Abstract:

Natural gas hydrate (NGH) is a type of ice-like compounds formed by natural gas and water at low temperatures and high pressures. It is widely distributed in the sediments of the seabed and frozen soil. It has huge resources and is expected to become a replacement energy in the future. Among the resources, NGH in the shallow layer or directly exposed on the seafloor has been widely discovered, and its formation process and stability law are still unclear. In order to reveal its stability law, the dissolving process of methane hydrate in quartz sand was experimentally studied. The dissolving efficiencies of methane hydrate in silica sand have been studied by injecting water and oil, respectively. The results show that both water and mineral oil can effectively dissolve methane hydrate in quartz sand, with the gas-to-liquid volume ratio about 2 for water injection and about 10 for oil injection. The cumulative gas production of water dissolving generally increases logarithmically with time, and the gas production rate fluctuates greatly, but it decreases gradually as a whole, and the gas-liquid ratio is stable at first and then decreases gradually; the cumulative gas production of oil dissolving generally increases in S shape with time, and the gas-liquid ratio increases first and then decreases gradually. At the same injection rate, the gas production rate and gas-liquid ratio of oil dissolving are significantly higher than those of water dissolution. The dissolving rate of hydrate is mainly affected by the type of dissolving medium (water or oil, in this work) and the contact status between dissolving medium and hydrate. Water and oil are easily channeled into quartz sand, forming a dominant seepage channel. The gas-to-liquid ratio decreases when water/oil forms dominant seepage channel. The results provide theoretical basis for stability studies of NGH in the shallow layer or directly exposed on the seafloor. It seems feasible in principle to exploit gas hydrate in the shallow layer or exposed to the sea floor by dissolving. It is suggested that systematic experimental and numerical simulation studies should be continued in the future.

Key words: methane, hydrate, dissolving, silica sand, experiment

摘要：

天然气水合物是天然气与水在低温高压的条件下形成的一种冰状物质，广泛分布于海底和冻土区的沉积物中，资源量巨大，有望成为未来接替能源。在已发现的资源中，有一种类型的天然气水合物位于海底浅表层或裸露于海底，其形成过程和稳定性规律尚不明确。为揭示其稳定性规律，实验研究了石英砂中甲烷水合物的溶解过程。结果表明，水和白油均能有效溶解石英砂中的甲烷水合物，注水溶解的气水体积比约为2，注油溶解的气液体积比约为10，溶解速率主要受液流-水合物的接触情况影响，随水合物饱和度升高而升高。水/油易在石英砂中窜进，形成优势渗流通道，随后气液比逐渐降低。实验结果为深入研究海底浅表层或裸露的天然气水合物的稳定机理提供了基础。

关键词: 甲烷, 水合物, 溶解, 石英砂, 实验