Effects of hydrate occurrence habits on the acoustic propagation of hydrate-bearing sediment

This study investigates the attenuation mechanisms of elastic waves in hydrate-bearing sediments (HBS) using a transmission method that accounts for wave impedance coupling, which is frequently neglected in conventional approaches. Our findings demonstrate that the presence of hydrate increases the acoustic impedance of HBS, thereby reducing the impedance mismatch between HBS and the transducer. This phenomenon weakens the coupling effect and enhances wave energy transmission, providing a plausible explanation for the discrepancies observed between the attenuation measurements in artificial HBS and those obtained from field sonic logging. The determined dissipation factors agree well with field logging data, confirming the reliability of our methodology. We further examined the acoustic characteristics of tetrahydrofuran (THF) HBS with different hydrate occurrence habits. The results reveal that pore-filling hydrates induce stronger attenuation than cementing hydrates, consistent with both previous laboratory and field observations. Comparison with field data reveals that the Shenhu HBS exhibits velocity characteristics similar to water-saturated HBS, but with less dissipation, suggesting a predominance of pore-filling hydrate structures. In contrast, the Mallik HBS resembles gas-saturated HBS in velocity but exhibits greater dissipation, implying the dominance of cementing hydrates. These interpretations are further supported by core and resistivity measurements. To elucidate these observations, a conceptual model based on viscous flow mechanisms is proposed. Pore-filling hydrates may compartmentalize fluid pathways and facilitate squirt flow, thereby enhancing viscous energy dissipation. Additionally, microscopic pores within large hydrate particles may allow submicro squirt flow, further enhancing energy attenuation.