Origin of Self-preservation Effect for Hydrate Decomposition: Coupling of Mass and Heat Transfer Resistances
Gas hydrates could show an unexpected high stability at conditions out of thermodynamic equilibrium, which is called the self-preservation effect. The mechanism of the effect for methane hydrates is here investigated via molecular dynamics simulations, in which an NVT/E method is introduced to represent different levels of heat transfer resistance. Our simulations suggest a coupling between the mass transfer resistance and heat transfer resistance as the driving mechanism for self-preservation effect. We found that the hydrate is initially melted from the interface, and then a solid-like water layer with temperature-dependent structures is formed next to the hydrate interface that exhibits fractal feature, followed by an increase of mass transfer resistance for the diffusion of methane from hydrate region. Furthermore, our results indicate that heat transfer resistance is a more fundamental factor, since it facilitates the formation of the solid-like layer and hence inhibits the further dissociation of the hydrates. The self-preservation effect is found to be enhanced with the increase of pressure and particularly the decrease of temperature. Kinetic equations based on heat balance calculations is also developed to describe the self-preservation effect, which reproduces our simulation results well and provides an association between microscopic and macroscopic properties.
Numerical study of gas production from methane hydrate deposits by depressurization at 274 K
Abstract As a potential new source of energy, gas hydrate has been the focus of research around the world. In this study, based on a summary of existing models, a one-dimensional mathematical model containing four phases (water, gas, hydrate, and ice phases) and three constituents (water, gas, hydrate) based on the finite difference method (FDM) was established for analysing methane hydrate decomposition at a relatively low temperature condition (approximately 274 K) by depressurization in porous media. This model can be used to investigate gas hydrate exploitation under a wider range of temperatures (e.g., deep seabed or permafrost conditions). When the initial temperature of the hydrate reservoir is approximately 274 K, ice generation occurs during exploitation. This investigation focused on the characteristics of hydrate decomposition, ice generation and ice distribution by changing the parameters of relevant settings. The analysis addressed the effects of ice generation on pressure, temperature, permeability, and cumulative gas production; the influence of other relevant parameters on each other; the influential factors and features of cumulative gas production and the instantaneous gas generation rate. The results showed that ice generation gradually increases during the hydrate decomposition process and occurs early and near the production well due to a large pressure gradient. As an unfavourable factor, ice generation causes the absolute permeability, instantaneous gas generation rate and local pressure to decline. The production well pressure is the determinant of ice generation. Moreover, the final cumulative gas production is determined by the hydrate characteristics, which include the hydrate saturation, reservoir porosity and permeability. Ice generation reduces the gas generation rate, but this does not affect the final cumulative gas production.
Surgical management of facial paralysis resulting from temporal bone fractures
Abstract Conclusion: To achieve good facial reanimation in cases with facial paralysis resulting from temporal bone fractures, the ideal timing for surgical intervention is at least within 1 month of injury and an appropriate surgical approach should be selected depending on the site of facial nerve injury. Objective: This paper aimed to address the ideal time for surgical intervention and the appropriate surgical approach for patients with facial paralysis resulting from temporal bone fractures. Methods: We retrospectively investigated 60 patients with facial paralysis due to temporal bone fractures who underwent facial nerve decompression via different operative approaches within 1 month after trauma, of which 48 were surgically treated by the middle cranial fossa approach (80%), 8 by a pure transmastoid approach (13.3%), and four by a combined transmastoid and middle cranial fossa approach (6.7%). The House–Brackmann (H-B) grading system was used to evaluate the recovery of facial nerve function. Results: The follow-up period for all the patients was 1 year. Among 60 patients who were surgically treated, 39 achieved grade I of facial nerve function, 18 achieved grade II, two achieved grade III, and one achieved grade IV according to the H-B grading system.