorption in various clay nanopores and analyze different approaches to characterize the absolute adsorption. As an important constituent of shale, clay minerals can have significant amount of nanopores, which greatly contribute to the gas-in-place in shale. In previous works, absolute adsorption is often calculated from the excess adsorption and bulk liquid phase density of absorbate. We find that methane adsorbed phase density keeps increasing with pressure up to 80?MPa. Even with updated adsorbed phase density from GCMC, there is a significant error in absolute adsorption calculation. Thus, we propose to use the excess adsorption and adsorbed phase volume to calculate absolute adsorption and reduce the discrepancy to less than 3% at high pressure conditions. We also find that the supercritical Dubinin-Radushkevich (SDR) fitting method which is commonly used in experiments to convert the excess adsorption to absolute adsorption may not have a solid physical foundation for methane adsorption. The methane excess and absolute adsorptions per specific surface area are similar for different clay minerals in line with previous experimental data. In mesopores, the excess and absolute adsorptions per specific surface area become insensitive to pore size. Our work should provide important fundamental understandings and insights into accurate estimation of gas-in-place in shale reservoirs.
Project description:Adsorbed methane is an important component of shale gas. Shale generally contains a certain amount of primary water, and isothermal adsorption experiments on wet samples show that water inhibits methane adsorption. Researches on methane adsorption mainly focus on the conditions of low pressure and water content. In this study, a hybrid GCMC-MD simulation method is proposed to study methane adsorption characteristics under high pressure and water content in pores of different sizes. This method can obtain the bulk pressure of the system while ensuring the simultaneous movement of methane and water molecules, and has high efficiency and reliability. It is found that the existence of water does not change the morphology of excess isotherm, and the relative decrease of adsorption capacity due to the existence of water is not sensitive to temperature. In ?3 nm pores, water molecules form water clusters and partially occupy wall adsorption sites, and the adsorption amount decreases linearly with increasing water saturation. In the 5 nm wide pore with 40% water saturation, water films formed and methane adsorption is strongly suppressed. It is expected these findings could provide guidance for the evaluation of the amount of adsorbed methane with primary water.
Project description:Shale gas has attracted increasing attention as a potential alternative gas in recent years. Because a large fraction of gas in shale formation is in an adsorbed state, knowledge of the supercritical methane adsorption behavior on shales is fundamental for gas-in-place predictions and optimum gas recovery. A practical model with rigorous physical significance is necessary to describe the methane adsorption behavior at high pressures and high temperatures on shales. In this study, methane adsorption experiments were carried out on three Lower Silurian Longmaxi shale samples from the Sichuan Basin, South China, at pressures of up to 30 MPa and temperatures of 40, 60, 80, and 100 °C. The simplified local density/Elliott-Suresh-Donohue model was adopted to fit the experimental data in this study and the published methane adsorption data. The results demonstrate that this model is suitable to represent the adsorption data from the experiments and literature for a wide range of temperatures and pressures, and the average absolute deviation is within 10%. The methane adsorption capacity of the Longmaxi shale exhibited a strong linear positive correlation with the total organic carbon content and a linear negative correlation with increasing temperature. The rate of decrease in the methane adsorption capacity with swing temperature increased with the total organic carbon content, indicating that the organic matter is sensitive to temperature.
Project description:Enhanced recovery of shale gas with CO2 injection has attracted extensive attention as it combines the advantages of improved efficiency of shale gas recovery and reduced greenhouse gas emissions via CO2 geological sequestration. On the other hand, the microscopic mechanism of enhanced shale gas recovery with CO2 injection and the influence of the subsurface water confined in the shale nanopores remain ambiguous. Here, we use grand canonical Monte Carlo (GCMC) simulations to investigate the effect of moisture on the shale gas recovery and CO2 sequestration by calculating the adsorption of CH4 and CO2 in dry and moist kerogen slit pores. Simulation results indicate that water accumulates in the form of clusters in the middle of the kerogen slit pore. Formation of water clusters in kerogen slit pores reduces pore filling by methane molecules, resulting in a decrease in the methane sorption capacity. For the sorption of CH4/CO2 binary mixtures in kerogen slit pores, the CH4 sorption capacity decreases as the moisture content increases, whereas the effect of moisture on CO2 sorption capacity is related to its mole fraction in the CH4/CO2 binary mixture. Furthermore, we propose a reference route for shale gas recovery and find that the pressure drawdown and CO2 injection exhibit different mechanisms for gas recovery. Pressure drawdown mainly extracts the CH4 molecules distributed in the middle of kerogen slit pores, while CO2 injection recovers CH4 molecules from the adsorption layer. When the water content increases, the recovery ratio of the pressure drawdown declines, while that of CO2 injection increases, especially in the first stage of CO2 injection. The CO2 sequestration efficiency is higher under higher water content. These findings provide the theoretical foundation for optimization of the shale gas recovery process, as well as effective CO2 sequestration in depleted gas reservoirs.
Project description:Shale gas is a promising energy source offering additional energy security over concerns of fossil fuel depletion. Injecting CO2 into depleted shale gas reservoirs might provide a feasible solution for CO2 storage and enhanced gas recovery. However, shale strain caused by the CO2 injection as well as CO2 sequestration in the reservoir needs to be considered during shale gas production. For this purpose, this paper examines the adsorption capacities, CO2-induced swelling, and He-induced strain of shales at 0-16 MPa and 35-75 °C. The maximum excess adsorption at different temperatures correlated with the bulk phase density: as the CO2 temperature increased, the maximum excess adsorption density decreased. The density of the adsorbed phase, obtained using the Dubinin-Radushkevich model, was used to fit the excess adsorption data. At low pressure, the CO2-induced strain on shale was caused by the gas adsorption, whereas at high pressure, it was caused by gas pressure. The absolute adsorption linearly correlated with the adsorption-induced strain.
Project description:Molecular dynamics simulation studies were employed to investigate the microscopic behaviors of CH<sub>4</sub> and CO<sub>2</sub> molecules in slit-nanopores (SNPs) with various surfaces and different compositions. Three kinds of SNPs were constructed by a pair-wise combination of graphene, silica, and the calcite surface. The grand canonical Monte Carlo and molecular dynamics simulation methods were used to investigate the adsorption and self-diffusion of the gases in the nanopores. It is found that in all three cases, the CH<sub>4</sub> molecules prefer to adsorb onto the graphene surface, whereas the CO<sub>2</sub> molecules prefer to adsorb onto the calcite surface. The adsorption intensity of gases adsorbed onto various surfaces, the adsorption distances, along with the details of adsorption orientations of CH<sub>4</sub> and CO<sub>2</sub> molecules on various surfaces are calculated. The surface characteristics, such as surface roughness and charge distribution, are analyzed to help understand the microscopic adsorption behaviors of the gases on the specific surface. It was found that competitive adsorptions of CO<sub>2</sub> over CH<sub>4</sub> broadly occurred, especially in the SNPs containing calcite, because of the strong adsorption interactions between the CO<sub>2</sub> molecules and the calcite surface. This work provides the microbehaviors of CH<sub>4</sub> and CO<sub>2</sub> in SNPs with various surfaces in different compositions to provide useful guidance for better understanding about the microstate of gases in complex nanoporous shale formation and to give out useful guidance for enhancing shale gas recovery by injecting CO<sub>2</sub>.
Project description:The adsorption characteristics of methane in shales play a critical role in the assessment of shale gas resources. The microscopic adsorption mechanism of methane considering the effect of moisture and especially salinity remains to be explored. In this work, combined molecular dynamics and grand canonical Monte Carlo simulations are conducted to investigate the adsorption behaviors of methane in the realistic kerogen matrixes containing different moisture contents (0-6 wt %) and various salinities (0-6 mol/L NaCl). Adsorption processes are simulated under realistic reservoir conditions at four temperatures in the range from 298.15 to 358.15 K and pressures up to 40 MPa. Effects of the moisture content on methane adsorption capacities are analyzed in detail. Simulation results show that the methane adsorption capacity declines as the moisture content increases. In comparison to the dry kerogen matrix, the reduction in the maximum CH4 adsorption capacity is as high as 42.5% in moist kerogen, with a moisture content of 6.0 wt % at 338.15 K. The overlap observed in the density distributions of water molecules and decrease in adsorbed methane indicates that the water molecules occupy the adsorption sites and, thus, lead to the reduction in methane adsorption capacity. Besides, the effects of salinity on CH4 adsorption isotherms are discussed. The salinity is found to have a negative influence on the methane adsorption capacity. The maximum CH4 adsorption capacity reduces around 6.0% under the salinity of 6 mol/L at 338.15 K. Adsorption of methane in kerogens of constant salinity but different moisture contents are further discussed. Results from the present study show that the moisture content has a greater impact on the adsorption of methane compared to that of salinity. The findings of this study have important implications for more accurate estimation of shale gas in place.
Project description:The large amount of nanoscale pores in shale results in the inability to apply Darcy's law. Moreover, the gas adsorption of shale increases the complexity of pore size characterization and thus decreases the accuracy of flow regime estimation. In this study, an apparent permeability model, which describes the adsorptive gas flow behavior in shale by considering the effects of gas adsorption, stress dependence, and non-Darcy flow, is proposed. The pore size distribution, methane adsorption capacity, pore compressibility, and matrix permeability of the Barnett and Eagle Ford shales are measured in the laboratory to determine the critical parameters of gas transport phenomena. The slip coefficients, tortuosity, and surface diffusivity are predicted via the regression analysis of the permeability data. The results indicate that the apparent permeability model, which considers second-order gas slippage, Knudsen diffusion, and surface diffusion, could describe the gas flow behavior in the transition flow regime for nanoporous shale. Second-order gas slippage and surface diffusion play key roles in the gas flow in nanopores for Knudsen numbers ranging from 0.18 to 0.5. Therefore, the gas adsorption and non-Darcy flow effects, which involve gas slippage, Knudsen diffusion, and surface diffusion, are indispensable parameters of the permeability model for shale.
Project description:Despite massive success of shale gas production in the US in the last few decades there are still major concerns with the steep decline in wellbore production and the large uncertainty in a long-term projection of decline curves. A reliable projection must rely on a mechanistic understanding of methane release in shale matrix-a limiting step in shale gas extraction. Using molecular simulations, we here show that methane release in nanoporous kerogen matrix is characterized by fast release of pressurized free gas (accounting for ~30-47% recovery) followed by slow release of adsorbed gas as the gas pressure decreases. The first stage is driven by the gas pressure gradient while the second stage is controlled by gas desorption and diffusion. We further show that diffusion of all methane in nanoporous kerogen behaves differently from the bulk phase, with much smaller diffusion coefficients. The MD simulations also indicate that a significant fraction (3-35%) of methane deposited in kerogen can potentially become trapped in isolated nanopores and thus not recoverable. Our results shed a new light on mechanistic understanding gas release and production decline in unconventional reservoirs. The long-term production decline appears controlled by the second stage of gas release.
Project description: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.
Project description:Developing the application of high-strength hydrogels has gained much attention in the fields of medical, pharmacy, and pollutant removal due to their versatility and stimulus-responsive properties. In this presentation, a high-strength freestanding elastic hydrogel membrane was constructed by clay nanosheets, N, N-dimethylacrylamide and 2-acrylamide-2-methylpropanesulfonic acid for adsorption of methylene blue and heavy metal ions. The maximum values of elongation and Young's modulus for 0.5% AMPSNa hydrogel were 1901% and 949.4 kPa, respectively, much higher than those of traditional hydrogels. The adsorptions were confirmed to follow pseudo-second kinetic equation and Langmuir isotherm model fits the data well. The maximum adsorption capacity of hydrogel towards methylene blue was 434.8 mg g(-1). The hydrogel also exhibited higher separation selectivity to Pb(2+) than Cu(2+). The methylene blue adsorbed onto the hydrogel membrane can be photocatalytically degraded by Fenton agent and the hydrogel membrane could be recycled at least five times without obvious loss in mechanical properties. In conclusion, this presentation demonstrates a convenient strategy to prepare tough and elastic clay nanocomposite hydrogel, which can not only be applied as recyclable membrane for the photocatalytic degradation of organic dye, but also for the recovery of valuables.