Numerical Simulation Of Heat And Mass Transfer Mechanism During Methane Hydrate Dissociation Inside Micro Pore By Depressurization

Methane hydrate has been listed as a new mineral by the State Council of China,its low temperature and high pressure storage environment and complex and variable inherent basic attributes have led to the complexity of hy drate mining.Depressurization method is currently the only way used by the marine gas hydrate mining test.Methane hydrate dissociation by depressurization is a multiphase and multicomponent fluid flow heat and mass transfer problem involving the interaction between solid ice and hydrate.The coupling relationship between the microscopic characteristics of hydrate dissociation,microscale effects,ice formation effects,ice melting effects,mass-transfer-limitation effects,fluid flow inside pore channels and hydrate dissociation effects are the key to break through the limitation of hydrate dissociation basic theory and promote the development of hydrate mining technology.In this paper,the numerical simulations were carried out on the microscopic characteristics of methane hydrate dissociation inside pore channels for the above scientific problems.The microscopic characteristics and mechanism of methane hydrate dissociation inside micro pores were investigated.The necessity of considering the interphase heat transfer and microscale effects in the model were proposed.The specific effects of ice formation and melting on hydrate dissociation and the interaction between the three were clarified.The leading role of mass-transfer-limitation on the microscopic mechanism of hydrate dissociation and the coupling relationship between mass-transfer-limitation,ice formation,ice melting and hydrate dissociation were revealed.The main work is as follows:Firstly,based on the basic properties of methane hydrate and comprehensive comparison of the existing dissociation model,a methane hydrate dissociation model consisting of four phases(the solid methane hydrate phase,gaseous methane phase,liquid aqueous phase,solid ice phase)and three components(methane hydrate component,methane component,water component)was proposed and verified.In the model,the interphase heat transfer,ice phase and slip flow were introduced,and the microscale effects were proposed based on this,and a theoretical basis for the study of the microscopic properties of hydrate dissociation inside micro pore channels was established.Secondly,based on the established and validated model,the microscale effects of methane hydrate dissociation inside real micro pore network section were simulated,and the situations of whether there is ice phase or not were discussed.It has been found that important parameters such as temperature,pressure,hydrate phase equilibrium pressure,volume fraction of each phase,ice phase distribution,hydrate decomposition rate,and mass transfer rate between water and ice are all affected seriously by microscale effects.This confirms the necessity of considering the microsacle effects into the model.In addition,based on the case ice-hydrate coexistence,the ice formation and melting effects were proposed,which laid the foundation for the next study.Thirdly,based on the study of the microscale effects on the methane hydrate dissociation inside the micro pore network,the research object was transformed from the pore network to the local micro pores,and methane hydrate dissociation under the condition of ice-hydrate coexistence inside a local pore was investigated,and three flow directions were considered.The coupling between ice formation,ice melting and hydrate dissociation effects was discussed.The results indicate that the presence of the ice phase increases the complexity of the hydrate dissociation,the ice formation rate is large near the dead-end of the micro pore channels.Ice formation is accompanied by the ice melting at other locations.The distribution of ice depends on the combination of ice formation and melting.The initial stage of ice formation promotes the hydrate dissociation,increases the pressure at local locations,and provides the driving force for the reformation of hydrate and the collapse of pores.Hydrate dissociation will be delayed and limited when the ice is enough.Whether the ice phase promotes the hydrate dissociation depends to some extent on the amount of ice at the current location.The change in heat inside the micro pore mainly depends on the endothermic of methane dissociation,the exothermic of ice formation,the endothermic of ice melting and interphase heat transfer,and the change in heat is seriously affected by ice formation and melting effects.The effects are most severe in the Typel flow regimes where the micro pore boundary is the outlet.Additionally,water and methane accumulate near the hydrate dissociation interface,and this aggregation is complicated especially in the presence of the ice phase.Based on this,mass-transfer-limitation effects is proposed,and the necessity of model improvement is clarified,the inevitability of further study on the mass-transfer-limitation effects during hydrate dissociation inside the micro pore is also clarified.Finally,based on the current model,the restriction effect of the water layer on mass transfer during methane hydrate dissociation was introduced.Combined with the basic theory of hydrate dissociation driving force,the mass-transfer-limitation effects of methane hydrate dissociation inside local pores was explored.The situations with the presence of the ice phase and ice-hydrate coexistence were discussed.It was found that hydrate dissociation is accompanied by mass-transfer-limitation effects in the cases with the presence or absence of the ice phase.For the case with the absence of the ice phase,the hydrate decomposes at a relatively high temperature,the mass-transfer-limitation effects have a great influence on the driving force of hydrate dissociation,which leads to a large influence on the cumulative methane production and the change in heat inside the micro pores.Under the condition of ice-hydrate coexistence,hydrate dissociation driving force is dominated by ice formation,ice melting,mass-transfer-limitation effects.The presence of the ice phase weakens the mass-transfer-limitation effects to a certain extent.