Elastic properties of gas hydrate‐bearing sediments

Abstract

Downhole‐measured compressional- and shear‐wave velocities acquired in the Mallik 2L-38 gas hydrate research well, northwestern Canada, reveal that the dominant effect of gas hydrate on the elastic properties of gas hydrate‐bearing sediments is as a pore‐filling constituent. As opposed to high elastic velocities predicted from a cementation theory, whereby a small amount of gas hydrate in the pore space significantly increases the elastic velocities, the velocity increase from gas hydrate saturation in the sediment pore space is small. Both the effective medium theory and a weighted equation predict a slight increase of velocities from gas hydrate concentration, similar to the field‐observed velocities; however, the weighted equation more accurately describes the compressional- and shear‐wave velocities of gas hydrate‐bearing sediments. A decrease of Poisson’s ratio with an increase in the gas hydrate concentration is similar to a decrease of Poisson’s ratio with a decrease in the sediment porosity. Poisson’s ratios greater than 0.33 for gas hydrate‐bearing sediments imply the unconsolidated nature of gas hydrate‐bearing sediments at this well site. The seismic characteristics of gas hydrate‐bearing sediments at this site can be used to compare and evaluate other gas hydrate‐bearing sediments in the Arctic.