Kerogen is the main host of methane in most shale formations. Although many studies focused on its isothermal adsorption, the relationship between its methane adsorption capacity and structural characteristics has yet been well addressed. The pore structures and methane adsorption of four kinds of type-II kerogens with different maturities and at a varying burial depth are studied using a combined approach of molecular dynamics and grand canonical Monte Carlo simulations. Attributed to the combined effect of temperature, pressure, and the surface structures of kerogen, the methane adsorption capacity varies with the kerogen maturity and the burial depth. A maximum methane adsorption is predicted at the depth of 1–3 km, which agrees with the observations. Meanwhile, a mismatch between the porosity change and the adsorption change of kerogens is noted and then interpreted by the surface structure evolution of kerogens. Analysis on the amorphous kerogens reveals that the surface and bulk compositions of kerogens are different, and the difference varies with the kerogen maturity. Methane adsorption depends not only on the porosity but also on the proportion of aromatic and aliphatic carbon atoms on the surface. The simulations are expected to be useful for the reserve prediction and interpretation of shale gas formations.

Spectral diffusion occurs in semiconductor quantum dots (QDs) under an external electric field, resulting from the transition energy shift of one single dot during optical absorption and emission processes. The classical electric polarization model fails to interpret the spectral diffusion in some cases when strong quantum confinement Stark effects and large Stokes shifts exist in the small-sized QDs. Based on the two-process (optical absorption and emission) four-state (the ground-and excited-states involved in each process) model, the field-induced spectral shifts of the (CdSe) 33 and (CdSe) 34 structures, which are computed with the first-principles calculations, are well fitted by the dipole moment and static polarizability changes in the separate processes. Furthermore, the linear and the quadratic variations of the field-induced spectral shifts are interpreted with the compressible and the incompressible electron−hole separation distances, which are computed from the Kohn−Sham levels involved in the excited states. Our study provides the direct pieces of evidence on the hypothesis summarized experimentally that the electron transition energy increases with the decreasing electron−hole separation. As a result, the spectral shifts of one single QD, as well as the spectral diffusion of QDs, are then adjustable by tuning the direction and strength of the external field.

Sulfur deposition usually occurs in the development of highsulfur gas reservoirs, threatening the production efficiency and mining safety by blocking the pore throats. Prediction on the solubility evolution of elemental sulfur in the sour gases is fundamental for controlling over sulfur deposition. It has been well-recognized that H 2 S is the main solvent in the sour gases. However, it remains unclear how the other components affect the sulfur dissolution in H 2 S. The solubility evolution of elemental sulfur (S 8 ) in the binary mixtures (H 2 S and CO 2 ), ternary mixtures (H 2 S, CO 2 , and CH 4 ), and mixtures up to five components is studied using molecular dynamics simulations under varying temperature, pressure, and component. On the basis of reproducing the experimentally observed dramatic solubility reduction, the antisolvent role of CO 2 and CH 4 in the sulfur dissolution is revealed. The solubility reduction stems from not only the low solubility in CO 2 and CH 4 but also the competitive interaction of CO 2 and CH 4 with H 2 S, and the latter is decisive. Our findings would be useful for the establishment of the sulfur dissolution/deposition model, which is essential for the development of high-sulfur natural gas reservoirs.

The dielectric properties in semiconductor quantum dots are crucial for exciton formation, migration and recombination. Different from 3D bulk materials, the dielectric response is however ambiguous for the small-sized 0D dots in which the effect of outer atoms on the inner atoms is usually described qualitatively. Based on the first-principles calculated electron density, the polarizability of the core-shell CdSe@ZnS wurtzite quantum dots is decomposed into the distributional contributions among which the dipole polarizability of the core is proposed to measure the shell effect on the dielectric properties of core-shell quantum dots. The shell thickness dependence on the shell effect is then studied, which is significant for the outermost shell, but decays rapidly in the additional shells. Moreover, this model gives explicit physical origins of the core dipole polarizability in the core-shell QDs, which is determined by the intra-shell polarization and inter-core-shell charge transfer. Our study proposes a new approach for studying the dielectric properties of core-shell quantum dots, which is effective and extendable for other low-dimensional structures.