PNNL

Abstract

Resonant ultrasound spectra were collected on natural geological core samples containing known amounts of water while under pressure with methane gas and cooled to sub-ambient temperatures such that methane hydrate formed in the pore space. Strong resonance peaks were observed using either compressional or shear mode transducers but only when gas hydrates were present. By using deuterated methane gas to form gas hydrate in a core sample obtained from the Mallik 5L-38 gas hydrate research well, resonance peak amplitude was conclusively shown to correlate with gas hydrate saturation. A pore water freezing model was developed that utilizes the known pore size distribution in a sample and pore water chemistry to predict gas hydrate saturations as a function of pressure and temperature. The model showed good agreement with the experimental measurements and demonstrated that pore water chemistry is the most important factor controlling equilibrium gas hydrate saturations in these sediments when gas hydrates are formed artificially in laboratory pressure vessels. With further development, the resonant ultrasound technique can provide a rapid, non-destructive, and field portable means of measuring the equilibrium P-T properties and dissociation kinetics of gas hydrates in porous media, determining gas hydrate saturations, and may provide new insights into the nature of gas hydrate formation mechanisms in geological materials.