Study On The Permeable Overburden Sealing And Exploitation By Inhibitor Injection Of Marine Hydrate Reservoir

Natural gas hydrates(referred to as hydrates)are ice-like substances formed by natural gas and water under low temperature and high pressure.It is an ideal energy source because of its large reserves,wide distribution,high energy density and cleanliness and non-pollution.The hydrate mining process involves the problems of complex flow,temperature,chemical and stress,and the disadvantages of complex construction and high costs.The production process are recognized as world problems.In this paper,taking the unconfined hydrate reservoir in the sea as the research object,a numerical model for the depressurization of unconfined hydrate reservoir in the sea considering seawater permeability is firstly established and numerical simulation operations are carried out.The dynamic characteristics of overburden mining in unconfined hydrate reservoirs under seawater permeability are investigated,and the main geological factors and production factors are analyzed to determine the influence of different factors on overburden mining.According to the results of the above work,the gas and water production of the unconfined hydrate reservoirs under seawater permeability is drastically reduced,so it is proposed to inject carbon dioxide into the cap rock to form carbon dioxide hydrate to achieve the purpose of confining the cap rock,and then to carry out the depressurization minster production.The specific measures are to add the carbon dioxide component and the kinetic equation of carbon dioxide hydrate reaction on the basis of the above work,and establish an improved numerical model.The simulation operation of the whole process is carried out to obtain the dynamic characteristics of the injection process and the production process.The results show that after completing the closure transformation of the hydrate reservoir,the cumulative gas production has been enhanced by up to 51%,which is remarkable,but there is still a large amount of undecomposed hydrate in the reservoir,which shows that a single depressurization method can not meet the demand of hydrate exploitation,and depressurization combined with other methods should be used to exploit the sea area unconfined hydrate reservoirs.In order to solve the problem of low recovery rate of hydrate reservoir by single depressurization method,this paper proposes to inject inhibitor to improve the problem of low recovery rate of depressurization production in hydrate reservoir.An experimental platform was built to experimentally study the synthesis process of natural gas hydrate under the sodium chloride inhibitor system.The results show that the presence of inhibitor inhibits the synthesis of hydrate and promotes the decomposition of hydrate,and the higher the concentration of inhibitor,the more obvious the effect.On the other hand,in the process of inhibiting hydrate synthesis,increasing the inhibitor concentration is more significant than increasing the temperature,which confirms the high efficiency of the inhibitor in mining hydrates.A model of injecting inhibitor mining in unconfined hydrate reservoirs at mine scale was established,and the hydrate mining process was simulated by the depressurization and combined inhibitor injection method to analyze the gas production law and the spatial distribution law of reservoir physical properties in the process of injecting inhibitor and to compare with the single depressurization and depressurization and combined heat injection methods,which showed that compared with the single depressurization method,the injection of inhibitor and heat injection both substantially increased the recovery rate of hydrate and the injection of inhibitor had a faster recovery rate in the middle and early stages.The results show that compared with the single depressurization method,the injection of inhibitor and heat injection both greatly improve the recovery rate of hydrate,and the injection of inhibitor has a faster gas production rate in the middle and early stages,which is a highly efficient means of extraction.Secondly,different injection concentrations,injection temperatures and injection pressures were investigated,and the results showed that higher injection salinity was favorable to improve the gas production in the middle and early stages,while the increase of injection temperature had less effect on the gas production in the middle and early stages,higher injection salinity could significantly improve the energy efficiency,and the increase of injection pressure could improve the gas production in the middle stage,but it substantially reduced the gas-water ratio,which was not favorable to the extraction.Then,a global sensitivity analysis of the above three parameters was conducted,and the results showed that increasing the injection salinity was the most extractive means.Finally,an intelligent optimization model was established on the basis of the numerical model of injection inhibitor to automatically optimize the injection parameters and complete the scheme preference,and the results showed that the optimal injection concentration,injection temperature and injection pressure were 24 wt %,23.50 ℃ and 17.05 MPa,respectively.The study reveals the mechanism of injection inhibitor in unconfined reservoirs,forms a new numerical model and simulation method,clarifies the injection inhibitor’s mechanism and optimizes the most reasonable mining scheme.Therefore,the research results are enriching the theory of the injection inhibitor mining in unconfined hydrate reservoirs and promoting the industrialized development of hydrates.