Experimental Study And Numerical Simulation On The Extraction Of Marine Gas Hydrates By Dissolving Method

Natural gas hydrate is an ice-like clathrate compound formed by natural gas and water under low temperature and high pressure.It has huge resources and is regarded as an important alternative energy source in the 21 st century.Since the 1970 s,the former Soviet Union,the United States,Japan,Canada,and China have successively carried out hydrate trial studies.The results show that the depressurization,thermal stimulation,displacement and inhibitor injection are technically feasible for the exploitation of natural gas hydrate.However,it faces bottlenecks such as slow production rate,massive sand production,small dissociation radius,and high cost.Therefore,exploring efficient and safe developing methods is the key direction of natural gas hydrate research,which has important theoretical and practical significance.In this paper,the process of dissolving and extracting of methane hydrate in quartz sand was studied through experiments,the influence of water injection rate,initial hydrate saturation and water flow direction on gas production was discussed,and the basic principle of hydrate extracting by dissolving method was analyzed.A numerical model for hydrate extracting by dissolving method was established,and the model parameters were optimized and verified for accuracy based on experimental data.The gas production of natural gas hydrate extracted by the dissolving method under different initial hydrate saturation and different water injection rate was predicted,and the principle of various factors in each stage of dissolving extraction had been analyzed.Innovatively designed a method of extracting seawater to dissolve shallow hydrates and conducted numerical simulation studies.The influence of different well types,well diameters and production pressure differences on the production process was discussed,and the law of gas production change in each stage of developing was analyzed.Experiments showed that the dissolving method can effectively extract methane hydrate in quartz sand,and the gas production process was continuous,stable and controllable.The recovery during water injection in the horizontal direction was greater than 43.9%,and greater than 67.1% during water injection in the vertical direction.Under the experimental temperature and pressure conditions,the gas-liquid ratio corresponding to the saturated solubility of methane in water was about 2.The development process of hydrates can be divided into three stages:(1)Establishment of dominant flow channels;(2)Wide and connection of flow channels;(3)Dissolve residual hydrates.The key factors affecting the rate of hydrate dissolving were the contact area of water and hydrate,contact time and dissolving reaction rate.Affected by gravity,the amount of hydrate formed in the upper part of the quartz sand was higher than that in the lower part,and the permeability of the upper part of the quartz sand was lower than that of the lower part,which resulted in a dramatic difference in the gas production when water injection from different directions.Using the numerical model established in this paper for hydrate extracting by the dissolving method,the gas production at different initial saturations was predicted and compared with the experimental results,and the final gas production error did not exceed 6.1%.The model believes that there is a hydrate dissolution zone in the reactor.When the water injection rate increases,the dissolution zone will be stretched.When the length of the dissolution zone exceeds that of the reactor,the produced water will not reach dissolution saturation.When the hydrate saturation increases,the dissolution zone will be narrowed accordingly.When extracting seawater to dissolve shallow hydrates on the seabed,the production efficiency is affected by the production pressure difference,completion length and well diameter.Proper profile control and water plugging of the formation near the production well can reduce the water production rate and increase the gas-liquid ratio.Taking measures to increase the permeability of hydrate reservoirs in the zone can improve the development efficiency.