The demand for energy in the world has increased dramatically, leading to an increase in the geological depth of unconventional natural gas development such as shale gas. Comprehension adsorption and diffusion of CH₄ at different geological depths is extremely significant, especially in the case of large depths. In this study, the molecular simulations were utilized to investigate the CH₄ adsorption, diffusion, and swelling in kaolinite up to 6 km. The outcomes show that the adsorption amount of CH₄ on kaolinite increases to the maximum at 2 MPa, while it decreases with increasing geological depth. This phenomenon is explained by the total interaction energy and adsorption isosteric heat between CH₄ and kaolinite. The hydrogen and oxygen atoms in kaolinite molecules are the strongly adsorbed sites of CH₄. Because of the wall effect, two strong adsorption layers of CH₄ molecules were formed adjacent to the kaolinite surface. The CH₄ coefficient, including that of self-diffusion and Fick diffusion, increased linearly with the geological depth, and the CH₄ diffusion activation energy in kaolinite is about 1.61 kJ/mol at 1–6 km. The kaolinite volume increases first, then slightly decreases, and finally obviously increases with increasing geological depth. By considering the results of adsorption, diffusion, and expansion, we infer that the optimal kaolinite-bearing shale gas reservoir is buried at a depth of 3–4 km.

中文翻译：

---

分子模拟对地质深度对高岭石中CH ₄吸附，扩散和溶胀的影响

世界上对能源的需求急剧增加，导致页岩气等非常规天然气开发的地质深度增加。CH ₄在不同地质深度的理解吸附和扩散极为重要，特别是在大深度情况下。在这项研究中，利用分子模拟研究了高岭石中长达6 km的CH ₄吸附，扩散和溶胀。结果表明，CH ₂在高岭石上的吸附量在2 MPa时增加到最大值，而随地质深度的增加而减少。CH ₄之间的总相互作用能和吸附等规热可以解释这种现象和高岭石。高岭石分子中的氢和氧原子是CH ₄的强吸附位。由于壁效应，在高岭石表面附近形成了两个CH ₄分子的强吸附层。CH ₄系数（包括自扩散和Fick扩散系数）随地质深度线性增加，在1-6 km处高岭石中CH ₄扩散活化能约为1.61 kJ / mol。高岭石的体积先增加，然后略有减少，最后随着地质深度的增加而明显增加。通过考虑吸附，扩散和膨胀的结果，我们推断出最佳的含高岭石页岩气藏被埋在3-4 km的深度。