| Natural gas hydrate is an important unconventional natural gas resource.It has the advantages of high energy density,large resource reserves,wide distribution,and clean combustion.It is recognized as a new energy with broad development prospects.Studies have shown that changes in hydrate saturation can have a significant impact on the physical properties of the reservoir.In 2015,my country’s hydrate sediment samples drilled from the South China Sea Shenhu waters had the highest average hydrate saturation of 45.2%.Therefore,it has more reference value to carry out research on the dissociation characteristics of hydrates with higher hydrate saturation.However,there are few studies on the depressurization dissociation characteristics of natural gas hydrate under different hydrate saturation conditions,and its dissociation law is still unclear.For this reason,this paper studies the dissociation characteristics of natural gas hydrate when the hydrate saturation changes within a large range and uses numerical simulation software to study the depressurization dissociation characteristics of natural gas hydrates,so as to provide a theoretical basis for depressurization exploitation of hydrate reservoirs with different saturations.The main research contents and conclusions of this paper are as follows:（1）The formation and depressurization dissociation experiments of natural gas hydrate under different conditions were carried out in the quartz sand system,and the optimal conditions for the synthesis of natural gas hydrate in the laboratory and the influence of pressure on the depressurization dissociation characteristics of the hydrate were obtained.The experimental conditions mainly include different gas inflow mode,different initial water saturations,and different dissociation pressures.The experimental results show that under the conditions of 44.5%initial water saturation and bottom inflow mode,the rate of hydrate formation is the highest,and this condition is the optimal hydrate formation condition;（2）The effects of different dissociation pressures on the depressurization dissociation characteristics of natural gas hydrate were studied.When the dissociation pressure is 2 MPa,the lowest temperature of the reservoir is lower,at 272 K.The reservoir can provide more sensible heat,which makes the hydrate gas production rate higher.At the same time,part of the water in the reservoir will freeze,which may affect the production efficiency in practice.（3）The effects of hydrates with different saturations on the depressurization dissociation characteristics of hydrates were studied.Hydrates with hydrate saturations of 31.4%,41.6%,and 46.4%were subjected to depressurization dissociation experiments,and the dissociation pressure was 2 MPa.The experimental results show that in the depressurization stage,the minimum temperature of the hydrate reservoir temperature（T1,T2,T3）of different saturations is similar,and the hydrate reservoir with a saturation of 41.6%has the highest gas production rate and the highest gas production;at the beginning of the constant pressure stage,the higher the hydrate saturation,the longer the reservoir temperature T1 and T3 remain unchanged,and the longer it takes for the reservoir temperature to return to the set temperature.The 46.4%hydrate has the highest gas production rate and gas production.At the same time,the depressurization dissociation characteristics of hydrates with different hydrate saturations（38.1%,42.1%,46.4%）under the decomposition pressure of 3 MPa were studied.Compared with the decomposition pressure of 2 MPa,the decomposition pressure of 3 MPa will increase the time for the temperature of the hydrate to rise to the set temperature and shorten the time required for the gas recovery rate to reach 100%during the depressurization dissociation process.（4）Numerical simulation research was carried out on the hydrate dissociation process.It mainly study the distribution and control mechanism of each phase material and reservoir temperature in hydrate reservoirs,and compare the numerical simulation results and experimental results.The results show that under the conditions of 2 MPa dissociation pressure and hydrate saturation of 46.4%,the temperature distribution of the hydrate reservoir results in a low temperature at the center of the reservoir and high temperature at the edge.This will cause a decrease in the heat transfer capacity of this area and a slower dissociation of a small amount of hydrate.The numerical simulation results of the gas production and CH4 recovery rate of hydrates with different saturations with a dissociation pressure of 2 MPa are basically consistent with the corresponding experimental results,which verifies the accuracy of the simulation model.However,the reservoir temperature change results of the numerical simulation are somewhat different from the corresponding experimental results. |
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