Estimation of Gas Hydrate Saturation Regarding the Hydrate Morphology in Hydrate-Bearing Sands in the Qiongdongnan Basin, South China Sea

• Published in: Pure and applied geophysics 2023-07, Vol.180 (7), p.2757-2773
• Main Authors: Wei, Deng, Jinqiang, Liang, Zenggui, Kuang, Yingfeng, Xie, Pin, Yan
• Format: Article
• Language: English
• Subjects: Bound water; Cementation; Cementing; Channel pores; Core analysis; Core sampling; Coring; Degassing; Earth and Environmental Science; Earth Sciences; Electrical resistivity; Estimation; Expeditions; Fluids; Gas hydrates; Geomechanics; Geophysics/Geodesy; Grain size; Hydrates; Logging; Morphology; P waves; Permeability; Quaternary; Sand; Saturation; Seismic wave velocities; Velocity; Wave velocity; Well logs
• ISSN: 0033-4553; 1420-9136
• DOI: 10.1007/s00024-023-03299-7

The GMGS6 hydrate expedition discovered hydrates in the Quaternary channel sand by logging and coring for the first time in the Qiongdongnan Basin, with an average sand grain size of ~ 70 μm and sandy content over 80%. Hydrate saturation exceeds 80%, and the P-wave velocity exceeds 3500 m/s. The quantitative link between hydrate saturation and velocity is not clear due to the complex hydrate morphologies. In this study, we analyzed the possible hydrate morphologies regarding the variation in velocity, resistivity, and permeability versus hydrate saturation based on in situ permeability measurements, pressure core degassing testing, and well logs, which indicates that both cementing and frame-supporting behaviors occur within the pores. Using the cementing model alone underestimates saturation, while the frame-supporting model overestimates saturation. As a result, the cementation model and three-phase Biot equation are jointly used to quantitatively invert the hydrate saturation based on the least-squares principle. The inverted results agree with the saturation from pressure cores and resistivity, and indicate that the prevalence of cementing hydrates greatly increases the velocity and strength of the hydrate-bearing sands, which is consistent with laboratory observations of the “gas excess” scene from previous studies. With the continuous supply of gas-bearing fluids, more hydrates cement sediment particles and previously formed hydrates, occupying most of the pore space and consuming the free and bound water in the pore space. This study thus provides a new method for estimating the saturation in hydrate-bearing sands, which is also important for estimating the geomechanical behavior of hydrate-bearing sands.