Formation and Production Characteristics of Shallow Marine Hydrates Considering Overlying Water Erosion

Abstract

Marine hydrates have received continuous worldwide attention owing to their great commercial exploitation prospects. Overlying water erosion is a common phenomenon within marine hydrate reservoirs, especially for the shallow hydrates that exist in submarine sediments without stratigraphic traps. However, the effect of overlying water erosion on the formation and decomposition of shallow marine hydrates remains unclear, presenting a challenge for economically viable marine hydrate exploitation. In this work, hydrate-bearing sediments with constant-pressure (∼9.5 MPa) overlying water were synthesized at different temperatures (274−279 K). The results show that the overlying water shortened the hydrate formation duration by more than 90%. This is attributed to the constant-pressure overlying water making the hydrate reservoir a dynamic water-excess system and providing adequate water supply sources to ensure sufficient gas−water contact for hydrate formation. In addition, the result indicates that the rate (percentage) of hydrate conversion is mainly determined by the temperature driving force (gas−water contacts). Furthermore, three different depressurization rates were adopted to decompose the hydrate in sediments with overlying water. As shown by the experimental results, the flowing overlying water helps to mitigate the reduction in reservoir temperature and accelerate hydrate decomposition by enhancing the heat and mass transfer in the hydrate reservoirs. In addition, the results show that the depressurization rate significantly affects real-time water production and hydrate reservoir temperature but has minimal impact on final cumulative water production. The findings obtained in this study contribute to advancing the exploration of shallow marine hydrate resources.