Pore-Scale Visualization of CH₄ Gas Hydrate Dissociation under Permafrost Conditions

Abstract

In this study, we demonstrate the effectiveness of combined production technique involving depressurization and thermal stimulation for gas production from CH4 gas hydrates in subzero temperature range between -3℃ to 0℃. CH4 gas hydrate phase transitions during formation, depressurization, re-formation, self -preservation, thermal stimulation stage were visualized using a high-pressure, water-wet, silicon-wafer micromodel with pore network of actual sandstone rock.A set of eight experiments were performed in which CH4 gas hydrate was formed at a constant pressure between 60 -85 bar and constant temperature between 0 °C -4°C. CH4 gas hydrate was then dissociated at constant system temperature between -3 °C to -2 °C by pressure depletion to study the effect of hydrate and fluid saturation on dissociation rate, self-preservation, and risk of ice formation. The dissociation rate and behaviour were heavily affected by the total hydrate saturation and initial hydrate distribution in the pore space. Additionally, the amount of produced CH4 gas was limited below 0 °C due to the rapid formation of ice from the liquid water that was liberated from the initial hydrate dissociation. The liberated CH4 gas was therefore immobilized and trapped by the formed ice and could not be produced without thermal stimulation. Thermal stimulation removed the blockage of pore space caused by ice and secondary hydrate formation and enhanced gas production. Visual observation showed self-preserved hydrates in metastable state dissociated before ice below subzero temperature providing experimental evidence of recently discovered methane leaking from gas hydrate deposits due to global warming. The results highlight the influence of heterogeneity in hydrate distribution and total saturation on the hydrate dissociation behaviour below 0 ℃ temperature. Micromodel observation provides direct insights into hydrate dissociation, self-preservation, fluid migration, gas coalescence, ice and secondary hydrate formation at pore scale below subzero temperature.

Chemically Influenced Self-Preservation Kinetics of CH4 Hydrates below the Sub-Zero Temperature

“…The hydrate morphology controls the total hydrate surface area, while the pore size distribution controls the hydrate stability. The pore-scale visualization studies have shown that hydrate remained stable in narrow pore throats outside of their stability zone [6,66]. In another study, the correlation of the size of the hydrate particles with self-preservation was investigated and the particle particles larger than 0.5 mm in diameter showed a high degree of self-preservation [67].…”

“…Energies 2021, 14, 6765 2 of 28 CH 4 recovery from hydrate reservoirs [4][5][6][7][8] and CH 4 -CO 2 replacement [9]. Information on rapid formation, high storage capacity, and dissociation behavior is essential for the success of these technologies.…”

“…Laboratory studies show that hydrate formation begins at the gas-liquid interface and that the hydrate formation process is probabilistic, with hydrate growth slowing over time because of the hydrate film-based gas-liquid mass transfer barrier. Mechanical techniques, such as agitation [6] and spraying through the nozzle [10], improve the formation and growth process, but increase power consumption and maintenance complexity. Chemical enhancement involves the application of chemicals, including thermodynamic promoters [11] and kinetic promoters [12,13], to enhance mass transfer under moderate pressure and temperature conditions without mechanical agitation.…”

“…Self-preservation is said to depend on guest molecules, co-guest molecules, additives, and grain size of hydrates [39]. Previous studies show that individual self-preservation is observed at temperatures between 240 K and 272 K and is present due to hydrate metastability, additional kinetic barrier, ice layer formation, and secondary hydrate layer [6,40].…”

Combining Optical Microscopy and X-ray Computed Tomography Reveals Novel Morphologies and Growth Processes of Methane Hydrate in Sand Pores

“…The presence of additives and substrate wettability also play a role in hydrate growth [38][39][40]. The most complex configurations observed so far by optical microscopy are those obtained in etched twodimensional silica glass or silicon wafer micromodels [50][51][52] or by using lab-on-a-chip technology [53][54][55].…”