Thermodynamic Stability Of Methane-Hydrogen Hydrate In Mariana Forearc Serpentine Mud Volcanic Area

As a potential energy source,natural gas hydrate is very abundant and which combustion products are cleaner.It is generally formed by natural gas and water at low temperature and high pressure,and it is an ice-like white solid substance.It is mainly distributed in the seafloor sediments and terrestrial permafrost zones of continental margins with a water depth of more than 500 meters.Natural gas hydrate is composed of an empty cage formed by hydrogen bonds of water molecules and small natural gas molecules enclosed in it.Because the small natural gas molecules are usually dominated by methane,they are also called methane hydrate.The vast ocean floor is barren and lacks the gas conditions to form hydrates.However,with the deepening of deep-sea investigation and research,it is found that the interaction of oceanic crust ultrabasic rocks and basalts with water can cause extensive serpentinization reactions.Serpentinization can provide ample methane and hydrogen for the ocean floor.The scientific community generally uses the Bottom Simulating Reflector（BSR）to identify whether natural gas hydrates exist in a certain area of nature.Typical geophysical evidence BSR for methane hydrate development was found on seismic profiles of the serpentinized region of the ultra-slowly expanding mid-ocean ridge in the Fram Strait between the North Atlantic and Arctic Oceans.Since the report of the phase diagram of hydrogen hydrate formation in 1999,there has been sufficient evidence that hydrogen can enter the water molecular cage to form hydrogen hydrate.For a serpentinized area with a large amount of methane and hydrogen,if the geological temperature and pressure are suitable,then the area is likely to develop a methane-hydrogen two-component hydrate.There are a lot of serpentinization phenomena in the Mariana fore-arc serpentine mud volcano area,which can provide gas supply for the formation of hydrates;the average depth of the seafloor of most mud volcanoes is more than 3000meters,which meets the temperature and pressure hydrate formation conditions of seabed sediments.In order to carry out methane-hydrogen hydrate research,the stability of methane-hydrogen hydrate and the supply of methane and hydrogen must first be determined.Solving these problems requires the establishment of a methane-hydrogen hydrate phase equilibrium model as well as a methane and hydrogen solubility model.At present,there is no methane-hydrogen hydrate three-phase equilibrium model and hydrogen solubility model related to the marine environment.The calculation model of methane solubility has mature theories,mainly based on the liquid electrolyte solution theory and the gas phase high-precision equation of state.There is no well-established theory for the calculation of hydrogen solubility.In this paper,by querying the experimental data of hydrogen solubility in relevant literatures at home and abroad,and then according to the collected experimental data of the solubility of hydrogen in pure water and sodium chloride solution,based on the methane solubility model theory,the relevant parameters of the hydrogen solubility model are finally obtained,and finally proposed a complete set of hydrogen solubility prediction models.In order to accurately predict the formation conditions of methane-hydrogen hydrate,the most reliable method at present is to establish a methane-hydrogen thermodynamic model based on classical adsorption theory combined with van der Waals theory and molecular potential energy model.By applying the gas solubility model and combining the adsorption theory,this paper finally establishes a thermodynamic model that can calculate the equilibrium temperature and pressure of the three-phase（hydrate-water-gas）three-phase（hydrate-water-gas）generated at different methane-hydrogen mole fraction ratios.The establishment of a methane-hydrogen thermodynamic model is helpful for judging whether methane-hydrogen hydrate exists in the Mariana serpentine mud volcano area,and provides a scientific basis for confirming the range of the methane-hydrogen hydrate stable zone and evaluating the hydrate resource content.Based on the simple fugacity coefficient equation for predicting water content in the gas phase,the high-precision equation of state for the gas phase and Pitzer’s liquid electrolyte solution theory,a model for the solubility of H₂ in pure water and Na Cl solution is established.The model is applicable to pure water:temperature It is273.15～423.15K,and the pressure is 0～1100Bar;the applicable range in 0～5mol/kg Na Cl solution:the temperature is 273.15～373.15K,and the pressure is 0～230Bar.The model can be extrapolated to seawater solution,and the calculated H₂ solubility at P=1atm and T<290K is in good agreement with the experimental data.Thermodynamic model based on classical adsorption theory,combined with van der Waals theory and molecular potential energy model.A thermodynamic model for the equilibrium temperature and pressure of methane-hydrogen hydrate three-phase（hydrate-liquid-vapor）was established.In pure aqueous solution,the predicted results are in good agreement with the experimental data when the gas phase mole fraction of hydrogen is4.55%,28.03%,and 33.85%.When the gas phase mole fraction of hydrogen is above40%,the predicted value is slightly lower than the experimental value.The results show that the three-phase equilibrium pressure increases gradually with the increase of temperature.At the same temperature,when the gas phase mole fraction of hydrogen increases,the pressure required to form hydrate increases;other conditions remain unchanged,with the continuous increase of sodium chloride concentration,the three-phase equilibrium pressure of methane-hydrogen hydrate increases significantly.Higher gas-phase hydrogen mole fractions or higher concentrations of sodium chloride inhibit the formation of methane-hydrogen hydrate.According to the 2018 IODP366 voyage reports,the drilling reports of the U1491site in the valley and the U1492 site in the peak of the Yinazao serpentine mud volcano in the Mariana fore-arc were selected.and the thermodynamic model of this paper was used to analyze the thermodynamic conditions for the formation of natural gas hydrate from methane and hydrogen.It is confirmed that Yinazao mud volcano has the conditions for methane-hydrogen hydrate formation,and the stable bottom depths of the two sites under different gas mole fraction ratios are determined.The bottom boundary of the hydrate stability zone at the U1491 and U1492 sites can reach 1087and 1815 meters respectively.The results show that with the same gas ratio,the bottom boundary depth of the stability zone at the U1491 station in the valley is 729 meters deeper than that of the U1492 station at the top of the mountain.