Study On Decomposition Characteristics Of Natural Gas Hydrate And Multiphysics Coupling

Natural gas hydrate,also called 'flammable ice',is a new energy source with high energy density and extremely high resource value.In order to reveal the influencing factors of natural gas hydrate gas production efficiency and the evolution of reservoir physical properties in test production projects,theoretical analysis,laboratory experiments and numerical simulations are combined to study the decomposition characteristics of natural gas hydrates and the multiphysics coupling problems in the pressure reduction process.The main results and conclusions are as follows:(1)Using a one-dimensional simulation experiment system of natural gas hydrate,the formation/decomposition experiment of natural gas hydrate in quartz sand with different particle sizes was carried out,and the phase equilibrium curve was tested based on the constant volume temperature scanning method.The experimental results show that the formation of natural gas hydrates can be divided into a mixed gas water cooling stage,a fast hydrate generation stage,a slow hydrate generation stage and a hydrate stable existence stage.During the generation process,the gas pressure drops by 3.38MPa and the temperature decreases by 14.5?.The decomposition of hydrates shows the characteristics of cyclic gas production,and the cumulative gas production curve is stepped,with a total gas production of 14694.4 ml;the 70-mesh and 300-mesh quartz sand particles have a large scale,and the capillary force between the particles has basically no effect on the hydrate generation and decomposition process.The phase balance curve is basically the same,and the relationship between pressure and temperature is y=10-11×e0.1033x.(2)Using a two-dimensional simulation experiment system of natural gas hydrate,the depressurization production process of natural gas hydrate under different well types was tested.The experimental results show that the temperature drop ranges of single vertical wells and single horizontal wells during decompression and decomposition are the same.The highest temperature drops are both 2.25?,the horizontal wells take 6000s to decrease,and the vertical wells take 15000s;the average gas production rates of vertical and horizontal wells are respectively 81.6 ml/s and 145 ml/s,the latter is 1.78 times that of the former.(3)Partial differential equations of multiple physical fields such as fully coupled natural gas hydrate chemical decomposition,gas-water two-phase flow and temperature are derived,a two-dimensional axisymmetric geometric model is established,and the numerical solution of the model is effectively calculated using COMSOL.By comparing the experimental results of the fully coupled model with Musuda,the reliability and applicability of the model are verified.Compared with the numerical models of other scholars,it simplifies the programming and calculation workload and improves the accuracy of the calculation results.(4)Based on the verified fully coupled model,the evolution of reservoir physical properties and gas production efficiency under different coupling relationships are studied,and the sensitivity of parameters is discussed.The results show that the T-H-C model has the lowest gas production rate among the three coupling relationships describing the depressurization of natural gas hydrate production,but comprehensively considers the physical fields such as heat transfer,gas-water two-phase seepage and chemical decomposition.There is no obvious interfacial effect on the parameter change curves of hydrate saturation,reservoir pressure and permeability.Peak gas production rate and cumulative gas production increase with the increase of production pressure difference and initial absolute permeability,increase with the left shift of the phase equilibrium curve,and decrease with the increase of initial water saturation.The rate of decrease in hydrate saturation and the rate of increase in permeability are positively related to the production pressure difference,initial absolute permeability and phase balance curve parameter e1,and negatively related to the initial water saturation.