Study On The Hydrate Formation And Dissociation Mechanism In Porous Media

On May 2017,China first time succeeded in exploiting natural gas hydrate in the South China Sea,which may lead to a global energy revolution.At present,natural gas hydrate has not been commercially exploited which the formation and decomposition mechanism of natural gas hydrate is still not clear.There is a great significance of experimental simulation studies on the formation and decomposition mechanism of gas hydrate in porous media for realizing hydrate reservoir identification and resource utilization.The paper studied the growth process of TBAB hydrates in a confined space to explore the effect of solution concentration and space size on the hydrate crystal morphologies.On the basis of confined space,the paper investigated the growth model of TBAB hydrates on the surface of sands and grain migration during multiple formation-decomposition cycles.The results showed that the morphology of TBAB hydrate crystals changed dramatically in the confined space.When the initial solution concentration was 35 wt.%,the TBAB hydrate crystal formed in the millimeter quartz tube displayed a perfect nephroid shape.The TBAB hydrate crystal grew in a spherical shape under the influence of the curved surface;when the tube diameter increases,the TBAB hydrate crystal grew in a conbined shape of spherical and cylindrical with the decrease of the curvature;the crystal growth rate increased with the tube diameter,which was 3.1×10^-2 to 3.8×10^-2?L/s.After introducing silica sands,it was found that TBAB hydrate grew along the surface of silica sands in the form of coating and floating firstly,and then it changed to the cementation.Meanwhile it was found that repeated formation and decomposition of hydrate could effectively guarante the stability of stratum.Furthermore,CH₄-C₃H₈ gas was introduced as the guest molecule.The paper studied the effects of media composition,grain size,salt solution and formation-decomposition cycles on the growth model and occurrence state of CH₄-C₃H₈ gas hydrates.The saturation of CH₄-C₃H₈mixture hydrates in porous media increased gradually with the formation-decomposition cycle.The hydrate first grew as a covered form,then it showed a conbined form of covered and suspended,and finally it grew as the suspended and supported form which showed the characteristics of pore-filling.After adding 3.5 wt.%NaCl solution,CH₄-C₃H₈ gas hydrates grew or decomposed gradually as spheric particles.In the paper,the effects of media composition,salt solution and decomposition temperature on cage characteristics of CH₄-C₃H₈ gas hydrate were investigated through Raman spectroscopy.Formation and dissociation kinetics equation of CH₄-C₃H₈ gas hydrates in porous media with brine were established in the paper.In the presence of sands,CH₄-C₃H₈ hydrates in pure water and brine systems showed sII,where CH₄ occupied 5¹² and 5¹²6⁴ cages,while C₃H₈just occupied 5¹²6⁴ cages.In the system of sand-pure water,the occupancy rate of 5¹²,5¹²6⁴cages and the hydrate number were 0.849-0.999,0.967-0.999,and 5.68-6.33,respectively;the amount of C₃H₈ in 5¹²6⁴ cages was 1.88-2.23 times that of CH₄.After adding salts,the occupancy rate of 5¹² cages decreased to 0.809-0.999.The amount of C₃H₈ in 5¹²6⁴ cages 5¹²6⁴cages increased.Salts lowered the occupancy rate of 5¹² cages.Meanwhile,the brine film hinders the escaping of gas during the decomposition of gas hydrate,which means negative effect of salts during gas hydrate exploitation need to be considered.Through dynamics analysis,it was found that,in the system of fine sand with 3.5 wt.%NaCl solution,growth of gas hydrate could be divided to the early stage and the later stage,which the apparent reaction activation energies were 80.4¹14.7 kJ/mol and 146.2¹98.2kJ/mol,respectively.CH₄ were prior to entering into the hydrate cage,while C₃H₈ entered in the later stage.The brine raised the activation energy of apparent formation reaction.During the decomposition of gas hydrates,the apparent decomposition activation energy decreased to62.8⁹4.1 kJ/mol under the influence of the salt and the sands.