Experimental study on 2-D acoustic characteristics and hydrate distribution in sand

Summary An experimental system was developed to measure the acoustic velocity of hydrate-bearing sands and to infer their 2-D velocity structure and hydrate content during hydrate formation. For this purpose, sands of two different grain sizes were chosen and arranged in alternating layers in a pressure vessel before saturating them with a solution of dissolved methane gas in a sodium dodecyl sulphate solution. During cooling and subsequent hydrate formation, acoustic velocities were measured with ultrasonic probes along the vessel wall. Hydrate formation was measured by time domain reflectometry. A straight ray-tracing method and an iterative algorithm based on the simultaneous iterative reconstruction technique algorithm were used to perform forward modelling and inversion of the ultrasonic tomography. The tomography results were used to obtain acoustic velocity profile distribution images of hydrate formation in loose sediments. The results show that the acoustic velocity in each layer increases rapidly when the hydrate saturation is less than 20 per cent. In contrast, the acoustic velocity increases slowly when the hydrate saturation is greater than 20 per cent. The effective medium theory was used to describe the changes of velocities with hydrate saturation. The empirical formula of P and S-wave velocity in hydrate-bearing sediments as well as the correlation between the wave velocity ratio and the hydrate saturation are also obtained. In the first stage of hydrate formation, the acoustic velocity is larger in the coarse sediment than in the fine sediment. At the end of hydrate formation, the acoustic velocities in different layers differ little and the hydrates are nearly homogeneously distributed in the reservoir. The 2-D velocity structure and inferred hydrate distribution indicate that in the longitudinal direction, the hydrate preferentially forms close to the gas source, thus the acoustic velocity is large in this area. In the transverse direction, the hydrate preferentially forms around the reactor wall, and so the acoustic velocities are larger than those in the internal sediments. The experimental results were used to infer how and where gas hydrate is formed over time and over space in the experimental setup.