Experimental Study On The Dissociation Of Frozen Gas Hydrate In Porous Media

Huge amount of natural gas hydrate resource has been confirmed in the global permafrost regions.The existence of ice is a key factor affecting the exploitation of the permafrost-associated gas hydrates.In fact,some hydrate reservoirs exist in permafrost,whose pores are filled by hydrate,ice and gas,and hydrate is usually frozen by ice,forming frozen gas hydrate.At present,most of the researchers mainly focus on the decomposition characteristics of hydrate above freezing point.There are relatively few researches on the decomposition of frozen gas hydrate in porous media below freezing point,and most of the researches focus on the self-protection dynamics of hydrate and its influencing factors.Few of the researchers carry out the research on the exploitation methods of frozen gas hydrate.Therefore,thesis mainly studies the formation and decomposition behavior of frozen gas hydrate in porous media below freezing point through experiments,and systematically evaluates the key factors controlling its decomposition kinetics.In thesis,the current exploitation method of natural gas hydrate is explored firstly,and the exploitation method of frozen gas hydrate is preliminarily selected through the effect of field trial exploitation in permafrost area.It is determined that the exploitation method of frozen gas hydrate is the combination of depressurization and electric heating.Then four runs of experiments on the formation and decomposition of frozen gas hydrate in porous media are designed,and the systematic characteristics of hydrate formation and decomposition are analyzed.By comparing the experimental results under different mining conditions,the influence of electric heating power,production pressure and initial ice saturation on mining effect is studied.And the production efficiency of frozen gas hydrate is evaluated by energy recovery efficiency.The research results mainly include:(1)In the process of forming hydrate-water-gas system,the average temperature of the system can rapidly drop to the target temperature.Three phase(hydrate,gas and liquid phase)saturation changes with time.The mass of free gas decreases and the mass of hydrate increases.The formation rate of hydrate is affected by temperature,pressure and gas-liquid contact area.(2)In the formation of hydrate-ice-gas system,the gas and water flow in the pores can induce secondary hydrate formation.The system pressure decreases sharply in the cooling stage due to the gas consumption by hydrate formation and the thermal contraction under the cooling effect.When the system temperature drops to 0?,even though the system pressure is much higher than the phase equilibrium pressure,the hydrate formation rate almost drops to 0.This shows that the liquid water begins to turn into solid ice,and the formation of ice film hinders the further contact between gas molecules and water molecules.(3)When the depressurization method is adopted,the sensible heat of the hydrate deposit cannot be effectively utilized by frozen gas hydrate due to the preservation effect of ice.Only a small amount of gas is obtained by depressurization,which is caused by the obstructive effect of the ice film on the mass transfer of methane molecules from the hydrate phase to the gas phase.This shows that the existence of ice can protect the hydrate well and make it in metastable state without decomposition.Therefore,the depressurization method presents little commercial value for the exploitation of frozen gas hydrate and the self-protection effect will be the main adverse factor hindering the decomposition of frozen gas hydrate.(4)The ice film on the surface of hydrate particles first absorbs heat when the method of depressurization combined with electric heating is used to decompose frozen hydrate,and then melts into liquid water.After the natural gas hydrate is exposed to low pressure environment,the decomposition of hydrate will not be hindered by ice.Therefore,electric heating can promote the decomposition of frozen hydrates below freezing point.Under the pressure of 1.50MPa,the equilibrium temperature of hydrate phase is-16.37?,and the liquid water produced by hydrate decomposition will be directly turned into solid ice.If there is no external heat to melt the ice,hydrate decomposition will stop again.Both the gas production and hydrate dissociation rates decline with time due to the gradually increased heat transfer resistance.(5)After the wellbore heating,the temperature of the system can rise above the freezing point,which shows that although the gas hydrate is completely surrounded and protected by solid ice in the reactor,the continuous heat injection can melt the ice film around the hydrate particles,thus eliminating the self-protection effect of the frozen hydrate.An inflection point is observed on each temperature curve,which is due to the heat transfer between these solid particles in the form of heat conduction before the ice melts and hydrate completely decomposes.Once they are completely transformed into liquid water and gas,heat convection starts to play a role in promoting the heat transfer rate,which results in faster temperature increase.(6)As the heat provided by electric heating is much higher than that required by ice melting and hydrate decomposition,E_net continues to increase over time.More and more energy is consumed in the form of the latent heat by ice melting,the sensible heat absorbed by the deposit,and the heat conduction across the low-temperature boundary.In addition,?and?rise to a peak value in a short time because of the fast hydrate dissociation rate in the early production period.Then they decline continuously due to the increased heat loss.(7)Parameter sensitivity analysis shows that lower electric heating power will slow down the heat transfer rate from the production well to hydrate sediment,and the destructive effect of wellbore heating on hydrate self-protection will also be weakened.The blocking effect of ice in the pore channels will reduce the permeability of sediment,and ultimately lead to the decrease of the decomposition rate of frozen gas hydrate.The reduction of production pressure has little effect on the overall mining performance.This shows that in general,higher electric heating power and lower ice saturation are more conducive to the exploitation of frozen gas hydrate.While due to the protection of hydrate particles by ice,the production pressure seems to have negligible influence on the overall production performance.Therefore,the dissociation of frozen natural gas hydrate does not depend on the production pressure.It is mainly dominated by the kinetics of its dissociation and ice melting as well as the heat transferred from the wellbore during the heating process.