Prevention Measures For Gas Hydrate Formation In Production Wells Of Deepwater Gas Fields

During the development of deep-water gas fields,due to the high pressure in the production wellbore and the low temperature conditions near the seabed,gas-liquid fluids are very easy to form natural gas hydrates in seabed wellheads,chokes and submarine pipelines,causing blockage of the wellbore,production reduction and even safety accidents.Therefore,it is of great significance to analyze the risk of hydrate formation in different working conditions and different links in the process of deep-water gas field development,and put forward safe,economic and effective hydrate prevention measures based on the risk analysis results,so as to ensure the efficient development of deep-water gas field.Aiming at the wellbore conditions of LS17-2 deep-water gas field in South China Sea,this paper uses the methods of literature research,theoretical analysis and laboratory experiments to predict the hydrate formation risk in the process of normal production,shut in,restart and submarine gas pipeline by using PIPESIM multiphase flow steady-state simulator,hydrate phase state prediction model and wellbore transient temperature and pressure field calculation model.Using self-designed high-pressure and low-temperature stirring hydrate experimental device and hydrate formation loop experimental device,the selection and effect evaluation experiments of kinetic inhibitor formula are carried out,and the kinetic inhibitor formula suitable for different working conditions is proposed,which provides the basis for the optimization of hydrate comprehensive prevention and control scheme for wellbore,subsea choke and subsea pipeline.Finally,based on the above risk analysis results and the proposed kinetic inhibitor formula,hydrate prevention programs under different working conditions are formulated and economic analysis is carried out.The results show that the higher the stable gas production or the lower the formation pressure in the normal gas production process,the smaller the risk of hydrate formation,and the influence of tubing size on the risk of hydrate formation needs to be determined in combination with the specific temperature and pressure conditions.The risk of hydrate formation decreases year by year with the increase of the production period,but due to gas-liquid separation,the risk of blockage is not significant in the entire production cycle.In the process of restart,the higher the opening rate,the lower the opening frequency or the higher the water gas ratio,the faster the wellbore temperature rises and the faster the risk of hydrate formation.During the shut-in-restart process,the greater the rate of well opening and the lower the frequency of well opening operations,or the higher the water gas ratio,the faster the temperature of the wellbore rises,and the faster the risk of hydrate formation from the wellbore is released.In addition,the induction time of the five kinetic inhibitor systems selected in the laboratory test can reach 200-2000 min under the condition of supercooling of 8.5-17.2℃,which can be applied to high pressure and low production,shut in restart and subsea gas pipeline.For the target gas field,there is no risk of hydrate formation during production at the designed gas-liquid output during the entire development cycle,but under the extreme special conditions of high pressure and low production,hydrate formation will occur when the gas production is less than 100,000 m³·d^-1.Thermodynamic or kinetic inhibitors can be used for prevention.Among them,methanol has the lowest cost.Compared with ethylene glycol,the lower the gas production,the more advantageous the kinetic inhibitor.Since continuous injection is required,it is recommended to inject ethylene glycol or Kinetic inhibitor.After shutting in the well at the initial stage of production,the wellhead subcooling degree can reach21℃,and the lower limit of the hydrate stability zone is about 520 m below the mudline.it is recommended to inject methanol for prevention and control.During the shut-in-restart process in the early stage of production,each gas well can be free from hydrate risk in 130-200 minutes,so methanol injection is recommended for prevention and control.During the shut-in-restart process in the middle and late stage of production,the scheme of injecting kinetic inhibitor can be adopted.The cost of kinetic inhibitor for hydrate control in submarine gas pipeline is lower than that of ethylene glycol,so it is suggested to adopt the scheme of kinetic inhibitor.