Natural gas hydrate dissolution accelerated by water flowing: Kinetics and implications

Abstract

Natural gas hydrate (NGH) dissolution may play a critical role in geological evolution and methane-induced climate change. However, the characteristics of hydrate dissolution in sediments remain poorly constrained. In this work, methane hydrate dissolution kinetics was quantitatively investigated by continuously passing methane-free water through hydrate-bearing sediments (HBS). The results showed that the methane release process can be divided into rapid and decayed stages. The hydrate dissolution process can be significantly affected by heterogeneity in the NGH distribution and dynamic variation in the flow channels. The average hydrate dissolution rate and methane flux were calculated based on the mass balance principle. With an increase in the water flux from 2.0 mL/cm2/min to 4.1 mL/cm2/min, the hydrate dissolution rate increased from 40.9 m/yr to 103.1 m/yr, and the methane flux increased from 23.3 mol/cm2/yr to 59.0 mol/cm2/yr. In comparison, the hydrate dissolution rate is less sensitive to variation in the hydrate saturation in the range of 12.8%–28.8%, with corresponding hydrate dissolution rate values ranging from 96.6 m/yr to 114.8 m/yr. The corresponding methane flux is in the range of 55.2–65.6 mol/cm2/yr. The NGH dissolution associated with the undersaturated water flowing through HBS is at least one order of magnitude faster than that in other reported hydrate dissolution environments (0.03–150 cm/yr). The rapid dissolution of local hydrate may contribute to geological evolution and methane release at hydrate sites with active seepage activity.