Influence of Flow Properties on Gas Productivity in Gas-Hydrate Reservoirs: What Can We Learn from Offshore Production Tests?

Gas hydrates are expected to be an energy resource in this century. Many countries, such as the United States, China, India, and Japan, have explored potential reservoirs and promising production methods. During the past decade, Japan has conducted the world’s first offshore production test in the eastern Nankai Trough, followed by China in the South China Sea. These offshore production tests have demonstrated that gas can be produced continuously from hydrate-bearing sand or silt-rich sediments. However, production tests in the eastern Nankai Trough have shown that gas production declines for a few days and then remains constant at a constant pressure in the bottom hole for at least a period of 4 days. This result is different from predicted behaviors by numerical simulations, showing a gradual increase in the gas production rate with time. The discrepancy between actual data and numerical simulations indicates that numerical simulators overestimate the response as a result of the lack of a proper system description in the models. Numerical simulation can reproduce the tendency of gas production stagnation by considering hydrate reformation in the reservoir. However, hydrate reformation is not expected to occur during the early stages of tests. Recent experimental and numerical studies have shown that sediment compaction and particle crushing by high effective stress, migration of fines leading to the formation of a skin, and changes in relative permeability are induced by depressurization during gas production. These phenomena result in dynamic permeability changes that can lead to gas production stagnation in the early stages.