Research On Exploitation Characteristics Of Methane Hydrate By The Combination Of Depressurization And Thermal Stimulation

Nature gas hydrates are widely distributed in the margins of marine continental shelf and permafrost region on land which satisfy the conditions of low temperature and high pressure.Due to its abundant reserves and pollution-free characteristics,nature gas hydrate are considered to be one of the important energy resources in the future.In this paper,hydrate exploitation characteristics are studied using depressurization combined with thermal stimulation for two types of natural gas hydrate deposits.The effects of different depressurization methods and thermal stimulation modes on reservoir temperature and pressure variations,gas and water production are analyzed during gas production.Therefore,it can provide certain theoretical support for the efficient exploitation of natural gas hydrates.First,a methane hydrate exploitation research experimental system was built to study the exploitation characteristics of methane hydrate by depressurization,warm water injection,electric heating,and any combination of them in gas-saturated and water-saturated methane hydrate deposits.The high pressure reactor is equipped with a power controllable electric heating rod to realize a new type of thermal stimulation — electric heating.And three 4-point thermocouples are evenly distributed in the reactor to measure temperature variations at different locations in the reservoir during hydrate exploitation.High-precision pressure measuring device is used so that acquire pressure data accurately.The reactor is designed with high pressure resistance.The experimental pipeline is built with imported Swagelok seamless steel pipe and joints.It has the characteristics of safety and good tightness.In addition,the experimental system is equipped with a multi-stage sand control device to solve the common sand plugging phenomenon in the experimental process.Secondly,methane hydrate formation and decomposition experiments were carried out under gas-saturation condition,and the combination method of depressurization and thermal stimulation were used in the hydrate decomposition experiment.The effect of "memory effect" on reducing the hydrate formation induction time was confirmed.Hydrate decomposition experiments was carried by depressurization combined with boundary heat transfer,warm water injection and electric heating.The effects of three different thermal stimulation modes on reservoir temperature and pressure variations,gas and water production were studied during methane hydrate decomposition under gas-saturation condition.It is found that when the hydrate is dissociated by depressurization combined with warm water injection,the pressure of the reservoir will increase during the warm water injection.The phenomenon of rising pressure will inhibit the hydrate decomposition.Therefore,compared with the simple depressurization,the average gas production rate is lower and the water production is higher.When electric heating is used to assist depressurization to dissociate hydrates,electric heating promotes decomposition of hydrates.The higher the electric heating power,the faster the hydrate decomposition rate,and the higher the average gas production rate.At the same time,there is little water production.Finally,decompression experiments of methane hydrate under water-saturation condition were carried out by pre-depressurization,depressurization combined with warm water injection and depressurization combined with electric heating.It is found that when the methane hydrate is dissociated by pre-depressurization,regardless of the depressurization mode,the reservoir pressure will drop rapidly to the vicinity of the phase equilibrium pressure after the formal depressurization starts,and then the pressure and temperature of the reservoir begin to decrease slowly.Compared with simple depressurization,the average gas production rate will be smaller when pre-depressurization mode was used to dissociate the hydrate.When the water-saturation methane hydrate is dissociated by depressurization combined thermal stimulation,warm water injection and electric heating process will increase the pressure of the reservoir,thereby inhibiting the decomposition of the hydrate.However,as far as the whole process is concerned,compared with the simple depressurization,thermal stimulation promotes hydrate decomposition during the hydrate decompression by depressurization.And the promotion of electric heating is more obvious,and it is more favorable in terms of reducing water production when the hydrate is dissociated by depressurization combined with electric heating.