Experimental Simulation Of Natural Gas Hydrates Production By Gas Injection Method And Depressurization Method And Optimization Of Exploitation Scheme

There are huge reserves of natural gas hydrate（NGH）resources in nature,which is far more than the sum of conventional fossil energy.And NGH,which is considered as an important clean energy,is widely distributed in the permafrost and continental slope sediments.At present,the production of oil and natural gas energy is insufficient in China,and foreign dependence ratio is high.So NGH becomes one of the best choices to ensure energy security and optimize energy structure.Several field trials around the world have revealed the short gas production cycle and high production cost of current production technologies.In this paper,in order to meet the context of"carbon peaking and carbon neutrality",a series of in-depth researches related to CO₂ replacement method have been conducted for the third class of NGH reservoirs without underlying free gas zone,including basic research on hydrate phase equilibrium of hydrogen-containing system,gas production characteristics,parameter optimization and enhanced gas recovery of NGH production process with CO₂/N₂ and CO₂/H₂ gas mixtures.Additionally,experimental research about the first class of NGH reservoir containing an underlying free gas zone,which is more easily to exploit,was conducted.The main research contents are as follows.（1）Study of hydrate phase equilibrium in hydrogen-containing system.Data of hydrate formation conditions for the binary system（CH₄+H₂）and ternary system（CH₄+CO₂+H₂）in pure water and brine（containing Na Cl and Na₂SO₄）were obtained.It is found that the increase of hydrogen concentration leads to stricter hydrate formation conditions,and the hydrate formation pressure is more sensitive to the gas composition with higher hydrogen concentration.Moreover,Na Cl have stronger inhibiting effect than Na₂SO₄with same mass fraction.In addition to experimental research,the original hydrate thermodynamic model was improved by replacing the vd W mixing rule with the WS mixing rule for calculating the gas-phase fugacity,and the model parameters were determined by a large number of experimental data.The overall average absolute deviation on formation pressure for 273 data points is reduced from 5.70%to 4.61%with the improved model.（2）Experimental study of CH₄-CO₂/N₂ replacement by dual horizontal wells.Replacement experiments were conducted in four different initial water saturation conditions（around 18%,35%,54%and 69%）,and two well patterns were compared in each water saturation.The results show that the free water in the reservoir limits the diffusion of injected gas,leading to negative effects on replacement process.Therefore,in reservoirs with high water saturation,the well pattern which set the production well below the injection well can deplace a large amount of free water and strengthen the replacement process.The methane recovery ratio is improved by 1.2-2.2 times.However,the large water production may lead to blockage of the pipeline caused by secondary hydrate formation,which must be monitored during gas production process.On the contrary,when the water saturation is low,it is more suitable to set the prodction well above the injection well,which results in higher methane concentration in the production gas.It significantly reduces the cost of subsequent gas separation,and improves CH₄ recovery and CO₂ sequestration rate.（3）Comparison of CO₂/N₂ and CO₂/H₂ replacement processes.The gas production behaviors of CO₂/N₂ and CO₂/H₂ replacement processes at three different CO₂concentrations（71,54 and 33 mol%）are compared.The results show that the decrease of CO₂ concentration of the injected gas is favorable to methane hydrate decomposition and unfavorable to CO₂ sequestration.Moreover,when the CO₂ concentration of the injected gas is 71 mol%or 54 mol%,the methane hydrate decomposition in CO₂/H₂replacement processes is more dramatic leading to a higher methane recovery ratio,because it is more difficult for hydrate formation in CO₂/H₂ system.However,both CO₂/N₂ and CO₂/H₂ can’t form hydrate in the cases with 33 mol%CO₂,and H₂ is more likely to diffuse upward,resulting in an earlier"gas short-cut"between the two wells.So the CH₄ concentration of output gas is lower in CO₂/H₂ replacement processes,which is not conducive to gas separation.（4）Enhancement of CO₂/N₂ replacement process.In order to solve the problem of low methane recovery ratio caused by slow replacement rate and limited replacement area,three methods were used to enhance CO₂/N₂ replacement process.The results show that the enhancement effect of increasing the injection gas temperature is not significant,but it can be used to alleviate the common problems of ice plugging or hydrate secondary formation during gas injection.Replacement combined with hot water injection can enhance methane recovery ratio,but CO₂ sequestration ratio also decreases significantly.And it is better to inject hot water before gas injection,which leads to higher methane recovery ratio.Gas injection in semi-continuous mode can expand the replacement area and improve methane recovery ratio,but the appropriate soaking duration must be used according to the reservoir properties to avoid the significant decrease of gas production efficiency.（5）Experimental study of multi-gas co-production.Experimental research on gas production in the NGH reservoir containing an underlying free gas zone with depressurization method was conducted.The results show that the underlying free gas can effectively enhance hydrate decomposition and significantly improve gas production efficiency.Gas production rate of such reservoirs in the depressurization stage is about 8 times higher than that of reservoirs without underlying free gas.Therefore,"multi-gas co-production"is more economical and technically feasible.Moreover,it is found that is better to set the wellbore at underlying free gas layer in the cases used a single vertical well.It is beneficial to the expansion of the NGH decomposition front,avoid lower local permeability near the wellbore,and improve gas production efficiency.