Experimental Study Of Methane Recovery From Hydrate-bearing Sedments By Combination Of Depressurization And Other Technologies

Hydrate production by depressurization includes two modes:direct depressurization by extracting pore fluid and indirect depressurization by gas sweeping to reduce the natural gas partial pressure.The second mode is usually called gas injection.In this paper,aiming at different hydrate occurrence states,we focused on the comparison of two depressurization modes,and carried out the simulation study of multi-method combined hydrate exploitation.In order to improve the mining efficiency and safety,the optimization of hydrate exploitation scheme and operation parameters was put forward.The main research contents are as follows:?1?Contrastive experiments of hydrate exploitation by the two depressurization methods were carried out for hydrate reservoirs in closed gas-rich or open water-rich environments.For closed gas-rich reservoirs,direct depressurization led to fracture and collapse.For the open water-rich reservoirs,the gas/water ratios of direct depressurization were extremely low,especially for layers and massive reservoirs.The methane recovery ratios were<10%and the gas-water ratios were<5.Pressure-retaining gas injection can effectively prevent overlying water from entering hydrate reservoir and enhance gas production.However,due to the poor dispersion of injected gas,the methane recovery ratio was?30%.The combination of depressurization and gas injection was further tried to enhance gas dispersion in the reservoir,and the methane recovery ratio was up to 80%.?2?For the open water-rich reservoirs,CO₂ injection was used to form a low-permeability CO₂ hydrate cap in the overlying layer.The result showed that the CO₂ hydrate cap could be stable at<8?and a pressure difference of<8 MPa.Compared with direct depressurization,CO₂ hydrate cap increased the gas-water ratio more than 10 times and the methane recovery ratio more than one times.The effect is positively correlated with the aging time of CO₂ hydrate cap.?3?The method of ethylene glycol?EG?pre-injection combined with depressurization is put forward to enhance hydrate production efficiency.The results showed that the gas production efficiency was increased more than 2 times of that of direct depressurization.The concentration and distribution of EG had a great influence on gas production efficiency.In the process of gas production,it is necessary to control the injection concentration,injection volume and injection rate reasonably to enhance the EG efficiency.?4?CO₂/H₂ pressure-retaining continuous injection-production?CIP?mode is proposed for hydrate production.The method could effectively protect reservoir stability by maintaining reservoir pressure and CO₂-CH₄ displacement.When the CO₂concentration of the injected gas was between 43-74%,1:1 replacement of the CO₂ and CH₄ could be realized.In the early stage of exploitation,methane recovery was mainly controlled by gas injection rate,while in the later stage methane recovery was mainly controlled by gas diffusion rate.Furthermore,based on the relations of injection-production ratio and methane recovery ratio,a gas injection control strategy was put forward to optimize the production process.?5?Aiming at the problems of low methane recovery ratio and methane concentration in continuous injection-production mode,a semi-continuous injection-production mode was further put forward.The results showed that methane recovery ratio and methane concentration were increased to 90%and 50%,respectively.Injection-production ratio was controlled at 1.5-2.5,which were significantly lower than continuous injection-production.In addition,it is found that by adjusting the frequency of injection-production interruption,hydrate dissociation rate and methane production rate can be significantly enhanced.