Dynamic behavior and permeability evolution of CO₂ hydrates during dissociation by depressurization: Insights from low-field nuclear magnetic resonance

Natural gas hydrates are considered a sustainable energy source. Despite their extraction in various countries, issues such as formation sanding and secondary hydrate generation frequently occur, leading to significant reductions in gas production or even production shutdowns. These challenges primarily arise from inadequate research on the formation and dissociation processes of hydrates influenced by pore medium properties and a limited understanding of how reservoir porosity and permeability evolve during hydrate extraction. This study investigates the dynamic properties of CO₂ hydrates during formation and dissociation using a low-field nuclear magnetic resonance (LF-NMR) system to analyze hydrate pore characteristics in porous media. The results indicate that hydrates form at the inlet before forming in the middle and outlet sections, causing blockages in the gas flow channels. During hydrate dissociation at 4°C and a pressure of 1.4 MPa, the hydrate decomposes axially from both ends toward the center and radially from the edges toward the center. The permeability evolution during hydrate dissociation, calculated using the SDR model and compared with other hydrate permeability models, shows that the Masuda model (N=8) fits the experimental data best. Most data points lie between the predictions of the parallel capillary model (pore-filling) and the Kozeny grain model (pore-filling), suggesting that hydrates preferentially dissociate starting from the edges of the pores.