| Natural gas hydrate is a clean energy with huge reserves.It is widely found in submarine slopes and plateau permafrost.Hydrate mining is a complex process with phase transition and multiphase migration in porous media.Meanwhile,hydrate formation and reformation during hydrate transportation are gas migration obstacles affecting hydrate mining efficiency.Therefore,the non-hydrate region(gas-water system)and the hydrate region(gas-water-hydrate system)in the seabed sediments were studied in a porous medium by means of a nuclear magnetic resonance(MRI)visualization experimental system and numerical simulation software.Hydrate formation and reformation properties under gas migration and its influencing factors were studied,and the inhibition method of hydrate reformation was proposed.That is great value for preventing hydrate reformation and improving of hydrate production efficiency.To clarify hydrate formation characteristics in gas-water system under gas migration conditions,the influence of gas migration direction,rate and pore water saturation on hydrate formation was analysed.It was found that gas migration can enhance the gas-water mass transfer and significantly increase pore water convection rate by 1.5-2.1 times.Gas migration direction affects the longitudinal distribution of pore water.The initial nucleation mode of hydrate affects hydrate formation rate,growth pattern and distribution in porous medium.When the hydrate nucleation mode is instantaneous,the hydrate formation rate is higher.Hydrate growth direction is extended from the reactor wall to the middle,and it is characterized by local aggregation in the porous medium.When hydrate formation is process nucleation,hydrate formation rate is slow.Hydrate formation position is random and finally distributed uniformly in the porous medium.The above studies provide an important basis for controlling the formation of hydrates under gas migration conditions and predicting the location of the plug after generation.In view of hydrate reformation under gas migration conditions in gas-water-hydrate system,the effects of gas migration rate,pressure,pore water saturation and residual hydrate on hydrate reformation were investigated.Those factors have a periodic impact on the hydrate reformation:residual hydrate is the main influencing factor in the initial stage;after that its influence gradually decreases and the influence of pressure increases.The pore water saturation has an inhibitory effect on the hydrate reformation saturation.As the pore water increases,the gas-liquid contact area decreases as well as hydrate reformation saturation.Therefore,a method of increasing pore water to suppress hydrate reformation is proposed.Residual hydrate is the dominant factor for hydrate reformation rate.Based on the influence of above factors,a method of increasing pore water saturation to inhibit hydrate reformation is proposed.In this paper,hydrate formation and reformation characteristics in gas-water system and gas-water-hydrate system under gas migration conditions were studied by numerical simulation method.The temperature and pressure distribution and seepage property in porous medium were supplemented,which could not be obtained in experiments.The effects of gas migration rate,gas pressure,pore water saturation and residual hydrate on hydrate formation and reformation were found consistent with experimental results.The inhibition method of secondary formation of hydrate proposed in the experiment was evaluated,and the negative linear relationship between the secondary generation saturation of pore water and hydrate was established.The results showed that the final hydrate saturation decreased 2%by every 10%increase of pore water saturation.This paper provides important theoretical support for improving the efficiency of natural gas hydrate production.This study provides important theoretical support for efficient and safe hydrate mining. |
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