A testing assembly for combination measurements on gas hydrate-bearing sediments using x-ray computed tomography and low-field nuclear magnetic resonance.

Abstract

Studying the pore-scale characteristics of gas hydrate-bearing sediments (GHBS) is very important for a deep understanding of (i) how fluid flows therein and (ii) the corresponding gas production. Micro X-ray computed tomography (X-CT) and low-field nuclear magnetic resonance (NMR) are often used independently to characterize the pore structure of GHBS. Here, we present a new testing assembly that combines X-CT scans and low-field NMR tests to determine the pore-scale characteristics of GHBS in situ. The main parts of the testing assembly are a removable core holder made of polyether ether ketone, an X-CT system, and a low-field NMR system. The core holder allows for independent pressure control for the formation/dissociation of gas hydrates, which is xenon hydrate here. X-CT scans and low-field NMR tests are conducted successively to obtain not only the hydrate pore-scale behavior but also the transverse relaxation time distributions of GHBS. Correlation analysis between the pore size distributions and the transverse relaxation time curves gives the transverse surface relaxivity of xenon hydrate-bearing sediments during hydrate dissociation. The results show that the hydrate pore occurs as a mixture of grain-coating, cementing, pore-filling, and patchy clusters in a gas-dissolved solution. The peak pore size at the maximum frequency ratio increases with decreasing hydrate saturation. In addition, the transverse surface relaxivity dependence on hydrate pore occurrences is in the range of 67.1-129.3 µm/s when the hydrate saturation is lower than 0.4. The combination measurements for GHBS have a promising potential in understanding the structure evaluation of pore space during gas recovery.