Experimental Study On The Effect Of Salt On The Stability Of Methane Hydrate

The thickness of the methane hydrate stable zone is mainly controlled by the stability of methane hydrate.The temperature,pressure,pore water salinity,and gas component factors affect the thickness of the hydrate stable zone.The formation of methane hydrate is closely related to formation water,while all kinds of salt ions in formation water(Cl^-,Na⁺,Mg²⁺,SO₄^2-,Ca²⁺,and transition metals Fe,Mn,Cu,Co,Ni)will affect the formation and decomposition conditions of natural gas hydrate.Many studies have experimented and obtained the effect of different salt systems on the stability of methane hydrate,but there is a dispute between the strength of inhibition and the influence of anion and cation on the stability of methane hydrate.There is still a gap between the present research results and the actual geological conditions.It is also necessary to carry out systematic research under real conditions and provide reference for the exploration and development of hydrate in the future.In this paper,the effects of different salts and different concentrations on the stability of methane hydrate are studied by high pressure visual capillary cell combined with laser Raman spectrometer.Some conclusions can be reached in this study,which are shown in the following text.?1?For the one type salt containning sysytem,effect of chloride,sulfate and carbonate on the stability of methane hydrate is compared.The three phase?CH₄·nH₂O_S-L-G?equilibrium curve of methane hydrate is shifted towards low temperature and high pressure along with the increase of salt concentration.?2?Under the same concentration condition,Inhibitory effect of different salts on methane hydrate formation is compared.With the increase of concentration,the inhibitory effect of MgCl₂ on methane hydrate is stronger than that of CaCl₂.The inhibition effect of CaCl₂ and MgCl₂ on methane hydrate is stronger,and KCl is weaker.For 0.5 mol%of sulfate,the inhibition intensity is K₂SO₄>NaSO₄?MgSO₄.For 0.5 mol%of carbonate,the inhibition intensity is Na₂CO₃?NaHCO₃.For concentration of 1 mol%of different salts on methane hydrate,the inhibition intensity is CaCl₂>MgCl₂>NaCl?KCl>NaHCO₃>MgSO₄.?3?Under the same ionic strength condition,the inhibition effect of anion and cation on methane hydrate is compared.With the increase of ionic strength,the inhibition effect of K⁺and Ca²⁺on methane hydrate will change.The inhibitory effect of Na⁺on methane hydrate is the strongest,and the inhibitory effect of Mg²⁺on methane hydrate is the weakest.The inhibition effect of anions on methane hydrate is Cl^->HCO₃^->CO₃^2->SO₄^2-.When the ionic strength is the same,the inhibitory effects of different salts on methane hydrate vary with the concentration.NaCl has the greatest inhibitory effect on methane hydrate,and the inhibitory effect of MgSO₄ on methane hydrate is the weakest.?4?The reasons for the effect of different salts on the stability of methane hydrate are analyzed.Na⁺is less affected by concentration,and the dominant factor affecting the stability of hydrates is the demand for energy in the decomposition of hydrates.The influence of the concentration of Ca²⁺on the equilibrium of methane hydrate phase is the main controlling factor of the change of the concentration of Ca²⁺.The effect of anions on methane hydrate is mainly controlled by the size of the anion radius.If the ionic radius is smaller,the effect on the phase equilibrium of methane hydrate becomes greater.Salt is a combination of anion and cation pair.NaCl has the greatest impact on the stability of methane hydrate,while MgSO₄has the smallest effect on the stability of methane hydrate.