Study On The Enhancement Technology And Application Of Natural Gas Hydrate Dissociation Process

Natural gas hydrate is not only a potential new energy,but also an efficient material of storing gas.Its dissociation characteristics play a critical role in process control of hydrate exploration and development,hydrate storage and transportation.At present,there is little research on the rapid dissociation of hydrate,especially in sediments.In order to solve the problem of slow dissociation,this paper firstly studies enhancement technology to raise the rate of dissociation,then the dynamic model of rapid hydrate dissociation is constructed and modified.Finally,a rapid hydrate saturation measurement system based on enhancement technology and dynamic model is designed for sampling analysis during exploration.The paper designed a platform of hydrate synthesizing,transferring and visible dissociation to study the influence of hydrate enhanced dissociation in the silica(with apparent volume of 7.6×17=771 cm³,porosity of 0.5 and hydrate saturation of 28±6%)by pressure reduction,fluid disturbance increase,heat injection,inhibitor addition and joint methods etc.The influence of water injection temperature(5.3?50.0?,driving force?T=85?130?),injection rate(1.54?6.10 m L/s),60 wt.%ethylene glycol solution and 3.5 wt.%sodium chloride solution injection on hydrate dissociation rate were mainly studied.The results show that the injection of flowing liquid effectively boosts hydrate dissociation;increasing temperature in water injection has a much better effect than raising injection rate;the enhancement effect on hydrate dissociation with inhibitor sodium chloride solution injection is basically the same as that of water injection,but much better than glycol solution.The maximum average gas production rate per unit volume of hydrate is 38.98 L/L_H·min^-1,and that of unit volume reservoir is 8.10 L/L_r·min^-1.In the early stage of hydrate dissociation,the hydrate dissociation is mainly controlled by heat transfer,indicating that the improvement in heat transfer rate is conducive to strengthening the hydrate dissociation.In the later stage,it is mainly controlled by mass transfer.Reducing the mass transfer resistance is conducive to strengthening hydrate dissociation.Based on the experimental results of rapid hydrate dissociation,the hydrate dissociation kinetics equation is established to describe the relationships between the dissociation rate of hydrate and its quantity,liquid temperature,and thermal properties under the condition of depressurization and liquid injection rate.It was obvious that the hydrate dissociation rate is greatly affected by temperature and thermal properties of the fluid,whereas the influence of the fluid injection rate is weak.In order to improve the dissociation rate,it is necessary to use the combined method of depressurization and liquid injection,injecting the liquid with high temperature,low viscosity,sufficient flow rate,and optimizing temperature and thermal properties of the injected fluid,then followed by the flow rate.Furthermore,the hydrate reservoir conditions were reconstructed by using the host sediments(silty clay)of natural hydrate core samples obtained from the South China Sea to make a more authentic cores of hydrate for enhanced dissociation experiments.The kinetic model of hydrate dissociation by depressurization combined with liquid injection was modified to be more practical.Finally,to meet the requirements of hydrate exploration in the South China Sea,the hydrate dissociation enhancement technology was applied to develop a rapid hydrate saturation measurement system,and a calculation software was equipped.The system can determine the accurate results in 6.3?29 min for samples with saturation less than 60%(absolute hydrate volume is 1.5mol,sample volume is 673 cm³),which shortens the measurement time by 71%?93.7%compared with depressurization dissociation,and the relative error is less than 11.7%.