Effect of Methane Gas on Acoustic Characteristics of Hydrate-Bearing Sediment-Model Analysis and Experimental Verification

Abstract

Gas leakage is an important consideration in natural systems that experience gas hydrate accumulation. A number of velocity models have been created to study hydrate-bearing sediments, including the BGTL theory, the weighted equation, the Wood equation, the K-T equation, and the effective medium theory. In previous work, we regarded water as the pore fluid, which meant its density and bulk modulus values were those of water. This approach ignores the presence of gas, which results in a biased calculation of the pore fluid’s bulk modulus and density. To take into account the effect of gas on the elastic wave velocity, it is necessary to recalculate the bulk modulus and density of an equivalent medium. Thus, a high-pressure reactor device for simulating leakage systems was developed to establish the relationship between wave velocity and hydrate saturation in methane-flux mode. A comparison of the values calculated by the velocity model with the experimental data obtained in this study indicates that the effective medium theory (EMT, which considers gas effects) is more applicable than other models. For hydrate saturations of 10%–30%, the result ranges between EMT-B (homogenous gas distribution) and EMT-B (patchy gas distribution). For hydrate saturations of 30%–60%, the results are similar to those of the EMT-B (homogenous gas distribution) mode, whereas hydrate saturations of 60%–70% yield results similar to those of the EMT-A mode. For hydrate saturations greater than 80%, the experimental results are similar to those of the EMT-B mode. These results have significance for hydrate exploitation in the South China Sea.