A method to quantitatively determine water content in unconsolidated sediments using low-field NMR

The low-field nuclear magnetic resonance (LF-NMR) determines the content of liquid water by measuring the T₂ spectrum of hydrogen.The water content was measured in unconsolidated sediments by the LF-NMR technique. The effects of particle size, mineral type and content, water salinity, temperature and gas pressure on the measured water content were investigated. Results show that the water content measured by LF-NMR was smaller than the actual value due to the surface relaxation of water in unconsolidated sediments. The correction factor C_m was introduced to correct the measured content of water. The characteristics of sediment and pore water have little influence on the measured content of water, with a relative error of <0.5% in precision of <0.20%. In contrast, temperature was negatively correlated with the measured content of water. The measured content of water increased by 10.71% when the temperature dropped from 25 to 1.7 ℃. The effect of pressure on the measured content of water depends on whether the gas contains hydrogen. The pressure change caused by the hydrogen-free gas had no effect on the measured content of water. For hydrogen gas, such as methane, the measured content of water increased linearly with the increase of pressure. The measured content of the water increased by 12.15% when the pressure increased from 0.10 to 5.05 MPa. Therefore, the effects of temperature and pressure on the measured content of water shall be considered when the change of water content is analyzed by LF-NMR during methane hydrate phase transition in sediment. The change in water content, which is monitored by LF-NMR under constant temperature and pressure, could accurately indicate the microscopic process of methane hydrate phase transition. Therefore, the method is expected to be widely used to study the microscopic kinetics of the formation and decomposition of methane hydrate.