| The natural gas hydrate resources are abundant in Chinese waters,and its safe and efficient exploitation has been listed as a major energy development project in the national13 th and 14 th Five-Year Plans.At present,the results of hydrate sampling show that the shallow surface natural gas hydrate reservoirs in the South China Sea are mostly nondiagenetic,and have the characteristics of shallow burial,no tight cap,loose mineral deposits,low degree of cementation and easy fragmentation.If conventional methods such as depressurization,thermal excitation and injection inhibitor are used for mining,it is easy to cause uncontrollable,unsafe and environmentally unfriendly problems such as difficult collection and leakage of decomposed gas.The solid fluidization exploitation method is a new technique proposed by Chinese scientists for the mining of nondiagenetic hydrate.The technology uses a mechanical device on the seabed to break the ore body containing hydrate into fine clastic particles,which are then transported to a surface platform for subsequent processing.However,as this technology is still in its infancy,the preliminary studies mainly focus on technical equipment,and there are few studies on the flow rules of the containing hydrate particles during the transportation process.Therefore,this paper takes hydrate-containing detrital particles as the research object,takes gas-liquid-solid multiphase flow in wellbore as the core,and aims to achieve the safe and efficient transportation of hydrate-containing detrital particles.The following studies are carried out:1.Based on a large gas-liquid-solid three-phase flow experimental system,the influences of key factors such as gas content,sand particle size,sand concentration and angle of inclination on sand migration and deposition were studied,and a new prediction model for critical sand carrying velocity under gas-liquid two-phase condition for solid fluidization exploitation was derived.The experimental and theoretical analysis results show that intermittent flow and agitated flow are more favorable to sand carrying than stratified flow.The critical sand carrying velocity is positively correlated with angle of inclination and sand particle size,but negatively correlated with gas content and sand concentration.The influence degree of sand particle size,angle of inclination,gas content and sand concentration on critical sand carrying velocity decreases successively.2.Based on the hydrate intrinsic kinetic model,considering the heat and mass transfer in the process of hydrate decomposition,combined with the thermodynamic model of hydrate phase equilibrium,a dynamic decomposition model of hydratecontaining particles under the flow state was established to study the decomposition behavior of hydrate.The effects of activation energy,flow velocity,temperature and pressure on decomposition rate,diameter and surface temperature of hydrate-containing particles were analyzed.The results show that the decomposition of hydrate-containing particles is a non-isothermal process.Compared with the mass transfer rate,the intrinsic dynamics and heat transfer rate of hydrate have more influence on the decomposition rate of hydrate-containing particles.Increasing the activation energy of the decomposition reaction,increasing the flow rate and the wellbore temperature will promote the decomposition of hydrate-containing particles.3.Based on the wellbore hydraulics model,considering the coupling relationship between multiphase flow,heat transfer and hydrate decomposition in the wellbore,combined with the dynamic decomposition model of hydrate-containing particles under the flow state,a gas-liquid-solid multiphase non-isothermal transient flow model was established considering the hydrate phase transition.The variation law of wellbore temperature,pressure,phase volume fraction and hydrate decomposition rate during solid fluidization mining is studied.The influences of mining rate,wellhead backpressure,injected seawater flow,salinity,temperature and wellbore diameter on the wellbore multiphase flow law are analyzed.The results show that with the increase of time,the critical well depth of hydrate decomposition increases gradually,the hydrate decomposition rate increases gradually,and the gas volume fraction increases continuously.Increasing the mining rate and the flow,salinity and temperature of injected seawater will promote the hydrate decomposition.The hydrate decomposition rate,critical depth and amount of hydrate-containing particles in wellbore can be controlled by adjusting wellhead backpressure.4.Based on the gas-liquid-solid multiphase non-isothermal transient flow model and combined with the new prediction model of critical sand carrying velocity under gasliquid two-phase condition,the optimized design model of key parameters of the natural gas hydrate solid fluidization exploitation was established.The influence laws of key parameters such as mining rate,wellhead backpressure,injected seawater temperature,flow rate,salinity and wellbore diameter on gas-solid production were analyzed,and the optimal combination of mining parameters was designed to ensure wellbore safety and maximize gas-solid production.The results show that increasing the flow rate,salinity and temperature of injected seawater can improve the gas production.Increasing wellhead backpressure and wellbore diameter can increase solid phase production.Increasing the production rate will shorten the time for the critical depth of hydrate decomposition to reach the stable.In this paper,the gas-liquid-solid three-phase flow law in wellbore during the natural gas hydrate solid fluidization exploitation is studied from three aspects: laboratory experiment,theoretical model and optimization design.The research results can provide theoretical and methodical reference for ensuring the safe and efficient transportation of hydrate-containing detrital particles during solid fluidization exploitation.It is of great significance to perfect the hydrate solid fluidization exploitation technology. |
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