| In order to successfully achieve the goal of carbon peak and carbon neutralization,my country has always advocated energy conservation and emission reduction measures such as increasing the proportion of clean energy to promote the harmonious coexistence of man and nature and ensure the sustainable development of the country.Natural gas(methane)hydrate is known as one of the most potential unconventional clean energy sources in the 21st century because of its wide distribution,huge reserves,and clean combustion products,and it is the object of research by researchers from all over the world.The ubiquitous thermodynamic inhibitors such as salts and alcohols are the primary factors that determine the formation and decomposition conditions of methane hydrate in the formation and transportation environment.Changes in their composition and concentration directly affect the phase equilibrium thermodynamic conditions of methane hydrate,which is related to hydrate exploration and evaluation,commercial development,engineering maintenance and even production and life.Therefore,it is of great significance to study the phase equilibrium of methane hydrate under the inhibition system for the safe,economical and efficient development and utilization of hydrate.In this paper,the phase equilibrium conditions of methane hydrate in four salt solutions were measured and the decomposition enthalpy of hydrate was calculated.The influence of the composition and concentration of salt inhibitors on the thermodynamic properties of methane hydrate was analyzed.Reliability evaluation method of equilibrium data;extended the applicable range of Chen-Guo model in predicting the equilibrium conditions of methane hydrate in pure water and ice,and improved a method for calculating the water activity of solution liquid phase under inhibition system,and studied salt inhibition.Influence of methane hydrate phase equilibrium conditions on methane hydrate phase equilibrium conditions.Aiming at the phenomenon that the published data of methane hydrate phase equilibrium under the same inhibition system is quite different,a hydrate phase equilibrium thermodynamics experimental system was established,and the constant volume method was used to measure the phase equilibrium conditions of methane hydrate in four salt solutions.The experimental results show that the effect of magnesium chloride on the equilibrium temperature of methane hydrate is the strongest under the same mass fraction,and is stronger than that of calcium chloride,sodium chloride and potassium chloride in turn;with the increase of solution concentration,the phase equilibrium condition of methane hydrate tends to Move in the direction of higher pressure,lower temperature.In addition,a reliability evaluation method of hydrate phase equilibrium data is introduced to evaluate the effectiveness of measured data and published literature data,which lays a foundation for the thermodynamic modeling of methane hydrate phase equilibrium under inhibition system.In order to successfully carry out the thermodynamic modeling of methane hydrate phase equilibrium,the Chen-Guo model was determined to be the basis for improving the modeling of methane hydrate phase equilibrium in pure water and ice by comparing the prediction accuracy and computational efficiency.The influence of the equation of state on the calculation accuracy of the model is discussed,and the Patel-Teja equation of state is found to be the most suitable for methane hydrate.The Chen Guo model is improved by increasing the molar volume difference between water and basic hydrate at high pressure AV(P)and the temperature correction coefficient D(T)to improve the prediction accuracy.Compared with the published literature data,when the equilibrium pressure is greater than 100 MPa,the absolute average temperature error of the improved Chen Guo model is 0.61 K lower than that of the original Chen Guo model;When the equilibrium temperature is lower than 273.15 K,the corresponding error is reduced by 5.98 K.The improved method is also suitable for the phase equilibrium prediction of ethane and carbon dioxide hydrate.The above improved method expands the application scope of Chen Guo model in predicting hydrate phase equilibrium conditions in pure water and ice,meets the research needs of hydrate phase equilibrium under high pressure for deep and offshore oil and gas development,and also provides theoretical support for the application research of low-temperature hydrate in the future.Aiming at the poor generality of the conventional salt solution activity model,the phase equilibrium modeling of methane hydrate in salt solution under inhibition system was carried out.Based on the reliability analysis of the phase equilibrium data of methane hydrate in a large number of salt solutions,the data conforming to the reliability evaluation were introduced into the theoretical expression of liquid phase water activity,and the liquid phase water activity model of single salt solution was proposed.The model was used to predict the phase equilibrium temperature of methane hydrate in 10 kinds of salt solutions with mass fractions ranging from 3.5 to 26%in the range of 2.39 to 185.60 MPa,and the corresponding predicted value was only 0.44 K different from the experimental value on average.At the same time,the above activity model Both perform well in predicting the phase equilibria of ethane and carbon dioxide hydrate in monosalt solutions.It can provide a theoretical reference for the commercial exploitation of marine methane hydrate in the future.Aiming at the poor applicability of the conventional alcohol solution activity model,the phase equilibrium modeling process of methane hydrate in alcohol solution under inhibition system was studied.The reliability evaluation method was introduced into the evaluation of the phase equilibrium data of methane hydrate in alcohol solution,and the liquid phase water activity model of mono-alcohol solution was proposed based on the analysis of the published literature data.It has been verified that the activity model can effectively predict the phase equilibria of methane,ethane and carbon dioxide hydrate in methanol,ethanol,ethylene glycol,diethylene glycol,triethylene glycol and glycerol solutions with a mass fraction of 5～85%.temperature,the absolute mean error between the corresponding predicted value and the experimental value is only 0.76 K.It can provide a theoretical basis for the prevention and control of hydrate in the process of oil and gas development.Aiming at the problem that the actual effect of mixed solutes on solution activity does not match the model assumptions,the phase equilibrium conditions of methane hydrate when the solution contains multiple inhibitors are discussed.The study found that:for salt-salt mixed solution,considering the influence of single solute on water activity one by one,the phase equilibrium condition of hydrate in mixed solution can be more accurately predicted;for saltalcohol mixed solution,considering the interaction between different solutes.The effect of action on water activity may be beneficial for modeling hydrate phase equilibria.A new method for calculating the liquid-phase water activity of salt-alcohol mixed solution is proposed.Compared with the model results that do not consider the effect of solute interaction on water activity,the corresponding prediction accuracy of the model introduced by this method in predicting the phase equilibrium conditions of methane and carbon dioxide hydrate in the mixed solution of salt and alcohol is increased by 28.34%and 41.80%on average.It can provide a theoretical basis for the process design of hydrate development/control in complex environments. |
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