Nuclear Magnetic Resonance Transverse Surface Relaxivity in Quartzitic Sands Containing Gas Hydrate

The proton nuclear magnetic resonance (NMR) technique is becoming increasingly popular in various assessments of physical properties in hydrate-bearing sediments, and NMR transverse relaxation time T-2 spectra are normally converted into pore size distributions by using the NMR transverse surface relaxivity rho(2). However, how the NMR rho(2) value evolves during hydrate dissociation or formation remains elusive largely due to the lack of experimental data. Thus, combined measurements of low-field NMR and X-ray computed tomography (CT) are performed to quantify the value of NMR rho(2) in quartzitic sands with different xenon hydrate saturations. Effects of hydrate saturation and pore habits on the value of NMR rho(2) are analyzed, and theoretical models are correspondingly proposed. These experimental data and theoretical models are extended to NMR-based predictions of hydraulic permeabilities and water retention curves, and suggestions for the predictions are given. Results show that the value of NMR rho(2) increases first and then decreases due to the presence of xenon hydrate in pores of quartzitic sands, and the water-xenon-hydrate interface is inferred to relax water molecules more quickly than the water-quartzitic-sand interface. The value of NMR rho(2) changes when gas hydrate is of grain-coating, pore-filling, or grain-touching habit, and predictions of hydraulic permeabilities and water retention curves based on NMR T-2 spectra need to be modified under this condition. However, patchy hydrate widespread in natural coarse-grained sediments has little effect on the value of NMR rho(2), and there is no need of significant modifications for hydraulic permeability and water retention curve predictions. This study has a great potential to further hydrate related NMR applications both in artificial and natural environments.

Automated summary

The study utilized a combination of low-field NMR and X-ray CT to quantify the NMR rho(2) values in quartzitic sands with different xenon hydrate saturations. Results showed changes in NMR rho(2) values due to the presence of xenon hydrate in pores, and theoretical models were proposed based on different pore habits.