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Density of states of methane before and after it adsorbing on the surface of kerogen fragment RNH 2 .
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Organic matter plays a vital role in the adsorption of shale gas. The current study about adsorption capacity and adsorption law of organic matter have made abundant achievements, but the effect of functional groups on the adsorption of shale gas is still not clear. In this paper, the adsorption of CH4 on the surface of kerogen fragments with diffe...
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... B, H sites of kerogen fragment RH and RCH 3 , the adsorption energies are both close to zero, explaining the adsorption capacity for methane of two kerogens are very weak. By comparing the elements of the functional groups, it is not difficult to find that the presence of oxygen and nitrogen element makes the adsorption of kerogens more powerful. Fig. 5 shows both the density of states of CH 4 before and after it is adsorbed on the surface of kerogen fragment RNH 2 . Before the adsorption, three isolated peaks appear nearby the energy of -7.5eV, 0, 9eV in black curve. However, the distribution of three peaks significantly moves to the lower energy region once the adsorption occurs. ...
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Carbon neutrality is one of ultimate goals of global population. The detection of CO 2 , now, is a research hotspot, and two-dimensional materials are undoubtedly play an important role. In this paper, the first-principles approach based on density functional theory was used to study the adsorption behavior of CO 2 on intrinsic and defective g-GaN. The results are as follows. The adsorption energy is relatively bigger, the band gap and the structures of g-GaN and CO 2 have no obvious changes when CO 2 is adsorbed on the intrinsic g-GaN. It indicates that intrinsic g-GaN is inert to CO 2 . Defective g-GaN still maintains a planar structure, but g-GaN are transformed from semiconductors to half-metal and metals after the introduction of Ga and N single vacancies, respectively. The CO 2 adsorption energy and adsorption distance are reduced, the structure of defective g-GaN is obviously deformed when CO 2 is adsorbed on defective g-GaN. It indicates that the adsorption between g-GaN and CO 2 is stronger. CO 2 is physically adsorbed on these three structures from the perspective of charge exchange which is good for desorption. Defective g-GaN still remain half-metallic and metallic properties after CO 2 is adsorbed on it. From the adsorption energy, the introduction of Ga vacancy enhances the detection ability of g-GaN for CO 2 , and it is better than N vacancy. This provides theoretical support for g-GaN materials as a nanoscale gas sensor materials.
As a clean and efficient unconventional energy source, natural gas hydrate has been highly valued and vigorously developed by many countries in recent years. In order to solve the problem that the existing hydrate structure symmetry is not high, which leads the theoretical research to be restricted, it is imperative to explore a new type of methane hydrate structure with high symmetry. Using the first-principles method which is based on the density functional theory (DFT), the structure and electronic properties of N-methane hydrate are calculated in the generalized gradient approximation (GGA) for Grimme dispersion correction. The obtained results are shown below. 1) The water cage structure of N-methane hydrate is a truncated octahedron (4⁶6⁸), which is composed of 8 regular hexagons and 6 squares, and the average length of the hexagons and the average length of the squares are both 2.723 Å. The average bond length of water molecules is optimized to be 1.056 Å, and the average bond angle of water molecules is 107.738°. The average bond length of methane molecules is 1.0973 Å. The average distance from methane molecules to water molecules is 4.2831 Å that is longer than the distance in the I- methane hydrate. So N-methane hydrate can accommodate larger volumes of gas molecules. The symmetric group is \begin{document}${\rm{IM}}\bar 3{\rm{M}}$\end{document} for N-methane hydrate, which has a simple and strict periodic stable structure. 2) The lattice parameter of N-methane hydrate is 7.70 Å, and the density is 0.903 g/cm³, which is greater than I-, II- and H-type hydrate density. 3) The x-ray diffraction(XRD) pattern of N-methane hydrateis calculated and is close to that of of I-methane hydrate, while the water cage of N-methane hydrate is larger. 4) The interaction between methane molecules and the water cage is van der Waals force, and the formation energy of N- methane hydrate is –0.247 eV, which indicates that the N-methane hydrate is easy to form. Both the density of states and partial density of states indicate that the interaction between methane and water cage is weak, and it relies on molecular force. 5) In addition, N-methane hydrate is an insulator material with the energy gap greater than 5 eV.
Increasing CO2 emissions and global warming side effects have prompted the researchers to look for safe and reliable storage sites that have high capacity. Among the available CO2 capturing sinks, depleted gas reservoirs have high potential to sequester CO2. Depleted conventional and unconventional gas reservoirs have large pore space after natural gas production and pressure reduction. Moreover, their ability to store hydrocarbons for many years inside the sealed reservoir with impermeable cap rocks provides safer options than saline aquifers or other geological traps. Incremental recovery of residual natural gas after injecting CO2 could decrease the cost of the process. This review highlights the efforts made to investigate the CO2 adsorption/desorption for EGR applications under typical reservoir conditions in conventional depleted sandstone and carbonate reservoirs. Moreover, it analyzes the advances in CO2-EGR in unconventional resources such as coal beds and shale to extract the knowledge from these reservoirs. In addition, various factors that control the displacement efficiency of natural gas by injecting CO2 and the consequent influence of CO2 on rock integrity are discussed. Nanoscale basis of CO2-EGR using multiscale molecular simulation that could improve the design and operational conditions for CO2-EGR operations is overviewed. Furthermore, this article assesses the ecological and economic impact of storing CO2 in different types of reservoirs. Field pilot tests, as well as challenges in the application of the CO2-EGR technique, are also covered.
Adsorption in porous media has a direct impact on the reserves of fluid in shale gas reservoirs, but they are rarely studied in combination with molecular simulation and quantum mechanisms. The adsorption behaviors between methane, carbon dioxide and quartz are investigated with Grand Canonical Monte Carlo, Molecular Dynamics and Density Functional Theory in this work. Adsorption isotherm, density distribution, adsorption energy and Density of States (DOS) are calculated and analyzed. These results are as below. The adsorption isotherms of methane, carbon dioxide and their mixture (mole ratio 1:1) can be described as Langmuir type. The adsorption amounts increase with the rising of pressure, and the adsorption capacity of carbon dioxide is stronger than methane. The position near the wall is occupied by methane and carbon dioxide, and density distribution shows that the thickness of adsorption layer is 5 Å. The distribution of adsorption energy states clearly physical adsorption. DOS of methane systems have a little bit difference, and a relatively large difference of peak value is in carbon dioxide systems. DOS of adsorbents (maximum and minimum adsorption energy) are basically coincided with each other, while DOS of methane and carbon dioxide shift with different degrees to the lower energy region.
The low permeability of kerogen governs the storage and production of shale gas. The flexible kerogen constantly experiences mechanical deformation induced by reservoir environment and complex interplay with geofluids. However, the kerogen deformation associated with CH4/CO2 competitive sorption remains poorly understood. In this work, the effect of preloaded moisture on the deformation of kerogen with different organic types was investigated with molecular dynamics simulation. The kerogen deformation upon CH4/CO2 competitive sorption was quantified with the combination of grand canonical Monte Carlo simulations and poromechanics theory. The effects of various factors and their corresponding contributions were discussed in detail. The effects of kerogen deformation on CH4/CO2 diffusion were studied. Some implications for CO2 sequestration and enhanced gas recovery (CS-EGR) were proposed. Our results verify the theoretical feasibility of CS-EGR in shale gas reservoir. CO2 is observed to have a higher affinity with kerogen and a lower diffusion coefficient compared with CH4, facilitating it to replace CH4 and retain in the kerogen matrix. The rising CO2 composition can induce larger kerogen swelling, thus opening fluid flow pathways and increasing shale gas production. There are optimum moisture content and reservoir pressure corresponding to the maximum effective pore size in kerogen. It could be feasible to enhance the efficiency of CS-EGR by manipulating the reservoir moisture and CO2 injection timing. Thermal stimulation in deep shale reservoir may not be efficient for CS-EGR. CH4/CO2 competitive sorption can induce significant swelling of kerogen. The flexible nature of kerogen should be considered to improve the evaluation on both gas-in-place and CO2 storage capacity.
Kerogen is the source of hydrocarbons in shale. As a soft nanoporous matter, kerogen constantly experiences mechanical deformation induced by subsurface stress environment and interplay with geofluid, significantly affecting the percolation and production of hydrocarbons. However, the associated coupling deformation of kerogen in reservoir conditions remains poorly understood. Here we quantify the kerogen coupling deformation induced by moisture uptake, CH4 and CO2 sorption, and mechanical compression using the combination of molecular simulations and poromechanics theory. The effects of kerogen maturity and preloaded moisture are discussed in detail. Our results show that moisture induced deformation is governed by the dynamic distribution of water molecules and the physicochemical characteristics of kerogen. Moisture induced volumetric strain increases with decreasing kerogen maturity or rising moisture content. The porosity of kerogen skeleton increases upon moisture uptake, although the residual porosity accessible for fluid presents a linearly decreasing trend. The coupling deformation upon gas sorption results from the combined effect of sorption swelling and mechanical compression. The coupling volumetric strain increases with rising kerogen maturity or declining moisture content, contrary to results for moisture uptake. The total deformation, coupling sorption–pressure–moisture effect, rises with increasing kerogen maturity. The evolution the total deformation follows with moisture content depends on the kerogen maturity. Results of this work can help improve the estimation of fluid-in-place in a shale gas reservoir and the understanding of the chemomechanical coupling associated with CO2 sequestration and CO2 fracturing.
