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Theoretical and experimental investigations into the formation and accumulation of gas hydrates Rempel, Alan


The substantial volumes of gas hydrates found in the Arctic and in marine sediments are both a possible source of global climate change, and a potential future energy resource. The rate at which a hydrate layer forms, and the spatial distribution of hydrate in the layer are controlled by the physical conditions of the formation environment. To better understand the physical conditions that affect hydrate layer characteristics, I present a quantitative model for the formation of hydrates in a porous medium. The theory is tested using the the results of laboratory simulations of the modelled conditions. Conservation principles are used to derive the full set of governing equations using the minimum number of assumptions and simplifications. Scaling arguments, based on estimates of physical parameters in marine sediments, show that both heat and mass transport are dominated by diffusive processes, so advection may be neglected in most formation environments. Analytical solutions to the leading-order set of equations are obtained for the case of a porous half-space cooled on its boundary. These solutions provide estimates of the growth-rate of a hydrate layer and the volume fraction of hydrate present. The model predicts that the layer grows on the thermal diffusion timescale with the phase-change interface moving at a rate which is proportional to the square root of time. The predicted hydrate volume fraction is determined by the rate at which compositional diffusion can supply gas to the moving interface. For the formation of a methane hydrate layer, the model generally predicts a hydrate volume fraction that is less than 1%. The modelled conditions are simulated in a reaction chamber constructed from a cast acrylic tube, 0.7 m in length, with an inner diameter of 0.14 m. To test the apparatus, experiments were conducted in which the growth-rate of an ice layer in the sand-filled reaction chamber was monitored using RTD temperature probes. These experiments demonstrate that a simplified version of the hydrate formation model describes the formation of an ice layer in a porous medium. CO₂ was used as the hydrate former to test the predictions for the growth-rate of a hydrate layer and the layer's hydrate content. CO₂ has a high solubility in water, and the model predicts a much greater hydrate volume fraction for a CO₂ hydrate layer than that for a low solubility gas such as methane. During hydrate formation experiments, the temperature probes were unsuccessful at detecting the position of the hydrate phase-change interface. At the end of some experiments, the pressure was reduced to dissociate the hydrate so that the presence of hydrate might be detected. However, the temperature probes failed to definitively identify the associated consumption of latent heat. This is despite the recovery of an ice Thydrate mixture following one such experiment. Additional instrumentation employing acoustic or electrical techniques may be necessary to quantitatively assess the model predictions.

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