Experimental study on characteristics of pore water conversion during methane hydrates formation in unsaturated sand

Understanding the pore water conversion characteristics during hydrate formation in porous media is important to study the accumulation mechanism of marine gas hydrate. In this study, low-field NMR was used to study the pore water conversion characteristics during methane hydrate formation in unsaturated sand samples. Results show that the signal intensity of T₂ distribution isn’t affected by sediment type and pore pressure, but is affected by temperature. The increase in the pressure of hydrogen-containing gas can cause the increase in the signal intensity of T₂ distribution. The heterogeneity of pore structure is aggravated due to the hydrate formation in porous media. The water conversion rate fluctuates during the hydrate formation. The sand size affects the water conversion ratio and rate by affecting the specific surface of sand in unsaturated porous media. For the fine sand sample, the large specific surface causes a large gas-water contact area resulting in a higher water conversion rate, but causes a large water-sand contact area resulting in a low water conversion ratio (C_w=96.2%). The clay can reduce the water conversion rate and ratio, especially montmorillonite (C_w=95.8%). The crystal layer of montmorillonite affects the pore water conversion characteristics by hindering the conversion of interlayer water.©2022 China Geology Editorial Office.