Numerical Simulation Of The Decomposition Process Of Natural Gas Hydrate By Depressurization In Reactor

With the consumption of traditional fossil energy and the pollution of environmental,finding a new clean energy has become the concentrated issue of human society.Natural gas hydrate is an ice-like clathrate compound,which is considered as a potential and important clean energy because of its wide distribution and large reserves,high energy density and clean burning.There are abundant natural gas hydrate resources in China's sea and plateau area,and its reserves can support our country's energy demand for hundreds of years.So studing the decomposition process of natural gas hydrate in porous media shows great significance.From the point of view of the feasibility and economy among the different methods of developing natural gas hydrate,the depressurization is considered as the most effective and potential method.In this study,the TOUGH+HYDRATE software was used to simulate the decomposition process of natural gas hydrate in a laboratory-scale cylindrical reactor.The effects of wellhead pressure,freezing,reactor initial pressure and thermal conductivity of the porous media on the decomposition process of natural gas hydrate were investigated.The instantaneous evolvement laws of gas released from hydrate(V_R),cumulative gas production(V_P),the free gas remained in reservoir(V_F),the remained hydrate in reservoir(m_H)and the gas production rate(Q_G)during the decomposition process of natural gas hydrate in reactor were analyzed.The pressure and temperature at different axial and radial directions over time were studied.And the spatial distribution of temperature,pressure and the hydrate saturation were further revealed.The results indicated that in the hydrate decomposition process,the remained hydrate in reservoir,gas released from hydrate and the cumulative gas production gradually increased.The free gas remained in reservoir and the gas production rate were increased quickly and gradually reduced.The temperature of reactor decreased first and then increased,while the pressure of reactor decreased continuously.The spatial distribution of pressure is uniform due to its rapid propagation.The temperature of the reactor is reduced first and then gradually increased from the boundary to the interior.The sensible heat of the reservoir was consumed in the early stage of hydrate decomposition,so the hydrate decomposed simultaneously in the whole reactor.In the subsequent stage,the heat was got by the thermal conductivity at the reactor boundary,so the hydrate gradually decomposed from the boundary to the center of the reactor.With the decrease of wellhead pressure,the hydrate decomposition rate gradually increased,so the gas released from hydrate and final cumulative gas production increased and the free gas remained in reservoir decreased.Meanwhile,the pressure and temperature dropped faster due to the endothermic decomposition reaction.However,when the wellhead pressure reduced to 0.5MPa,a lot of ice was formed in the reactor,which inhibited the hydrate decomposition and reduced the gas production rate.With the decrease of reactor initial pressure,the hydrate decomposition rate gradually increased,so the gas production rate and final cumulative gas production increased.When the initial pressure increased from4.30MPa to 5.30MPa,the time required for the complete decomposition of hydrate increased by 3.23 times and the cumulative gas production decreased by 22.45%.With the increase of the thermal conductivity of the porous media,the gas production rate increased due to the improved heat transfer conditions of reservoir,anyhow the final cumulative gas production was the same.