Abstract
Background: Vast amounts of untapped natural gas are stored in the form of Natural Gas Hydrates (NGHs) which are encountered in nature in marine and permafrost settings. The global volume of natural gas trapped within NGH reserves is considered to be ranging around 1000-20,000 trillion cubic meters (TCM). India in particular is estimated to have around 1894 TCM of natural gas in the form of NGH reserves and even the possibility of harnessing 10% of the total energy present as this resource may fulfill the county’s energy demand for the next 200 years.
Methods: Taking into consideration the huge energy potential of NGHs, in the present work, experimental investigations have been conducted to study the kinetics of NGH formation and dissociation in simulated gas hydrate bearing sediments. The thermal stimulation method was used to decompose NGHs; hydrate dissociation was carried out at various dissociation temperatures and injection water flow rates.
Results: For the hydrate formation experiments, higher water to hydrate conversion was observed with the unconsolidated sediment (silica sand + quartz mixture) as compared to the consolidated sediment (Berea sandstone) at the same initial water saturation. Water to hydrate conversion is also dependent on initial water saturation in the sediments. Higher water to hydrate conversion is observed in partially water saturated sediment as compared to completely water saturated sediment. The initial rate of water saturation also follows the same trend. For the hydrate dissociation experiments, it was observed that as the temperature of the injected water increases, the initial gas recovery along with its rate also increases which saturates and settles down after a certain period of time. The gas recovery (%) ratios obtained after say 10 minutes of hydrate dissociation, for the faster heating (high flow-rate) to slower heating (low flow-rate) systems were 2.08:1; 1.77:1; and 1:1 for the dissociation temperatures of 293 K, 288 K and 283 K respectively.
Conclusion: The better hydrate saturation, gas uptake and hydrate growth kinetics obtained with the unconsolidated sediment may be down to a number of factors such as better gas-water contact, sediment porosity and spatial distribution of water in sediment while the partially water saturated sediment also leads to better dispersion of gas in the sediment along with the already present water thus resulting in higher hydrate formation rate. Gas recovery rate is directly dependent upon the temperature difference ( ΔT) between the injection water and the hydrate bearing zone (at the time of start of dissociation); at the dissociation temperature of 293 K, around 97% of gas recovery was achieved for the first 30 minutes of hydrate dissociation which is way better that the 67% and 43% achieved at 288 and 283 K respectively for the same time period. During thermal stimulation, the effect of the flow rate of injected water on the kinetics of hydrate dissociation becomes increasingly pronounced as the hydrate dissociation temperature (temperature of injected water) increases.
Keywords: Natural gas hydrates, test sediments, kinetics, thermal stimulation, flow-rate, gas recovery.
Graphical Abstract
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