Ultrasonic Wave Velocities Associated with Formation and Dissociation of Methane Hydrate in Artificial Sandy Sediment

摘要

To determine whether ultrasonic wave velocities are affected by the occurrence and amount of methane hydrate in the pore spaces of sediments, we measured ultrasonic velocities in artificial methane hydrate-bearing sandy sediments during the formation and dissociation processes of methane hydrate. An artificial core specimen was made based on the properties of a natural core recovered from a site in the Nankai Trough near Japan. A core holder was used that permitted the independent application of both confining and pore pressures to the artificial core using a rubber sleeve and two syringe pumps. Piezoelectric ceramic sensors were positioned at both end faces of the core and used to oscillate and detect the compressional and shear ultrasonic waves. The formation of methane hydrate, by pressurizing water in the pore spaces of the core sample using methane gas, was associated with an increase in both wave velocities. The increase in ultrasonic velocities was particularly marked at saturations of methane hydrate in the pore spaces above 30% and the maximum velocity of the compressional wave exceeded 3,000 m/s. While ultrasonic wave velocities decreased with the dissociation of methane hydrate due to depressurization or heating, they exhibit hysteresis during formation and dissociation. These findings indicate that the velocities depend on the occurrence of methane hydrate in pore spaces as well as the concentration.