Molecular dynamics (MD) simulations were performed to determine the three-phase coexistence line of sI methane hydrates. The MD simulations were carried out at four different pressures (4, 10, 40, and 100 MPa) by using the direct phase coexistence method. In current simulations, water was described by either TIP4P/Ice or “optimal” point charge (OPC) models and methane was described as a simple Lennard-Jones interaction site. Lorentz–Berthelot (LB) combining rules were used to calculate the parameters of the cross interactions. For the OPC model, positive deviations from the energetic LB rule were also considered based on the solubility of methane in water. For the TIP4P/Ice water model, the obtained three phase coexistence temperatures showed good agreement with experiment data at higher pressures, which is consistent with previous predictions. For the OPC water model, simulations using the classic and the modified LB parameters both showed negative deviations to the experimental values. Our results also indicated that the deviation of the T₃ prediction by the OPC model was not closely correlated with the predicted melting point of ice. At 4 MPa, the modified OPC model showed a better prediction of hydrate equilibrium temperature, even better than the prediction by TIP4P/Ice. Considering the relatively higher accuracy in biomolecular MD of the OPC model, it is suggested that this model may have a better performance in hydrate MD simulations of biomolecule-based additives.

CH₄/CO₂ replacement is of great significance for the exploitation of natural gas hydrate resources and CO₂ storage. The feasibility of this method relies on our understanding of the CH₄/CO₂ replacement efficiency and mechanism. In this study, CH₄/CO₂ replacement experiments were carried out to study the distribution characteristics of CH₄ and CO₂ in hydrate-bearing sediments during and after replacement. Similar to previously reported data, our experiments also implied that the CH₄/CO₂ replacement process could be divided into two stages: fast reaction and slow reaction, representing CH₄/CO₂ replacement in the hydrate-gas interface and bidirectional CH₄/CO₂ diffusion caused replacement, respectively. After replacement, the CO₂ content gradually decreased, and the methane content gradually increased with the increase of sediment depth. Higher replacement percentage can be achieved with higher replacement temperature and lower initial saturation of methane hydrate. Based on the calculation of CO₂ consumption amounts, it was found that the replacement mainly took place in the fast reaction stage while the formation of CO₂ hydrate by gaseous CO₂ and water almost runs through the whole experimental process. Thus, the pore scale CH₄/CO₂ replacement process in sediments can be summarized in the following steps: CO₂ injection, CO₂ diffusing into sedimentary layer, occurrence of CH₄/CO₂ replacement and CO₂ hydrate formation, wrapping of methane hydrate by mixed CH₄-CO₂ hydrate, continuous CO₂ hydrate formation, and almost stagnant CH₄/CO₂ replacement.

The pore structure of marine natural gas-hydrate-bearing sediments is a key factor related to the physical properties of reservoirs. However, the resolution of micro-computed tomography (micro-CT) images is unsuitable for the analysis of pore structures in fine-grained sediments. In this regard, super-resolution (SR) reconstruction technology is expected to improve the spatial resolution of micro-CT images. We present a self-supervised learning method that does not require high-resolution datasets as input images to complete the training and reconstruction processes. This method is an end-to-end network consisting of two subnetworks: an SR network and a downscaling network. We trained on a self-built dataset of hydrate samples from three different particle sizes. Compared with typical methods, the SR results indicate that our method provides high resolution while improving clarity. In addition, it has the highest consistency with the liquid saturation method with the subsequent calculation of porosity parameters. This study contributes to the investigation of seepage and energy transfer in sediments containing natural gas hydrates, which is particularly important for the exploration and development of marine natural gas hydrate resources.