Investigation Of Replacement Combined With In-situ Autogenic Heating Enhanced Exploitation Of Natural Gas Hydrates

Carbon dioxide replacement method can realize resource exploitation and carbon sequestration at the same time,which is the most promising method for gas hydrate exploitation.However,carbon dioxide replacement method has low recovery efficiency,lower energy efficiency,which is still far from industrial application.To solve the above problems,this paper combined the CO₂ replacement method with the heat stimulation,and proposed the step-by-step heat injection and the self-generated heat injection enhanced replacement recovery technology to improve the recovery efficiency and energy efficiency.HYSYS software was used to simulate the two recovery method schemes,and the recovery energy efficiency of the two methods was calculated and compared.In order to improve the recovery percentage of CH₄ and reduce the loss of heat in the process of heat transportation,in-situ intermittent heat injection and enhanced replacement were firstly combined to exploited CH₄ hydrate.Compared with decreasing intermittent heat injection,increasing intermittent heat injection modes make the CH₄ recovery percentage increased from 8.60%to 16.96%,the production-injection ratio increased from 0.041 to 0.076,and the CH₄ concentration of export gas increased from 7.71%to 15.13%.In the range of 2.16-3.7 MPa,reducing the initial replacement pressure is beneficial to improve the exploitation effect of CH₄ hydrate.The optimal exploitation conditions are 275.15 K,2.16 MPa incremental heat injection replacement,methane recovery percentage is 25.4%,production-injection ratio is 0.53.Then,in order to further reduce energy consumption and improve CO₂ storage percentage,in-situ Ca O hydration heat is used to enhance CO₂ replacement exploitation.Compared with the blank control experiment,the methane recovery rate increased from 8.64%to 21.0%after Ca O injection,and the production-injection ratio increased from 0.055 to 0.124.When the replacement pressure decreases from 3.4 MPa to 2.6MPa,the methane recovery rate increases from 20.96%to 31.43%,and the methane concentration increases from 15.9%to31.1%.Under the condition of 281.15 K,3MPa,the methane recovery percentage of self-generated heat replacement by Ca O injection is 69.81%,and the strengthening effect is significant.Based on the experiments results,the recovery process was designed by HYSYS and the energy return on investment（EROI）ratio of the two recovery schemes was compared.The results of energy efficiency analysis show that the EROI value of increasing intermittent heat injection replacement recovery is 5.23～11.90,and the minimum energy consumption is 0.851k Wh/Nm³-CH₄.The EROI value of methane hydrate exploitation by Ca O injection enhanced replacement is 4.68-18.96,which is higher than increasing heat injection enhanced replacement,.At 279.15 K and 3.00 MPa,the EROI value of 3.00 MPa replacement mining is 18.96,which is higher than the theoretical EROI value of 17.3 for heat stimulation exploitation.The corresponding unit energy consumption is 0.536 k Wh/Nm³-CH₄.After the introduction of Ca O injection,the energy consumption per unit of the enhanced mining process is reduced by37.01%.