Formation of Metastability of Pore Gas Hydrates in Frozen Sediments: Experimental Evidence

Abstract

The Arctic permafrost and zones of hydrate stability may evolve to the conditions that allow gas hydrates to remain metastable for a long time due to self-preservation within 150 m depths. The behavior of relict (metastable) gas hydrates in frozen sediments is controlled externally by pressure and temperature and internally by the properties of hydrate particles and sediments. The sensitivity of the dissociation and self-preservation of pore gas hydrates to different factors is investigated in laboratory experiments. The observations focus on time-dependent changes in methane hydrate saturation in frozen sand samples upon the pressure dropping below phase equilibrium in the gas–hydrate–ice system. The preservation of pore gas hydrates in these conditions mainly depends on the initial hydrate and ice saturation, clay contents and mineralogy, salinity, and texture of sediments, which affect the size, shape, and structure distortion of hydrate inclusions. The self-preservation mechanism works well at high initial contents of pore ice and hydrate, low salinity, relatively low percentages of clay particles, temperatures below −4 °C, and above-equilibrium pressures. Nuclear magnetic resonance (NMR) measurements reveal considerable amounts of unfrozen pore water in frozen sediments that may hold for several days after the pressure drop, which controls the dissociation and self-preservation processes. Metastable gas hydrates in frozen sand may occupy up to 25% of the pore space, and their dissociation upon permafrost thawing and pressure drops may release up to 16 m3 of methane into the atmosphere per 1 m3 of hydrate-bearing permafrost.

Migration of Salt Ions in Frozen Hydrate-Saturated Sand: Effect of Silt and Clay Particles

“…To calculate the parameters of the hydrate content of the sediment, the hydrate number (n) for methane hydrate used was 5.9. 51 The mass of hydrated water m g ( , )…”

Experimental and Simulation Study for the Dissociation Behavior of Gas Hydrates – Part I: CH4 Hydrates

“…In addition, an assumption was made that the metastable gas hydrate could exist for a long period based on field and laboratory data due to the generated ice film which covers the hydrate surface. This was introduced as self-preservation phenomena which play a crucial role in the kinetics of hydrate dissociation, so that the gas hydrate may partly convert to ice and back in a zone of recrystallization at the ice–hydrate boundary. − Considering the environmental effects, a proper understanding of CH 4 hydrate dissociation behavior with respect to climate change is urgently needed for an appropriate evaluation of the stabilities of natural gas hydrate deposits.…”

Mathematical Model of the Decomposition of Unstable Gas Hydrate Accumulations in the Cryolithozone

“…One of the clearest examples of massive methane discharge and the pockmark formation in the Barents Sea is the study [32], which substantiates the hydrate destabilization as a result of the sheet glacier destruction during the Holocene. Geodynamic factors can also play an important triggering role in the process of disturbing the metastable equilibrium of relict gas hydrates that experienced partial dissociation, which stopped as a result of the formation of ice films sealing the free gas inside gas hydrate microparticles, known as hydrate self-preservation [1][2][3]33]. Under self-preservation conditions, the metastable gas hydrates can break down when additional shear stresses occur, which may destroy thin ice films and cause free gas release, filtration and, ultimately, emission.…”