Investigation Of The Characteristics Of Methane Hydrate Formation And Decomposition In Marine Muddy Sediments

Natural gas hydrate is a new type of high-efficiency clean energy.The resources in theSouth China Sea reaches about 80 billion tons of oil equivalent.Realizing its safe and efficient exploitation is of great significance to China's future energy strategic security.Natural gas hydrates in the South China Sea behave muddy,low permeability and weak cementation characteristics.Based on the characteristics of real South China Sea hydrate sediments,the formation and decomposition characteristics of methane hydrates in marine muddy reservoirs are investigated,and optimized and efficient exploitation method is proposed,which is vital to the exploitation and utilization of natural gas hydrate resources in the South China Sea.In this paper,based on the muddy sediments in the Shenhu area of the South China Sea,standard samples of hydrate reservoirs are prepared.And a standard method for sample preparation are proposed.In connection with the low-permeability characteristics of natural sediments,the study of hydrate formation characteristics in marine mud sediments are carried out,which reveals a significant difference in hydrate formation behaviors between muddy and sandy reservoirs.Among them,the induction time of hydrate formation in muddy reservoirs is about 2.5 times that in sand and the formation process is more slowly;the dynamic control mechanism of this difference is analyzed using the phase equilibrium curve decomposition segment method,it is found that the driving force required for the formation is greater.Meanwhile,in view of the muddy hydrate reservoirs,the hydrate decomposition characteristics are investigated from the aspects of back pressure control and reservoir hydrate accumulation.It is found that the tight muddy sediments hinder the mass transfer of hydrate decomposition gas production process.It confirms the importance of reasonable gas production back pressure control and sieve hole size selection for sand block;the kinetic factors of low gas production back pressure promoting hydrate decomposition are analyzed;the risk of reservoir freezing or secondary formation caused by low thermal conductivity of muddy sediments during the exploitation process is clarified.Through the regional difference of the minimum temperature in the reservoir,the promotion of the surrounding heat transfer and the increase of the sensible heat of the reservoir duo to the high hydrate saturation from the perspective of energy supply are analyzed.In order to increase the temperature of the reservoir during the hydrate dissociation process and inhibit the freezing of the reservoir,this paper develops a optimized step-wide depressurization method,and introduces Stefan(Ste)number and average gas production rate to establish the relationship between thermodynamics and kinetics of the hydrate decomposition process.The results show that the reservoir heat absorption process during the low gas production stage is intensified,which effectively avoids the excessive heat loss of the reservoir,and promotes hydrate decomposition fully and stably.The small gradient depressurization operation benefits for heat transfer and enhances the gas production rate from hydrate decomposition.When the depressurization gradient is 0.5 MPa,the minimum temperature of the central reservoir is promoted from-0.5 ? to 0.27 ?,which can effectively avoid the ice formation;The analysis results of Ste number and average gas production rate show a positive correlation between them.It indicates that the promotion of sensible heat of the reservoir can accelerate the hydrate dissociation rate.It reveals that the sensible heat of the reservoir dominates the gas production process in the low thermal conductivity hydrate reservoir.