Research On Decomposition Characteristics In Solid Fluidization Exploitation Of Marine Non-Diagenetic Gas Hydrates

Natural gas hydrate is considered as one of the most potential "future energy".In the South China Sea,there are huge reserves of marine natural gas hydrate,having characteristics of shallow-buried,non-dense cover,weak cementation and easy fragmentation,which was defined as non-diagenesis hydrate.At present,the experimental and theoretical research of natural gas hydrate in pure water system has been very mature,and also there are many studies on gas hydrate formation and decomposition in porous media,but the experimental research on the decomposition kinetics of the marine non-diagenetic hydrate has just started.In this paper,we have not only carried out laboratory design and theoretical calculation,but also studied the decomposition law and influencing factors of marine non-diagenetic hydrates,to enhance the prospects of the independent new technology in the development and utilization of flammable ice resources.In this paper,the s I hydrate lattice model has been established.The bonding energy and electron density of state(DOS)of gas hydrates,especially of methane hydrate,have been studied by using density functional theory(DFT).We have built the marine non-diagenetic hydrate experiment apparatus and designed new experiment methodology.A crushing and stirring device has been designed to simulate the "entry-breaking”process in the solid fluidization technology.A seawater compensation device has been designed to remove the free gas in the reactor before the decomposition reaction and to accurately measure the rate of gas hydrate decomposition reaction.Twenty groups of single factor experiments have been designed to study the effect of six factors on the decomposition reaction,including pressure difference,ambient temperature,salinity,particle size of the sediment,stirring speed and concentration of ethylene glycol.Sixteen groups of multivariate experiments have been designed to take into account the effects of pressure difference,ambient temperature and stirring speed on the decomposition of non-diagenetic hydrate under the interaction of three indexes.On the basis of decomposition kinetics experiments,the decomposition rate model of marine non-diagenetic hydrate has been derived.The main conclusions and achievements are as follows:(1)A new device which can be used to simulate the solid fluidization exploitation of marine non-diagenetic hydrate has been established to reveal the impact of solid fluidization technology on the characteristics of marine non-diagenetic hydrates' decomposition.(2)The decomposition rules of marine non-diagenetic hydrate in solid fluidization exploitation were studied through single-factor variable experiments and multivariable comprehensive experiments.Pressure is an important factor affecting gas hydrate phase equilibrium.The greater the drop pressure is,the faster the decomposition reaction could be,and the more obvious the temperature would drop.Salt ions in seawater are natural chemical inhibitors,which weaken the stability of hydrate structure and promote the decomposition of hydrate.In the process of solid fluidization exploitation,a large number of hydrate decompose within the narrow pipe,the pure water will reduce the salinity of hydrate slurry,and along with the rapid fall of system temperature,resulting in the decrease of hydrate decomposition efficiency.Salt water injection into the pipeline can be considered to maintain a high concentration of inorganic salt ions,leading to efficient hydrate decomposition.The decomposition and activation energy of non-diagenetic hydrate in the South China Sea is about 74.13 kJ/mol.(3)Different from the conventional understanding,the decomposition rate of hydrate in solid fluidized exploitation experiment is little affected by the sediment particle size.The reason is that in the solid fluidization experiment,the loose non-diagenetic hydrate have been broken up firstly,and then the hydrate was converted into a large number of tiny hydrate particles,leading to significant increasing in the surface area of hydrate.Therefore,the pore surface area in porous media detennined by sediment particle size and pore size is no longer the main factor restricting hydrate decomposition.(4)Based on solid fluidization technology,this paper introduced the index of "stirring speed" in the decomposition experiment of non-diagenetic hydrate.Stirring helps hydrate spun off from the sedimentary sand body,and large hydrate particles have been broken into smaller ones to further increase the decomposition surface area.At the same time,it can also speed up the heat and mass transfer.The temperature,ion concentration in all parts of the system would become balance as soon as possible,which indirectly promote hydrate decomposition rate.(5)Injection of decomposition promoter(ethylene glycol)has effectively improved the decomposition reaction rate of non-diagenetic hydrate.The higher the concentration of the promoter is,the faster the decomposition reaction would be,but the improvement effect gradually decreases.When the concentration of ethylene glycol reaches 60%or so,the improvement effect reaches the limit.In fact,only 10%?20%of the concentration of ethylene glycol solution can obtain a good decomposition acceleration effect.(6)The decomposition rate equation of non-diagenetic hydrate in solid fluidized exploitation is-dnH/dt=KdcnHn,Kdc=F(?P,T,R,S,M,C).In the experiment,the decomposition reaction of non-diagenetic hydrate is the first order reaction.The degree of depressurization,ambient temperature and stirring speed are three major indicators in solid fluidization exploitation process.With the method of multivariate nonlinear regression,we have established the hydrate decomposition rate constant model in the process of solid fluidization exploitation Kdc=|0.322?P+0.011e(0.5T)+0.02R-1.043|.The model passes the check,and the match accuracy is high.(7)In the microscopic structure of hydrate,the structural stability of carbon dioxide hydrate is stronger than that of methane hydrate,and the binding energy(-2.36 eV)is lower than that of methane hydrate(-0.58 eV).The results of calculation and macroscopic experiments confirm each other and provide a scientific explanation for macroscopic experimental phenomena from the microscopic mechanism.