Influence of heterogeneous hydrate distribution on the compressional wave velocity of hydrate-bearing sediment specimens

This paper reports the results of a series of compressional wave velocity tests on specimens in which gas hydrate in the pores are substituted for iced pure water. In these specimens, ice is heterogeneously distributed by separating the ice-bearing sample into high- and low-saturation segments to simulate hydrate accumulation on one side of the specimen during synthesis of the hydrate-bearing sediment samples in laboratory. The length of high- and low-saturation segment was adjusted to simulate the different degrees of local accumulation of the synthesized hydrate-bearing sediment samples. The calculated saturation of the ice-bearing specimen from 15% to 40% with 5% saturation gradient was studied by the direct and indirect measurement methods. Ultrasonic measurements were performed to determine the compressional wave velocities, and results show that the heterogeneous distribution of ice in porous specimen remarkably influences the compressional wave velocity of the ice-bearing sample, especially for low-saturation samples. The compressional wave velocity increases with the increasing of the length of high saturation segment, and the maximum difference of the compressional wave velocity is up to 0.247 km/s (direct measurement) and 0.199 km/s (indirect measurement) at 30% calculated-saturation. This difference causes >20% uncertainty in the saturation of the ice-bearing sample using the Cementation Theory Model. Thus, the heterogeneous distribution of hydrate leads to a considerable measurement error for the compressional wave velocity of hydrate-bearing sediment sample. A larger error could occur during the estimation of hydrate saturation in the natural gas hydrate-bearing stratum, when the corresponding model of the hydrate saturation and compressional wave velocity corrected by the laboratory data are used to interpret the sonic logging data and seismic data.