Study On Natural Gas Hydrate Exploitation By Depressurization Combined With In-situ Electric Heating

Natural gas hydrate(NGH)is a kind of ice like cage solid substance formed by methane and water under high pressure and low temperature.Because of its high energy density and large reserves,it is considered to be the future alternative energy and has attracted the attention of all countries.A lot of research work has been carried out on the exploitation of natural gas hydrate at home and abroad.The existing research shows that the depressurization method is the most economical exploitation method,but the exploitation rate needs to be improved;the traditional heat injection method has the ability to improve the gas production rate,but there is a large amount of heat loss in the process of heat injection,which leads to the prominent problem of low energy utilization efficiency.Therefore,in this paper,the direct heating method of hydrate reservoir based on in-situ electric heating principle is proposed to reduce the energy loss during heat injection process.Focusing on the key scientific problem of the thermal fluid coupling mechanism of the phase transition process in porous media,for the study of depressurization method and depressurization combined with in-situ electric heating method(Combination method),laboratory scale experiments and numerical simulation are carried out,and field scale simulation is carried out according to the actual gas hydrate deposits in Shenhu area of South China Sea to study the exploitation effect of depressurization and combination method in field scale.The main research contents and conclusions are as follows:(1)The analysis of hydrate formation process in porous media shows that the hydrate formation rate is faster in the early stage of hydrate formation process,and decreases significantly in the late stage.In the later formation stage,the formation rate is mainly determined by the diffusion rate of methane gas through hydrate to free water and the heat transfer of external water bath.The hydrate formed is unevenly distributed in the Cuboid Pressure Vessel(CPV).(2)In the laboratory scale experiments and simulations,by depressurization,the driving force of pressure difference is the main factor that affects the gas production rate of hydrate reservoir,and the heat transfer from environment to reservoir has a significant impact on hydrate production.In a certain pressure range,the lower production pressure leads to the rapid decrease of the temperature around the wellbore in the reservoir,which significantly increases the temperature difference between the hydrate reservoir around the wellbore and the water bath,and this promotes the heat transfer rate from the water bath to the hydrate reservoir.After adding electric heating to depressurization method,depressurization combined with in-situ electric heating method with dual decomposition driving force shows better exploitation effect than depressurization method.By optimizing the heating power and production pressure,under the condition of reasonable consumption of electric heating heat,lower production pressure can significantly improve the energy recovery rate and ensure higher energy efficiency.At the same time,optimizing the electric heating mode and adopting huff and puff method with intermittent heating in combination method are conducive to further improving the energy utilization efficiency in the exploitation process.(3)In the laboratory scale experiments and simulation,for the effect of wellbore location on production,by depressurization method,there is no difference in pressure difference within the reservoir,the decomposition driving force provided by depressurization is the same in any area of the reservoir,the heat transfer is controlled by the heat transfer from the water bath to the CPV,and the heat transfer direction is always from the reactor boundary to the central area of the CPV.The influence of different moving distance and flow direction of gas and liquid flows caused by the location of the production well on heat transfer is not obvious,and the gas production is almost not affected by the location of production well.After adding electric heating,the electric heating well is used as the heat source,the heat transfer direction diffuses from the heating well to the reservoir,and the heat transfer is controlled by electric heating.The heat taken by gas and liquid produced outside from the central well has no obvious weakening effect on electric heating.In the combined method of double-side well with electric heating well located at the reservoir boundary,the heat carried by gas and liquid in the reservoir does not show obvious thermal convection effect,while part of the electric heating heat transferred to the water bath environment has a strong weakening effect on the decomposition driving force.Therefore,when choosing the location of the in-situ electric heating well,the heating well should be placed in the center of the reservoir as far as possible.(4)In the field scale simulation of hydrate exploitation,due to the influence of porous media permeability,the influence range of depressurization pressure in the reservoir is limited,and the reduction of production pressure has little effect on hydrate decomposition and gas production.When electric heating is added to depressurization method,due to the poor heat transfer characteristics of reservoir porous media,the heat transfer and influence range of electric heating in reservoir are limited,and the promotion effect of electric heating on hydrate decomposition and gas production is not obvious.When a single well is arranged for electric heating and gas production at the same time,the heat transfer is weakened by the effect of gas and water flow;when the double well is arranged to separate the production well from the heating well to a certain distance,the heat loss of electric heating can be reduced beacause the gas and liquid flow does not pass through the heating well,and the heat convection effect generated by gas and water flow can obviously promote the hydrate decomposition.(5)Compared exploitation effect in laboratory scale with that in field scale,in field scale,porous media reservoir properties(permeability,thermal conductivity,etc.)have a significant impact on heat transfer and pressure distribution,thus hindering hydrate decomposition and gas production.On the laboratory scale,the blocking effect of porous media reservoir properties on fluid flow and the weakening effect on heat transfer are affected by the scale effect,which is not obvious;and the surrounding water bath transfers enough heat to the reservoir for hydrate decomposition,and heat transfer in the reservoir is controlled by the surrounding water bath.The hydrate production rule obtained in laboratory scale is limited by reservoir size and boundary effect to a certain extent.