Study On The Formation And Dissociation As Well As The Heat Transfer Behaviors Of Natural Gas Hydrate In Porous Media

With the fast consumption and exhaustion of the fossil fuels such as coal and oil,the exploitation of energy is undergoing a transition from the conventional fossil fuels to other clean and environmental-friendly energy sources.Natural gas hydrate(NGH,also called “combustible ice”)is a kind of clean energy sources with high energy density and abundant reserves in the world.As one of the promising energy resources in the future,it is of great importance to exploit NGH safely and efficiently for adjusting and optimizing the energy supply structure as well as ensuring the energy security of China.It is necessary to investigate the formation and dissociation properties as well as the key affecting factors of gas hydrate in porous media,which is of great academic significance and engineering value for efficient and safe exploitation of gas hydrate.This study aims to investigate the formation and distribution behaviors of gas hydrate in porous media and the control mechanism of heat transfer on hydrate dissociation by experimental and numerical simulation methods.Based on the theory of enhanced heat transfer,a new exploitation method involving multiple strategies has been proposed for hydrate exploitation.The main purpose of this study is to provide theoretical guidance and technical support for field-scale hydrate exploitation in the future,and the main results are as follows:(1)The methane hydrate formation process in a pilot-scale reactor is investigated by numerical simulation.The spatial distributions of methane gas,water,and hydrate during the formation process have been obtained.Results show that the hydrate formation rate is mainly controlled by the mass transfer at the gas-water interface.The mass transfer rate is related to the pressure driving force and the distribution pattern of gas and water in the pores.The formed hydrates are found to be distributed layer by layer in the deposit,and the spatial distributions of the three phases are all found to be very uneven.Such heterogeneity is more pronounced along the vertical direction.(2)Gas hydrate samples are prepared in a high-pressure reactor by injecting fluids through multiple points.Then the heat transfer behaviors during hydrate dissociation by depressurization have been studied in a single vertical well by experimental and numerical simulations.The evolutions of the heat transferred from the boundaries,the heat consumed by hydrate dissociation,the sensible heat change of the deposit,and the heat loss are analyzed.It is found that the hydrate dissociation is dominated by the heat transfer from the surroundings.The location of the production well shows little influence on gas production performance.The amount of lost heat is equal to that of the sensible heat increase of the deposit.A lower wellbore pressure will result in faster heat transfer and hydrate dissociation rates,which leads to faster energy recovery rate.(3)The heat transfer behaviors during hydrate dissociation by depressurization and thermal stimulation have been studied using a single vertical well by experimental and numerical simulations.The synergistic controlling mechanism of depressurization and thermal stimulation driving forces on hydrate dissociation has been obtained.Results show that there are two dissociation interfaces in the vessel,and the heat conduction at the boundary and the electric heating at the wellbore can be used to characterize the depressurization and thermal stimulation driving forces,respectively.Heat conduction at the boundary is inhibited by the wellbore heating.In addition,the heat loss will increase sharply when the total amount of heat across the boundary becomes negative.(4)The key factors affecting the hydrate dissociation and heat transfer properties under depressurization and thermal stimulation have been analyzed.Results show that the energy recovery rate becomes faster under lager wellbore heating rate,while the depressurization driving force becomes less notable.A lower production pressure is more favorable for faster heat transfer in the deposit.The depressurization driving force can be maintained above zero during the whole exploitation period if the production pressure is set low enough.Comparing with continuous heat injection mode,intermittent heat injection can control the heat absorption rate of the hydrate deposit and thus shorten the exploitation time and increase the net energy gain.(5)A novel combined exploitation mode has been proposed.The gas production and heat transfer behaviors under different production modes have been experimentally studied.Results show that the highest net energy recovery rate(about 8 k J/min)and a relatively high energy efficiency(71.59%)are obtained with the new method.For depressurization combined with wellbore heating,the heat transfer rate is limited by the thermal conductivity of the deposit.Thermal convection becomes the key factor controlling the heat transfer with water injection,while the heat loss during fluid transportation is inevitable.The new method has the advantage of lowering the heat loss and enhancing the heat transfer,thus making the energy recovery rate the highest.