Experimental And Numerical Studies On Natural Gas Hydrate Dissociation Via Depressurization

With the rapid development of human society,the consumption of energy is also increasing,which brings increasingly serious environmental pollution problems.People need to find a new type of clean energy to replace the traditional energy.Natural gas hydrates(NGH),also known as “combustible ice”,have many advantages such as huge reserves,high energy density,environmentally friendly,etc.,which makes it have very high potential mining value and attract widespread attention worldwide.Depressurization method has the advantages of low cost and high energy efficiency.In this paper,the depressurization decomposition system of NGH designed by our research group was used to study the depressurization decomposition law of natural gas hydrate in porous media under salt conditions.At the same time,physical and mathematical models are established based on the experimental system.Numerical simulation method is used to study the depressurization of natural gas hydrate in different permeability and salinity environments,and the spatial distribution and variation law of the parameters are discussed.The conclusions are as follows:1.The decomposing experiments of natural gas hydrate in quartz sand under different salinities were carried out.The results show that compared with pure water,the decomposition rate of hydrate in Na Cl solution is faster,and the average decomposition rate of hydrate increases with the increase of salinity.At the same time,it is found that the lower the wellhead back pressure is,the higher the hydrate decomposition rate is,but under the salt condition,the difference of hydrate decomposition rate under different back pressure becomes smaller.Under the same decomposition back pressure,the higher the average decomposition rate of hydrate,the lower the average temperature in the reactor.2.The numerical simulation of decomposing NGH in porous media under different salinity conditions shows that the higher the salinity,the shorter the decomposing time and the faster the decomposing rate under the same durations,which is consistent with the experimental results.The heat transfer has an important influence on the hydrate decomposition reaction.Due to the heating effect of the constant temperature boundary,the heat of outer side the simulation area is more sufficient,which lead to higher temperature and faster hydrate decomposition rate.While the temperature in the center of the reactor decreases faster and rises slower,and the hydrate decomposition rate is slow.3.The numerical simulation of depressurization decomposition under different permeability conditions shows that low permeability will affect the flow performance of gas and water.On the one hand,it reduces the heat and mass transfer rate,which leads to the longer total decomposition durations and the slower average decomposition rate.On the other hand,it also makes the pressure and temperature distribution uneven.And it is found that the second generation of hydrate is easy to occur near the wellhead of production wells.