Mechanism Study On The Decomposition Evolution Of Methane Hydrate During Exploitation Process By Fluidization

Marine gas hydrate is a kind of potential energy with abundant reserves.Realizing its efficient exploitation is the focus of energy research that the world is generally concerned about.The solid-state fluidization exploitation method is a new technology proposed for nondiagenetic and shallow hydrate storage.The main process is to fluidize the submarine hydrate and pipe it to an offshore platform for decomposition.An increase in the temperature of the hydrate fluidization process system will cause the hydrate to decompose,and the generated gas will cause abnormally complex flow in the pipeline transportation process and increase transportation risks.The inherent safety requirement of fluidization exploitation is to avoid a large amount of hydrate decomposition in the process of fluidization and pipeline transportation.The key scientific problem is to clarify the influence law of complex parameters and conditions on the hydrate decomposition evolution during fluidized exploitation.This article focuses on the hydrate phase transition characteristics in fluidization and pipeline transportation.The thermodynamic stability,thermal decomposition characteristics of methane hydrate,and its decomposition law during fluidization and pipeline transportation have been emphatically studied,providing a theoretical basis for hydrate fluidization exploitation technology.First of all,research has been carried out on the phase equilibrium characteristics of methane hydrate under in-situ conditions in the South China Sea sediments.The size of the sediments,the types and concentrations of conventional salt ions in the South China Sea at a depth of 122 m at the Liwan 3 station have been carefully analyzed.The phase equilibrium data of natural gas hydrate in the three kinds of water content(30 wt%,35 wt%,40 wt%)are obtained.The changing characteristics of the methane hydrate cages proportion under constant temperature and pressure conditions have been experimentally studied.The hydration number of the synthetic sample in the laboratory does not reach 5.75(the ideal value for type I methane hydrate).The influence of temperature and pressure driving force on the proportion of methane hydrate cage is proved.The variation range(5.80～8.22)of the methane hydrate hydration number under different temperature and pressure conditions has been experimentally determined,which provides basic data for parameter setting of fluidization exploitation process.Secondly,the evolution of temperature and pressure during the methane hydrate thermal decomposition process has been experimentally studied.The temperature and pressure curves of the methane hydrate dynamic decomposition process were obtained by the constant-volume heating method.It is found that the increase in heating rate has a greater impact on the temperature than the pressure.The existence of the limit decomposition rate of methane hydrate itself has been revealed.The staged heating hydrate decomposition process experiment was carried out.It is found that during the decomposition process,when the temperature stops,the hydrate evolves from a non-equilibrium state of decomposition to a stable equilibrium state.The stable equilibrium curve of methane hydrate under different heating holding conditions has been determined.At the same time,the heating process of the methane hydrate-ice coexistence system was studied.It is found that ice controls the temperature of the system stronger than that of hydrates.The methane hydrate decomposition rate in the ice melting stage is significantly reduced.By comparing the hydrate decomposition rate at the constant volume and constant pressure stages,it is found that the gas production of the constant pressure open system promotes the methane hydrate decomposition,which provides a theoretical basis for the decomposition of methane hydrate in the fluidization exploitation process.Finally,a high-pressure storage fluidization system and a low-temperature and highpressure pipeline circulation system were built.The experimental simulations of fluidization and pipeline transportation have been realized separately.The three control stages of pressure control,concentration control and balance in the fluidization process have been discovered.The influence of methane hydrate cage proportion on its phase transition evolution direction is clarified.The influence of fluidization parameters(rate,duration,temperature)on the methane hydrate decomposition amount is quantitatively revealed.The similarity between the methane hydrate formation curve and the decomposition curve in the pipeline transportation of marine sediment slurry system has been found.The thermodynamic transition interval of the formation and decomposition state of methane hydrate is obtained.The influence of pipeline transportation conditions(flow rate,slurry water content)on the formation and decomposition of methane hydrate has been quantitatively revealed.