Research On Multiphase Unbalanced Pipe Flow In Solid Fluidized Mining Of Marine Gas Hydrate Reservoirs

The resource of marine natural gas hydrate is about 2.83 × 1015 m3,which is more than 100 times of that on the land.The total resource of hydrate is as much as 1.56 times of natural gas reserves in the world.Currently,the characteristics of acquired marine natural gas hydrate samples are poorly cemented in shallow and surface layer.They do not have rock frame fabric or strong cementation.The Academician of Chinese Academy Engineering has put forward a method for submarine poorly cemented natural gas hydrate reservoir,which is called solid fluidized excavation.That is an innovative technology for hopeful exploitation of that hydrate reservoir.In solid fluidized excavation,hydrate is excavated in solid with no change of initial temperature and pressure and it will be transported to offshore platfonn.Through this method,this resource is exploited effectively and safety risks are avoided like production,equipment and environment.But the main scientific technical problems are:complex multiphase nonequilibrium pipe flow regular patterns,operation parameters optimization method,well control safety and experimental evaluation method.Based on this,the studies on multiphase nonequilibrium pipe flow in marine natural gas hydrate reservoir solid fluidized excavation have been accomplished in this thesis.The main works are shown as follows:(1)After full investigation and research,reservoir characteristics,exploitation risks and technical problems of submarine poorly cemented hydrate in shallow and surface layer have been analyzed.Through adaptability analysis,technology advantages are compared with conventional exploitation methods.Then critical scientific issues are summed up.(2)Based on hydrate decomposition mechanism,natural gas hydrate decomposition mathematical model is established in pipe flow.Also,assistant models like decomposition area,decomposition rate constant,fugacity of methane gas and phase equilibrium equation of hydrate are established.Then numerical simulation method is used to get influence laws of temperature,pressure and pipe flow rate on hydrate decomposition rate.And molecular dynamics simulation method is selected to study natural gas hydrate decomposition change regulation in microcosmic view.(3)Then wellbore temperature and pressure,multiphase flow mathematical models are established in solid fluidized excavation.At the same time,solid phase starting models considering hydrate cohesive force in horizontal wellbore and solid phase migration model in vertical wellbore are established.Then through numerical simulation method,the influence laws of hydrate saturation,particle diameter,liquid phase density and viscosity are analyzed for starting critical flow rate.Considering the coupling interactions of hydrate nonequilibrium decomposition and multiphase flow in wellbore,complex media nonequilibrium multiphase flow characteristic parameters prediction method is established.According to initial and boundary condition,finite difference method is used for numerical simulation.And the regularity understandings are got by sensitivity analysis of key construction parameters like delivery rate,rate of penetrate,drilling mud density,hydrate reservoir drilling cuttings diameter,marine riser diameter and wellhead back pressure.Then the parameters optimization evaluation system is established.(4)Using complex media nonequilibrium multiphase flow characteristic parameters prediction method,based on pipe flow safety,following safe and efficient exploitation principle,operation parameters are designed and optimized.(5)Existing multiphase flow test benches are used for multiphase flow and cutting carrying experimentation in vertical wellbore and solid phase migration experimentation in horizontal wellbore.The accuracies of solid phase migration mathematical models are confirmed in vertical and horizontal wellbore.At the same time,marine natural gas hydrate reservoir in shallow and surface layer solid fluidized excavation large physical modelling test bench is designed,and experimental program are introduced.The studies of this thesis perfect the solid fluidized excavation technical proposal,and form a targeted applicability analysis method,a complex media multiphase flow coupling hydrate decomposition analysis method and an operation parameters optimum design method,which provide important positive significances for safe,reasonable and efficient excavation of marine natural gas hydrate reservoirs and extension of hydrodynamics industrial application.