Comparison of methane mass balance and X-ray computed tomographic methods for calculation of gas hydrate content of pressure cores

Abstract

Gas hydrate saturation was calculated for twelve pressure cores taken during National Gas Hydrate Program (NGHP) Expedition 01 in the Krishna–Godavari Basin, Bay of Bengal, at a location where thin gas hydrate veins were common. One of two methods were used to calculate gas hydrate saturation for each core: methane mass balance after depressurization and gas collection, considered the “gold standard” for measurement of gas hydrate saturation; or voxel intensity analysis (rather than binary segmentation) of X-ray computed tomographic (CT) reconstructions. Gas hydrate saturation in cores measured by methane mass balance was calculated to be 17.8%, 10.9%, 11.9%, 13.6%, 9.5%, 1.4%, and 0% by percent of core volume. Gas hydrate saturation in similar cores measured by intensity analysis of CT reconstructions was 17.2%, 9.1%, 6.7%, 7.8%, and 3.1% by percent of core volume. This quantitative CT intensity analysis contained systematic errors and therefore the calculated hydrate saturations are lower bounds. The systematic errors can be removed from the quantitative CT analysis by converting the CT intensities to real densities, though this was not possible for this study. All pressure core gas hydrate saturations were similar in magnitude to each other as well as to independent estimates of gas hydrate saturation from porewater freshening, and all variations in saturation could be explained by natural variation between samples. CT intensity (or preferably density) analysis of pressure cores showed promise for calculation of the saturation of vein hydrate in natural samples, allowing pressure cores to be used for further analyses under pressure after hydrate quantification. Theoretical examination of CT density analysis showed that this method would be unable to detect pore-filling gas hydrate; judicious examination of the results from CT density analysis versus other hydrate quantification methods on the same samples might allow quantification of pore-filling hydrate.