Research On The Dissociation Rules Of Methane Hydrate With Inorganic Salts

Thermodynamic inhibitors are often used in the prevention of hydrate formation and production of gas hydrate reservoirs in oil field,because they are easy to obtain and it can decompose the gas hydrate quickly.However,they are still a lot of problems in the application process of thermodynamic inhibitors like inorganic salts and alcohols:the typical problems include over-dosage,blindness in choosing the thermodynamic inhibitors,etc,and can lead to an increasing cost in hydrate exploitation and an aggravated environment around oil field.Therefore,researchers need to conduct in-depth studies on the decomposition mechanism of gas hydrate with thermodynamics inhibitors,in order to provide theoretical and technical support to the prevention and production of gas hydrate in oil filed.In this thesis,we studied the step-down decomposition process of methane hydrate in three typical inorganic salt?sodium chloride,potassium chloride and calcium chloride? solutions on the bench scale.Results showed that,with the increase of decompression range,the differential pressure between the reaction kettle and the three-phase equilibrium pressure increased,the driving force promoted the hydrate decomposition was enhanced,the methane hydrate decomposition rate also increased;with the increase of the initial injection mass concentration of inorganic salts solution,when the decompression range was small,the activity coefficient of water molecules in solution reduced,the diffusion coefficient also decreased,the methane hydrate decomposition rate was accelerated;when the decompression range was big,due to the hydrates decomposition reaction being an endothermic reaction,a small amount of ice might generate in the reaction process,which might delay the decomposition of methane hydrate,and thus the decomposition speed of methane hydrate speeded up first,and then slowed down;the decomposition rate of methane hydrate in inorganic salt solutions followed the rule as:NaCl>KCl>CaCl₂,and the bigger the mass concentration was,the closer the decomposition rate was among the inorganic salts.Based on the experimental results,and the assumption that hydrates decomposition process was not affected by the influence of heat transfer and mass transfer effect,I derived the kinetic model of hydrate decomposition,and introduced the parameters A,which relates to?the type of the injection solution and the initial injection mass concentration of the solution?,according to Kim-Bishnoi model,and obtained the relation between the cumulative gas production nH and time t,which important for the precise calculation of decomposition time and the decomposition amount of methane gas.Finally,we analyzed the main factors influencing the accuracy of the kinetic model for the methane hydrate decomposition:when methane hydrate was able to decompose completely,the main factor was the heat transfer in the process of methane hydrates decomposition;when methane hydrate was not able to decompose completely,the main factor was the difference between the calculated value P_e and real value of the three-phase equilibrium pressure.