Abstract: Natural gas hydrate is considered as a clean energy resource with great potential, and more than 90% of hydrates are distributed in deep-sea sediments. Aiming at the current status of hydrate mining technology, the paper proposes a new exploitation method of injecting normal temperature seawater to enhance the heating efficiency of low-frequency electric field in marine hydrate reservoirs, which has multiple advantages such as depressurization, in-situ heat generation by electric field, thermal convection and in-situ seawater extraction. By means of numerical simulation, the paper analyzes the feasibility and production mechanism of the new method for improving the production efficiency of marine hydrate reservoirs, and explores the effects of seawater injection rate and voltage on the production performance. The research results show that, based on the electric field heating, the injection of normal temperature seawater further improves the gas production rate, gas-to-water ratio and energy efficiency ratio, and the stimulation mechanism lies in strengthening the heat transfer efficiency by seawater convection, providing gas flow driving force and enhancing the reservoir electrical conductivity. With the increase of injection rate, the convective heat transfer effect is further enhanced and the peak gas production is improved, but the gas-to-water ratio decreases continuously due to the increase of water production, and the energy efficiency ratio first increases rapidly and then stabilizes. Increasing voltage can significantly improve gas production and gas-to-water ratio, but it also increases the heat consumption in hydrate reservoirs, thus leading to a decreasing energy efficiency ratio. It is suggested that the electric heating system should be optimized based on the comprehensive consideration of gas production and energy efficiency ratio.

Key words: natural gas hydrate, low-frequency electric field heating, seawater injection, thermal convection, depressurization, production efficiency

摘要： 天然 气水合物被视为具有巨大潜力的清洁能源资源,90% 以上的水合物分布在深海沉积物中。针对目前水合物开采技术现状,提出了注入常温海水强化海域水合物藏低频电场加热效率的新型开采方法,可发挥降压、电场原位生热、热对流以及海水就地取材等多重优势。通过数值模拟手段,分析了新方法提高海域水合物藏生产效率的可行性及生产机理,探讨了海水注入速率和电压对生产性能的影响。研究结果表明,在电场加热的基础上,注入常温海水进一步提升了产气速率、生产气水比及能效比,增产机理在于海水对流强化传热效率、提供气体流动驱动力以及增强储层导电性。随着注入速率的增加,对流传热效果进一步加强,产气峰值得到提高,但受产水量增加的影响,生产气水比不断减小,能效比先快速增加后趋于稳定。增加电压可显著提高产气量和生产气水比,但也加大了水合物储层内的热耗,导致能效比不断减小,建议在综合考虑气体产量与能效比的基础上对电加热制 度进行优化。

关键词: 天然气水合物, 低频电场加热, 海水注入, 热对流, 降压, 生产效率