Laboratory Measurements of the Effects of Methane/Tetrahydrofuran Concentration and Grain Size on the P-Wave Velocity of Hydrate-Bearing Sand

An experimental apparatus was developed to measure P-wave velocity (V-p) of gas-hydrate-bearing sediment. Tetrahydrofuran (TIE-IF) was added to quicken the hydrate formation in the porous media and to synthesize hydrate-bearing sediments with uniform distribution. Methane acted as a free gas to participate in the hydrate formation. Five experimental runs were performed to examine the influence of sediment grain size and THF concentration on V-p. The P-wave velocity and the amplitude for the first arrival wave signal were collected in real time during hydrate formation process. The experimental data showed that V-p increases monotonically with the increase of hydrate saturation in the sediment pore space and finally tends to be a constant value. This final V-p value increases with the increase of initial THF content, but the effect of sand grain size on V-p is inconclusive. The variations of amplitude for the first arrival wave signal with elapsed time during hydrate formation illustrates that the amplitude increases with the increase of hydrate saturation until it attains a maximum value and then decreases gradually due to the effect of free methane gas penetrating into the hydrate-bearing sediment. The acoustic velocity of THF-hydrate filled sediment was also predicted based on the extended contact cement theory. The predicted results were close to the experimental data obtained in this work