Methane hydrate phase transition in marine clayey sediments: Enhanced structure change and solid migration

Natural gas hydrates are abundant in marine sediments and have great potential for resource development. However, there is still a lack of clarity regarding the impact of hydrate phase transition on the structure change of marine sediments. Hence, we conducted in-situ microscale experiments of hydrate phase transition in clayey sediments obtained from the active Haima cold seep area in South China Sea. By utilizing advanced X-ray Computed Tomography technology and a controlled method of hydrate phase transition, we successfully observed the dynamic behaviors of hydrate formation and decomposition, and revealed their effects on sediment structure and solid migration. Our findings suggested that methane hydrates initially occupy primary pores while generating new pores and fractures in clayey sediments. These processes lead to changes in pore morphology and anisotropy, characterized by a decrease in the degree of anisotropy and an increase in fractural and shape factor of pores. During hydrate decomposition, methane hydrates tend to be reformed in clayey sediments before initiating the decomposition process. More significantly, the combining effects of hydrates reformation and decomposition lead to occurrence of enlarged pores in sediments, resulting in unpredictable structural failure of the sediment. Furthermore, the simulation results of solid migration in sediments revealed that the solid particles migrate towards the bottom of the sediments during hydrate formation, leading to the compaction of sediment structure. Conversely, during hydrate decomposition, the solid particles move significantly towards the upper region of the sediment due to fluid seepage through newly generated and enlarged pores. The structure change and solid migration mechanisms revealed in this study highlight the potential risks during hydrate phase transition such as sediment instability and sand production, offering valuable insights into efficient and secure hydrate exploitation.