Sound velocity measurement methods for porous sandstone. Measurements,   finite element modelling, and diffraction correction
  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. 
Mathias S{\ae}ther, Per Lunde, Geir Ersland
physics.ins-det physics.geo-ph