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Gas hydrates are crystalline ice-like solids that form when water and sufficient quantities of certain gases, relatively small in molecular size, are combined under the right conditions of temperature and pressure. Under these conditions, the amount of gas stored in a given volume of hydrate is 170 times higher than when the gas is at standard conditions. This fact contributes to its potential as an economically recoverable energy source.Substantial amounts of hydrates have been found in the earth's sediment beneath the permafrost, in Arctic basins, and in Ocean bottom sediments along the continental margins of the United States. Due to this potential resource, the problems associated with the production of natural gas from these hydrate zones have become of greater interest to the hydrocarbon industry. Preliminary simulation studies for the production of gas from hydrate reservoirs using thermal injection, depressurization, and in-situ combustion have been done by McGuire (1981), Holder et al. (1982, and others. However, one area which is important but has received little attention is the rates of heat and mass transfer during hydrate dissociation. Kamath et al. (1 984) viewed hydrate dissociation as a nucleate boiling phenomenon. Selim and Sloan (1 985) viewed heat transfer during hydrate dissociation as a moving-boundary ablation process and showed that their model fits hydrate dissociation data to within 10% (Ullerich et al., 1987). Results of these studies are only useful in modeling pure hydrate dissociation processes.For gas hydrates in the earth's sediment, the rates of heat and mass transfer are strongly influenced by the presence of the surrounding sediment. In the present work, a physical model that describes hydrate dissociation under thermal stimulation in porous media, is presented. The model views the dissociation as a process whereby gas and water are produced at a moving boundary. The latter separates the dissociated zone, which contains gas and water, from the undissociated zone, which contains the hydrate.Consider, for instance, a uniform distribution of hydrates in a porous medium which is initially a t a uniform temperature, T,, and occupies the semi-infinite region, 0 < x < m. Initially, the hydrates are assumed to fill the entire pore volume. At time t = 0, the temperature a t the boundary, x = 0, is raised to a new temperature, To, which is higher than T,, and is held constant thereafter. Hydrate dissociation commences and, as a result, there is a moving interface a t some distance, x = X(,f), which separates the water/gas region from the undissociatecl hydrate region. Thus a t any time t > 0, the water/gas phase (dissociated hydrate zone) occupies the region, 0 < x < X ( t ) , while the undissociated hydrate zone occupies the region, X ( t ) . : x < m; these regions are designated as I and 11, respectively.The water resulting from the dissociation process is assumed to remain motionless and is retained within the pores of the dissociated zone. This assumption restricts the analysis to hy...

An analytical model that describes hydrate dissociation under thermal stimulation in porous media is presented. The model views the dissociation as a process in which gas and water are produced at a moving dissociation boundary. The boundary separates the dissociated zone containing gas and water from the undissociated zone containing the hydrate. A similarity solution to the conservation equations is derived, and results are presented in graphical forms that are useful in numerical computations. In particular, heat fluxes, temperature profiles, and gas pressure distributions are presented for two cases that simulate saturated-steam and hot-water thermal stimulation. A parametric study showed that the dissociation rate is a strong function of the thermal properties of the system and the porosity of the porous medium. The energy efficiency of the dissociation process, defined as the ratio of the heating value of the gas produced relative to the heat input, was also computed. For hydrate thermal stimulation, an energy efficiency value of about nine was found, which appears encouraging for natural gas production from hydrate.

in Wiley InterScience (www.interscience.wiley.com).It was shown that flowline hydrate plugs dissociate radially not horizontally; this has significant implications for the remediation of a hydrate blockage, a major flow assurance problem. Over a decade of measurements has enabled models to estimate the dissociation time for hydrate plugs in flowlines, using radial heat transfer with two moving boundaries. Three different plug dissociation scenarios were modeled: singlesided depressurization, two-sided depressurization, and dissociation by radial electrical heating. The models were able to replicate the experimental observations with no fitted parameters. Structure I hydrate was found to dissociate faster than structure II; this was attributed to the different latent heats between the structures. These results indicate that hydrate dissociation in these systems is

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