Study On Secondary Growth Characteristics And Influencers Of Methane Hydrate In Porous Media

Natural gas hydrate is a kind of crystalline compound formed under low temperature and high pressure.It is widely stored in permafrost and seabed sediments.It is an important source of natural gas in the future and an important form of natural gas storage and transportation.Proving the formation kinetics of natural gas hydrate is the premise and basis for revealing the mode of occurrence of natural gas hydrate deposits and improving the efficiency of natural gas hydrate storage and transportation.The key scientific question is how to qualitatively and quantitatively clarify the characteristics of natural gas hydrate formation and its influence mechanism.Since hydrate formation is a reaction process that nucleates first and then grows in time,and the randomness of hydrate nucleation time and space determines the simultaneous existence of multiple hydrate nucleation and growth in space,traditional research often neglects the formation of hydrates.Interaction between nucleus and growth process.For this reason,this paper proposes a new method to study the hydrate formation kinetics based on the natural gas hydrate regrowth process,which is the process of further growth of in-situ hydrate.It can clarify the control mechanism of hydrate growth rate under the premise of eliminating the influence of nucleation stage,and provide basic theoretical support for elucidating the kinetics of natural gas hydrate formation.This paper is based on the low-temperature and high-pressure natural gas hydrate experimental system.First,natural gas hydrate is formed in a porous medium with an initial pore water content of 30%,a temperature of 273.65 K,and a pressure of 5 MPa(nucleation growth method).Nucleation growth induction time ranging from tens of minutes to hours,and there is still a large driving force(greater than 1 MPa)and incompletely converted water(greater than 20%)after nucleation growth;Water injection in the medium induces the hydrate that has been formed to be further formed(regrowth mode).It is found that the injected water consumes methane gas immediately,indicating that there is no induction time for the hydrate regrowth,and the system approaches the equilibrium pressure or when the hydrate regrowth ends.Almost all the injected water is converted into hydrates,indicating that hydrate regrowth is a reaction process with a small driving force;however,the gas consumption rate in the regrowth process of natural gas hydrate is slower than that of nucleation growth.When the pressure in the regrowth process increases,experimental observations when the random nucleation growth process occurs,the gas consumption characteristics are compared to clarify the occurrence time of the hydrate regrowth process nucleation growth and the corresponding amount of hydrate formation.Based on the nuclear magnetic resonance imaging system,this paper has proved the hemispherical agglomeration characteristics of the injected water in the porous media.The experimental results show that the gas consumption rate is positively correlated with the saturation of the existing hydrate and is affected by the particle size and properties of the porous media in the hydrate regrowth process;Further analysis of the methane consumption characteristics of the regrowth process,it is found that hydrate regrowth occurs firstly on the surface of the water ball(three-phase interface).And according to the linear relationship between the constant hydrate regrowth rate inside the water sphere(two-phase interface)and the existing hydrate saturation,the gas consumption rate of the microscopic hydrate reaction is obtained.Finally,the three-stage growth mode and control mechanism of natural gas hydrate regrowth in porous media are proposed(Growth of hydrate film on the surface of the water ball,re-growth of hydrate inside the water ball,and re-growth of hydrate that is not restricted by mass transfer).