String Structure Design And Flow Safety Guarantee For Depressurization Of Natural Gas Hydrate In Marine

Natural gas hydrate is regarded as an important strategic reserve resource in the future because of its large amount of resources,high efficiency.At the same time,low gas production,poor gas-water separation effect,large fluctuation of bottom hole pressure,secondary formation of hydrate blocking production string and other operational risks appear in the trial production of natural gas hydrate in the sea area,and the trial production is seriously even interrupted.Therefore,based on the goal of safe and efficient exploitation of marine hydrate,through numerical simulation,reasonable prediction of hydrate production capacity change trend,optimization of hydrate exploitation well string structure design,establishment of hydrate formation area quantitative prediction method considering complex string temperature and pressure profile,combined with the characteristics of marine hydrate development,the flow guarantee scheme of exploitation well is given.Firstly,a numerical model of depressurization is established in this paper,and the model is calculated by TOUGH+HYDRATE,and compared with the experimental results of hydrate depressurization by other scholars,the model in this paper is in good agreement with the experimental results.On this basis,the gas and water production of hydrate depressurization horizontal wells are predicted.Under the simulation conditions,the average gas and water production of the hydrate production well is 73238 m³·d^-1 and 295 m³·d^-1,respectively.The predicted gas and water production provides a basis for the design of string structure and prevention of secondary hydrate formation.Secondly,based on the water production of natural gas hydrate,the string structure of natural gas hydrate exploitation well is designed,and the designed string structure is optimized by numerical simulation software.It mainly includes three parts:the pressure drop and separation efficiency of gas production pipeline are the key evaluation parameters.Under the simulation conditions in this paper,the optimal pitch of spiral gas-liquid separator is 34 mm and the number of turns is 15;the electric submersible pump in the production pipeline is mainly used to optimize the high-yield water in the early stage of mining,so as to efficiently and timely transport the produced water in the wellbore to the platform,and then stabilize the bottom hole pressure;in the gas-liquid mixed pipeline,the unsteady pressure drop caused by gas-liquid two-phase flow can be changed into stable pressure drop through the vertical pipe with coil,and the liquid carrying efficiency can be improved.Under the simulation conditions in this paper,the optimal pitch of coil is 50 mm and the diameter is 30 mm.The design and optimization of string structure lay a foundation for the calculation of temperature and pressure field and the prediction of hydrate formation area.Finally,based on the calculation models of temperature and pressure fields of different pipe strings and the calculation of gas hydrate phase state in the process of hydrate exploitation,the quantitative prediction method of hydrate formation area in exploitation wells is established.Under the condition of gas and water production in this paper,the risk of secondary hydrate formation in different strings is predicted.For the string structure in this paper,there is a high risk of secondary formation of hydrate in the gas production pipeline.The greater the pressure drop,the smaller the risk of hydrate formation.Under the average gas production,there is a risk of secondary formation of hydrate from the water depth of 400 m to the bottom of the well.The flow guarantee methods for different production are given:for average gas production,30 wt%ethylene glycols should be injected;For high gas production,heating 30℃is used or 20 wt%ethylene glycols.This paper can provide a reference for the exploitation of natural gas hydrate in the sea area.