Stability analysis of the water bridge in organic shale nanopores: A molecular dynamic study

In the last decades, shale gas development has relieved the global energy crisis and slowed global warming problems. The water bridge plays an important role in the process of shale gas diffusion, but the stability of the water bridge in the shale nanochannel has not been revealed. In this work, the molecular dynamics method is applied to study the interaction between shale gas and water bridge, and the stability can be tested accordingly. CO₂ can diffuse into the liquid H₂O phase, but CH₄ only diffuses at the boundary of the H₂O phase. Due to the polarity of H₂O molecules, the water bridge presents the wetting condition according to model snapshots and one-dimensional analyses, but the main body of the water bridge in the two-dimensional contour shows the non-wetting condition, which is reasonable. Due to the effect of the molecular polarity, CO₂ prefers to diffuse into kerogen matrixes and the bulk phase of water bridge. In the bulk of the water bridge, where the interaction is weaker, CO₂ has a lower energy state, implies that it has a good solubility in the liquid H₂O phase. Higher temperature does not facilitate the diffusion of CO₂ molecules, and higher pressure brings more CO₂ molecules and enhances the solubility of CO₂ in the H₂O phase, in addition, a larger ratio of CO₂ increases its content, which does the same effects with higher pressures. The stability of the water bridge is disturbed by diffused CO₂, and its waist is the weakest position by the potential energy distribution.