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

摘要

An experimental apparatus was developed to measure P-wave velocity (V_P) of gas-hydrate-bearing sediment. Tetrahydrofuran (THF) 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.