Quantitative determination of pore‐structure change and permeability estimation under hydrate phase transition by NMR

Dynamic Dissociation Behaviors of sII Hydrates in Liquid Water by Heating: A Molecular Dynamics Simulation Approach

An understanding of the dynamic behavior of subtle hydrate dissociation in the liquid water phase is fundamental for gas production from marine hydrate reservoirs. Molecular dynamics simulations are performed in this study to investigate the dissociation kinetics of pure propane and binary propane + methane sII hydrates in a liquid water environment. The results show that faster hydrate dissociation rates are observed at higher initial temperatures. The hydrate phase dissociates from the cluster surface to the inside in a layer-by-layer manner under the simulation temperature conditions, which is similar to the behavior of sI hydrates and is independent of the hydrate crystal type. Compared to the binary sII hydrate, the pure sII hydrate dissociates more easily under the same initial temperature conditions, which can be attributed to the stabilizing effect of guest molecules in the hydrate cages. The empty cages collapse in one step, in contrast to the two-step pathway induced by the guest-host interaction. In addition, a hydrocarbon phase forms in the binary hydrate dissociation system instead of nanobubbles. These results can provide molecular-level insights into the dynamic mechanism of hydrate dissociation and theoretical guidance for gas recovery by thermal injection from marine hydrate reservoirs.

Advances in Characterizing Gas Hydrate Formation in Sediments with NMR Transverse Relaxation Time

The formation process, structure, and distribution of gas hydrate in sediments have become focal points in exploring and exploiting natural gas hydrate. To better understand the dynamic behavior of gas hydrate formation in sediments, transverse relaxation time (T2) of nuclear magnetic resonance (NMR) is widely used to quantitatively characterize the formation process of gas hydrate and the change in pore characteristics of sediments. NMR T2 has been considered as a rapid and non-destructive method to distinguish the phase states of water, gas, and gas hydrate, estimate the saturations of water and gas hydrate, and analyze the kinetics of gas hydrate formation in sediments. NMR T2 is also widely employed to specify the pore structure in sediments in terms of pore size distribution, porosity, and permeability. For the recognition of the advantages and shortage of NMR T2 method, comparisons with other methods as X-ray CT, cryo-SEM, etc., are made regarding the application characteristics including resolution, phase recognition, and scanning time. As a future perspective, combining NMR T2 with other techniques can more effectively characterize the dynamic behavior of gas hydrate formation and pore structure in sediments.