  Acoustic material parameters of gas hydrate bearing porous rocks are
important for evaluation of methods to exploit the vast methane gas resources
present in the earth's subsurface, potentially combined with CO2 injection. A
solid buffer method for measuring changes of the compressional wave velocity in
porous rocks with changing methane hydrate contents under high-pressure
hydrate-forming conditions, is tested and evaluated with respect to effects
influencing on the measurement accuracy. The limited space available in the
pressure chamber represents a challenge for the measurement method. Several
effects affect the measured compressional wave velocity, such as interference
from sidewall reflections, diffraction effects, the amount of torque (force)
used to achieve acoustic coupling, and water draining of the watersaturated
rock specimen. Test measurements using the solid buffer method in the pressure
chamber at atmospheric conditions are compared to independent measurements
using a water-bath immersion measurement method. Compressional wave velocity
measurements have been done in the steady state region at frequency 500 kHz for
various specimen made of plexiglas and Bentheim sandstone. Finite element
simulations of the solid buffer measurement method with plexiglas specimen have
been used for comparison with the measurements, and to aid in the design,
control, and evaluation of the measurement method and results. Highly favorable
agreement between the two measurement methods has been obtained, also with
respect to repeatability and reproducibility. The results indicate that the
solid buffer method may be suitable for use in the pressure chamber with
Bentheim sandstone and changing methane hydrate contents under high-pressure
hydrate-forming conditions, for quantitative measurements of the compressional
wave velocity in such rock core samples at these frequencies.
