Experimental Simulation And Energy Efficiency Analysis Of Gas Hydrates Production By Gas Injection Method

The natural gas trapped into gas hydrates is estimated to be between 0.2×10^l5 and1.12×10^l8 m³,which is also kind of valuable clean energy resource.Although its reserves are very attractive,field test verified that the cost of gas production from gas hydrates is still very high and may cause environmental and safety issues.It is a huge challenge to exploit natural gas from gas hydrates right now.Here we study the decomposition conditions of methane hydrate in quartz sand,the rule of gas production from hydrates by two different gas injection methods,and the energy efficiency ratio?EER?of different mining methods.The main achievements of this work are as follows:?1?After the preparation of methane hydrate in quartz sand,20¹00 mesh,the decomposition conditions of methane hydrate were determined by a pressure-searching method.The results show there are several decomposition pressures at specific temperature,and the difference between the maximum and minimum pressure is less than 0.5 MPa.In addition,the final decomposition pressure closes to the bulk phase equilibrium pressure.The influences of particle size of quartz sand and water saturation on hydrate dissociation conditions were investigated simultaneously.Compared with the phase equilibrium conditions of CH₄ hydrate in quartz sand,the bulk phase equilibrium conditions of CH₄ hydrate calculated by CSMgem and PVTsim model is the same,while calculated by Chen-Guo model is a little higher when temperature is above 276 K,calculated by Multiflash model is samller.?2?The hydrate formation conditions of two kinds of ternary gas mixture?N₂/CH₄/CO₂,H₂/CH₄/CO₂?in pure water were measured a by pressure-searching method,respectively.The formation conditions of N₂/CH₄/CO₂ hydrate determined in experiment is the same with the values calculated by Chen-Guo,CSMgem and Multiflash models,but different with the calculated values by PVTsim model.The cage occupancy of H₂ in small cavity ranges from 0.3 to 2.3%,while in large cavity is located in?2.16¹3.6?×10^-7%.Even the conditions are in hydrate stable zone,the lower the temperature is,the higher the cage occupancy of H₂ in small cavities would be.?3?Here the rule of gas production from gas hydrates by N₂ sweep was studied in lab.During gas injection,the mole fraction of N₂ in pore space increases with elapsed time,leading to the increase of fugacity difference for methane between hydrate phase and gas phase,which further promotes the dissociation of methane hydrate.The gas production efficiency of N₂ sweep is much higher than that of direct depressurization method.Hence,gas sweep can be regarded as an enhanced depressurization method.The increase of gas injection rate would result in the improvement of gas production efficiency,but also increase the injection-production ratio,means more N₂ is needed when recover per unit volume of methane.The gas production efficiency of continuous injection model is higher than that of batch injection mode,while the injection-production ratios of those two runs perform same.For the exploitation of low saturation hydrates,N₂ sweep method can obtain relatively high methane recovery and low injection-production ratio,which is not capability in other mining methods.?4?This work supplied a novel hydrates production method by CO₂/H₂replacement and simultaneously studied the mechanism of gas production caused by gas sweep and gas replacement.In gas replacement stage,when the mole fraction of CO₂ in feed gas is greater than 0.72,the increase of CO₂ mole fraction would extend the time used for gas exchange and increase CO₂ sequestration ratio,but it would decrease the cumulative gas production ratio.If the mole fraction of CO₂ in feed gas is less than 0.18,the decrease of CO₂ mole fraction would further increase the gas production rate and cumulative gas production ratio,while the CO₂ sequestration ratio would decrease conversely.Notably,when the mole fraction of the CO₂ ranges from 0.55⁰.72,the amount of CO₂ trapped into hydrate phase is close to the amount of methane dissociated from hydrate,This can keep the balance of total amount of gas hydrates,in this way to maintain the stiffness of hydrate sediment skeleton.Even though the CO₂ sequestration ratio and cumulative gas production ratio are not the highest in this range,it can meet the dual function of CO₂ replacement.In addition,the increase of initial CH₄concentration would reduce the CO₂ sequestration ratio and cumulative methane production ratio during gas replacement stage.In order to achieve higher methane production ratio,it is better to reduce the mole fraction of CH₄ as much as possible.?5?Based on thermodynamics analysis,the chemical exergy of CH₄ is 822 kJ/mol at standard conditions.This work also supplies a way to calculate the exergy production,exergy consumption and energy efficiency ratio?EER?of different hydrate exploitation methods.During gas production,the EER linearly increase with the amount of CH₄ in the recovered gas.Depressurization method has the highest EER,which is 29.31.On the contrary,the EER of the CO₂/N₂ replacement is one magnitude order smaller,which is4.93.The EER of pure gaseous CO₂ replacement,N₂ sweep and H₂/CO₂ replacement methods are ranges from 9.59 to 19.82.The EER is highly depended on the composition of the recovered gas.If the composition of the recovered gas changes,the energy efficiency ratio would fluctuate accordingly.