Numerical Simulation Of Heat And Mass Transfer During Gas Hydrate Decomposition

Since twenty-first century,China's economy has developed rapidly,the contradiction between economic development and energy shortage has become increasingly prominent.In addition,China is still dominated by fossil fuels such as coal,oil and natural gas,renewable energy accounts for a little proportion,environmental problem is imminent.Therefore,it is of great significance to look for alternative new energy resource with no pollution.Natural gas hydrate is a kind of new resource which exists in deep-sea sediments or permafrost areas in land.Because of its abundant reserves,high energy density,combustion characteristics with no pollution,the international energy sector regards it as the most potential alternative energy resource in the future.Natural gas hydrate dissociation process is a complex process involving heat and mass transfer and phase transitions,the gas production rate will be controlled by some factors such as gas-liquid two-phase flow,heat transfer effect and hydrate decomposition kinetics.Through the research of a large number of literatures,a two-dimensional mathematical model of hydrate decomposition is established.In the process of depressurization-induced decomposition,because there is no additional heat injected into the hydrate-bearing layers,the heat from over-and underburden,heat transfer and bottom hole pressure(BHP)are the important factors that influence hydrate decomposition,the related parameters are separated in order to simulate gas hydrate dissociation by single vertical depressurization well under different initial conditions such as temperatures of over-and underburden,thermal conductivities of the porous medium and BHP;in the process of dissociation by hot water injection,the water injection rate and temperature both are important factors that influence dissociation process.Choosing different injection speeds and temperatures of hot water,we first simulate the hydrate decomposition in a simple one-dimensional numerical simulation model,and then in a two-dimensional numerical simulation model,the spatial distributions of temperature and decomposition front and the rules of gas production rate and cumulative gas production are obtained.From the research results we can get some conclusions as follows:1.The higher the temperature of the over-and underburden,the faster the rate of decomposition front penetrating into the reservoir center,the greater the gas production rate and cumulative gas production,and the decomposition process lasts longer as well.2.The greater the thermal conductivity of the porous medium,the lower the overall temperature of the reservoir and the more uniform of temperature distribution.With the increase of thermal conductivity,the influence of thermal conductivity on cumulative gas production is gradually weakened.3.The temperature in the decomposition front will decrease with the decline of the BHP,which indicates that more sensible heat can be used for the hydrate decomposition;when the temperature of over-and underburden is lower,the influence of the temperature of over-and underburden on cumulative gas production is more significant,however,with the increase of the temperature of over-and underburden,the influence of BHP will be gradually weakened.4.In the process of thermal-stimulated decomposition,the hydrate decomposition process can be regarded as a moving boundary ablation problem and the temperature in the decomposition boundary changes fastest,the whole reservoir can be divided into three areas:decomposed area,decomposing area and non-decomposed area and the decomposing area only exist in a narrow area.Increasing water injection rate or temperature will increase gas production rate,but have little effect on final gas production.