Research On The Depressurization Decomposition Characteristics Of Methane Hydrate Under Different Occurrence Conditions

With the continuous increase of energy consumption and the gradual depletion of traditional fossil fuels,Natural Gas Hydrate(NGH)has triggered great interest of many countries.However,by present,the researches of the gas hydrate exploitation have still remained at the stage of field tests and experimental simulations.In the process of realizing the commercial exploitation of NGH,how to efficiently control the sediment deformation and guarantee the safety during production has become a research focus.To investigate the decomposition behaviors of methane hydrate by depressurization and study the sediment deformation during hydrate decomposition,in this work,series of experiments,which simulate the reservoir in the South China Sea,were carried in a novel three-dimensional(3-D)high-pressure reactor with a quick-opening component.Two groups of contrast experiments were successively carried out with this apparatus.One was in two kinds of sediments,which were sandy sediment and marine silty clay sediment,and the other was with two distribution states of block and dispersion.(1)For hydrate in sandy sediment,the pressure-temperature(P-T)relationship is consistent with that of bulk hydrate.However,in silty clay sediment,the hydrate decomposition conditions shift into a higher pressure region than that in sandy sediment,which is mainly due to the small pore particle size and the presence of salinity;(2)It is considered that there is a variable equilibrium for the methane hydrate deposits in silty clay sediment.Hydrate decomposition conditions in silty clay sediment vary at different positions in the reactor because of the effects of salinity,organics,minerals and the uneven distribution of the pores and pore particles.;(3)The formation of methane hydrate in ice powder mainly undergoes four processes including the free diffusion and adsorption of gas molecules,the formation of the intermediate complex(Intermediate of Methane Hydrate,IMH),the diffusion and bonding of IMH and the hydrate formation.Furthermore,the decomposition of this kind of hydrate is the inverse process of the formation reaction.In block hydrate deposit,because the reaction core formed by H2 O molecules was quite complicated,the complex number ‘n' of IMH had a large span.Thus there were many kinds of intermediates,which made the depressurization process during the block hydrate decomposition present multiple stages;(4)Through opening the reactor after the hydrate decomposition experiments,fractures,subsidence and faults were observed in the sediments.The fractures are considered to be a combined consequence of gas seepage and the cementation effect of hydrate and this phenomenon is called Radial Shrinkage Effect of Hydrate Decomposition(RASHEHD).