Review of rock-physics studies on natural gas hydrate-bearing sediment attenuation

Attenuation is an essential reservoir property, and understanding its mechanism in gas hydrate-bearing sediments is important for predicting gas hydrate saturation. Natural gas hydrates mainly accumulate in coarse-grained sands or fine-grained clays with different morphologies, and the attenuation characteristics of these gas hydrate-bearing sediments are inconsistent. Many rock-physics models have been proposed in recent years to elucidate the attenuation mechanisms of gas hydrate occurrence. There are two types of attenuation models for gas hydrates in sands. One of these models is based on the three-phase Biot theory and contains multiple mechanisms that describe the contact effects between hydrate and sediment grains, such as grain cementation and squirt flows caused by microcracks. This model can reasonably reproduce the enhanced attenuation observed with increasing hydrate saturation in sonic-logging data. Attenuation described in the effective grain model is dominated by the viscous flow between the water in hydrate pores and free water. These types of models agree with the seismic attenuation observed in gas hydrates in sands. Moreover, the attenuation model of gas hydrates in clay is formulated by first establishing an attenuation model for background sediments, which employs the effective grain model to characterize attenuation in clay minerals; then, the properties of pure gas hydrate and the effects of hydrate occurrence on sediments are quantified. This model has also been successfully applied to seismic reflection data in the seismic frequency band. The above models have achieved significant progress in attenuation studies on gas hydrate-bearing sediments, and they have aided the quantitative interpretation of observed attenuation in field data and attenuation-related constraints for gas hydrate saturation. However, the application of these models is limited to certain extents: the attenuation predicted by the gas hydrate-bearing sands model failed to match that observed in the field data. Additionally, the gas hydrate-bearing clays model does not consider fracture shapes, and its performance has not been verified at the ultrasonic frequency band. To improve the accuracy and applicability of these models, the relationship between hydrate morphology and attenuation must be elucidated, and further rock-physics studies based on field data must be conducted.