Influences of stress state on compressional wave velocity of sandy hydrate-bearing sediment: Experiments and modeling

Marine natural gas hydrate (NGH) reservoir usually undergoes three-dimensional stress. The geophysical model that ignores the influences of stress might cause discrepancies between the model-based results and observed data during reservoir characterization and inverse modeling. In this study, we carried out systematic acoustic measurements on sandy hydrate-bearing sediment (HBS) under various stress states. The results revealed that the growth rate of the observed compressional velocity decays continuously with increasing hydrate saturation, which suggests that hydrate morphology is in the contact-cementing mode under the excess gas method. The confining/pore pressure correlations of the compressional velocity vary with hydrate saturation. The compressional velocity for the HBS is suppressed by increasing pore pressure. The compressional velocity increases with increasing confining pressure at under low hydrate saturation (i.e., ≤30 %), and conversely, decreases with increasing confining pressure when hydrate saturation exceeds 30 %. It is anticipated that the influence of pore/confining pressure on the compressional velocity shall be attributed to the pore structurale changes and hydrate morphology transitions. To be more specific, the pore structure variation plays a dominant role when hydrate saturation is relatively low, whereas the hydrate morphology transition plays a dominant role once the hydrate saturation exceeds ∼30 %. Finally, a compressional velocity model for sandy HBS was proposed, which considers the influence of stress states. And comparing the model-based results with observed data, the fitted correlation coefficient R2 are all greater than 0.9, and the deviation range is only 0.25–0.59 % at various saturation. It implies that the model has a high accuracy and is able to describe the acoustic response of the HBS under multiple factors, including hydrate saturation, pore pressure, and confining pressure. The results might have some significance on the prediction of NGH reserves in sandy reservoirs.