Numerical Study On Optimization Producing Scheme Of Hydrate Reservoirs In Shenhu Area

Natural gas hydrate is a kind of ice like solid material by the reaction of low molecular gas(such as methane,ethane and carbon dioxide)and water in low temperature and high pressure formed.It is non stoichiometric clathrate compounds,favored by the world for its large amount of resources,wide distribution,high density of energy and its cleanliness.Since 1998,the investigation of natural gas hydrate in the South China Sea has been carried out.Northern slope of the South China Sea is the regional focus on the investigation of gas hydrate in China,while the Shenhu area is considered to be one of the most promising areas.At present,the main methods of hydrate exploitation have depressurization,heat injection and depressurization+heat injection combined.On the basis of the three main methods,this paper designed four methods including depressurization,thermal displacement,depressurization+thermal displacement and depressurization+well heating to study the hydrate production of Shenhu area.With the combination of numerical simulation method to evaluate productivity and economic evaluation enthod,this paper evaluated the production effect of four kinds of methods,finally giving the production effect of four methods from better to bad as following:depressurization,depressurization+thermal displacement,depressurization+well heating,and thermal displacement.Finally,the most suitable methods for Shenhu hydrate production selected is depressurization.Considering the maximum cumulative gas production characteristics of depressurization+thermal displacement,the trial production schemes of depressurization method and depressurization+thermal displacement method are all optimized in this paper.At first,this paper studied the depressurization production dynamic of Shenhu hydrate,and then studied burden layer's effect on hydrate production,finally analyzed the influence of depressurization amplitude and depressurization rate on the gas production capacity.The results show that the burden layer can promote the hydrate decomposition,and the burden layer's thickness in the numerical study should be at least 30m,and its thermal conductivity should be taken according to the formation conditions.Higher the depressurization amplitude,faster the depressurization speed,and greater the gas production.Finally,based on the orthogonal design,the influence of depressurization amplitude to cumulative production gas is bigger than depressurization rate.The best producing scheme of depressurization given is the depressurization amplitude as 0.783P_i(Bottom Hole Pressure,BHP=3MPa)and the depressurization rate as 8.83MPa/d.After 20 years production,this scheme can get the cumulative production gas of 1.2318×10~8 m~3,the hydrate can be completely decomposed,and the hydrate reservoir recovery is 89.54%.For the depressurization+thermal displacement method,this paper first studied the exploitation status of Shenhu hydrate,and then analyzed the effects of depressurization amplitude,depressurization rate,injecting temperature and injecting rate on gas production capacity.The results show that higher the depressurization amplitude,faster the depressurization rate,higher the injecting temperature and slower the injecting rate,the cumulative gas production will be greater.But when considered economic efficiency and energy efficiency,the recommended injecting temperature is 25~60?and the the recommended injecting temperature is 40~60m~3/d.After the orthogonal design,the influence of the mining parameters from large to small is the following:injecting temperature,injecting rate,depressurization amplitude,and depressurization rate.According to the orthogonal design results,the best producing scheme by depressurization+thermal displacement method is recommended as follows:the depressurization amplitude is 0.783P_i(BHP=3MPa),the depressurization rate is 8.83MPa/d,the injecting temperature is 25?and the injecting rate is40m~3/d.After 20 years'production,the cumulative gas production can reach to 1.3675×10~8 m~3,the hydrate can be completely decomposed,the recovery of hydrate reservoir is 99.41%,and the energy efficiency can reach to 91.27.The research results of this paper have an important guiding significance to the further hydrate in Shenhu area.