Multi-phase Flow And Productivity Assessment For Hydrate Reserviors With Argillaceous Siltstones

The results of natural gas hydrate pilot test in Shenhu region have shown that hydrate resources of this region are abundant and have the prospect of industrial development.Hydrate-bearing sediments in Shenhu region of the South China Sea are featured with unconsolidated argillaceous siltstones,which can pose a great challenge for gas production due to the migration of fine clay and silt particles(or sandout)that can restrict sustainable gas flow.It is urgent to find out the multi-phase flow mechanism(gas,water,clay,sand,hydrate)in hydrate-bearing sediments with unconsolidated argillaceous siltstones and its effect on gas production for optimizing sand control technology and improving production capacity of hydrate reservoir.In this paper,the methods of fluid mechanics,oil and gas development and numerical simulation are used.Based on the geological characteristics of unconsolidated argillaceous siltstones,pilot test results in typical subsea sediment and kinetic model of hydrate decomposition,the multi-phase flow model of sand-hydrate-gas-water in the process of gas production from hydrate reservoirs has been established,considering sand control method,sand/clay migration,change of porosity and permeability and multi-component flow and thermodynamic model.The multi-phase flow mechanism and its influence on productivity are analysed.The methods to improve the production capacity of hydrate reservoir have been proposed,providing theoretical support for the moderate sand control,the use of vertical wells,horizontal wells and multi-well network development technology.The research results indicate that sandout or sand control is critical for sustainable gas production from hydrate-bearing argillaceous siltstone sediments.An improper sand control design criteria can induce substantial silts migration and led to clay clogging in near well formations,restricting gas production capability.For the vertical well with effective length of70 m,the cumulative gas production(using 6?m screens)is only 8.17×10~6 sm~3 in 5 years,which can be improved to 23×10~6 sm~3 after optimization.The result of limited sand production method is strictly controlled by the improvement of permeability of formation near the well.The simulation results show that it is a key measure to control the production and establish the regulation mechanism of controlled sandout by using reasonable depressurization method.Aiming at the design of 6?m sand control with serious clay clogging,the production capacity can be increased by 3.2 times and the amount of sand can be reduced by 54%with segmented depressurization method.Overall,it still faces the problem of insufficient productivity by the use of single vertical well to extract hydrate reservoir of this type.Therefore,for the commercial exploitation of hydrate reservoirs with unconsolidated argillaceous siltstones,innovative measures to enhance production are essential,including horizontal wells and heat stimulation.The simulation results show that using horizontal well(200 m),compared with single vertical well(70 m),the production capacity can increase by about 221%.In addition,the use of vertical well network with heat stimulation can effectively improve the production capacity of hydrate reservoir,but its economy needs to be further evaluated.