Study On Fluid-solid Coupling Numerical Model Of Sanding In Hydrate Reservoir

Gas hydrate is regarded as the most potential clean energy to replace conventional fossil fuels,such as oil,coal,etc.,in the 21 st century,due to its large reserves,high energy and low pollution in the world.With the proposal of China's marine power strategy,the safe and efficient production of natural gas hydrate is of great significance.Depressurization is the most widely used promising method for hydrate production.The basic principle of depressurization is to drop the reservoir pore pressure and induce the solid hydrate changes into liquid phase,and then flows into the wellbore.The above process always causes the change of physical properties of reservoir and multiphase flow process.On the other hand,the sand particles can migrate with the fluid in the pore space,which is called sanding.A large amount of sand produced blocks the seepage channels of hydrate and thereby affects the sustainable commercial exploitation of hydrate.Therefore,it is of great theoretical and practical significance to study the problems of multiphase flow and sand particle migration in the reservoir caused by hydrate phase transition.In this paper,the problems of multiphase flow and sand particle migration in the reservoir caused by hydrate phase transition during hydrate exploitation by depressurization were studied by fluid-solid coupling numerical simulation.Firstly,a mathematical model of coupling the kinetic decomposition of hydrate and unsaturated seepage in porous media was established by comprehensively considering the coupling between hydrate decomposition and multiphase flow;the Masuda hydrate decomposition was numerically simulated by COMSOL multi-physical field coupling finite element software and compared with Masuda hydrate decomposition test to verify the rationality of the model.The sensitivity analysis results show that the peak gas production rate of hydrate increases with the increase of permeability and pressure drop amplitude.According to VG model,the increase of initial water saturation leads to the decrease of relative permeability of gas phase as well as the decrease of the peak gas production rate;the decrease rate of hydrate saturation increases with the increase of reservoir permeability and depressurization amplitude and decreases with the increase of initial water saturation.Secondly,the movement model of sand particles in the saturated and unsaturated states in porous media was established by considering the interaction between particle migration and flow field,and the particle migration in Yousif porous media was simulated to verify the rationality of the model.It is concluded from calculation and analysis of the migration sensitivity of sand particles in porous media that With increasing flow rate and sand particle size,the migration rate of sand particles increase very rapidly,showing the sand production increases in the same time;the increase of particle concentration promotes the pore blockage and reduces the migration rate of particles in pores,but more particles migrate to the outlet and the sand production increases due to an increase of particle content in porous media.Finally,based on the actual production test data of Nankai Trough in Japan,a fluidsolid coupling model for sand production process in which the heterogeneity of actual hydrate layer was fully considered,was established.The model can reflect the influence of reservoir heterogeneity on physical parameters and sand particle migration during hydrate production.It is found from the study that the heterogeneity of hydrate reservoir leads to vertical unsaturated flow and interlayer migration of particles.The model is more suitable for actual production conditions of hydrate and of great significance to improve the theory of industrial production of hydrate.