Acoustic characteristics and micro-distribution prediction during hydrate dissociation in sediments from the South China Sea

Different micro-distribution patterns in gas hydrates may produce different acoustic effects on the deposition medium. The influence of hydrate distributions on the reservoir and the acoustic response of the hydrate during the dissociation process are very important. In this paper, ultrasonic velocity and hydrate saturation were measured by ultrasonic measurement and time domain reflection (TDR) techniques, respectively. The relationship between the wave velocities and saturation of the hydrate along with X-ray computed tomography (XCT) scanning results were used to analyze hydrate distribution patterns and their acoustic response characteristics. The results show that the rate of decrease in hydrate saturation in the dissociation stage remains basically unchanged, and the hydrate dissociation rate remains constant. During the process of hydrate dissociation, the hydrate first disrupts the stable sedimentary skeleton structure, and then breaks down at the contacts with sediment particles. Then, the hydrate gradually decomposes into the pore space away from the particle contacts and the hydrate is mainly present in suspended mode. Finally, hydrate decomposes entirely and free bubbles are in contact with the hydrate. In the early stage of hydrate dissociation, compressional and shear wave velocities decreased rapidly by 453 m/s and 307 m/s, respectively. In the later stage of hydrate dissociation, compressional and shear wave velocities decreased by 271 m/s and 119 m/s, respectively, with smaller decreases in wave velocities than in the early hydrate dissociation stage. The results show that the measured changes in P-wave velocity are consistent with results obtained from a site in the Shenhu area. The micro-distribution modes during the hydrate dissociation process were inferred. M the initial stage of dissociation, the hydrate decomposed at the contacts with sediment particles, whereas the hydrate was mainly suspended in the pores in the later stage.