Abstract
Abstract Methane hydrate is a promising energy resource in the near future. One of the processes of its development in the sub-seabed sediments is depressurisation: pumping-up water through a well depressurises down to the extent lower than the equilibrium, and hydrate dissociates. However, hydrate dissociation is endothermic and temperature may become lower than the equilibrium point near a well and, hence, hydrate reforms. This may cause blockage of gas-water flow in the sediment. In this study, heat and mass transfer in water-gas-sand three-phases was analysed numerically by modelling them using a threedimensional lattice Boltzmann method. Sand grains were assumed to be spherical for simplicity and the dissolution of methane into water through the interface was also analysed. At the same time, volumetric rate of hydrate formation was obtained by comparing the model results with those of measurement. Introduction Methane hydrate is an ice-like crystal that consists of water as host cage and gas as guest molecules, and exists widely in the sub-seabed sediments and in the permafrost layer, which are under low temperature and high pressure. It is confirmed that methane hydrate contains methane more than that in the form of gas on the earth and has been paid attention as a new energy resource in recent years. To predict the technical and economical feasibilities of its production process, numerical simulations are necessary. One of the processes of its development in the sub-seabed sediments is depressurisation: pumping-up water through a well depressurises down to the extent lower than the equilibrium, and hydrate dissociates. However, hydrate dissociation is endothermic and temperature may become lower than the equilibrium point near a well and, hence, hydrate reforms. This may cause blockage of gas-water flow in the sediment. Therefore, formation of gas hydrate in the sediment should be understood and modelled numerically for the design of its production system. For modelling hydrate formation, its rate is particularly important. Therefore, it is necessary to estimate the formation of hydrate and the consequent reduction of permeability accurately to predict the production rate of methane gas. In this study, a small high-pressure cell was filled with fine glass beads, methane gas, and small amount of water, and gas hydrate is formed in it by increasing pressure. The same was simulated by a numerical model. As a result, by comparing the both results, a hydrate formation rate was extracted.
Published Version
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