Since huge amounts of CH 4 are bound in natural gas hydrates occurring at active and passive continental margins and in permafrost regions, the production of natural gas from hydrate-bearing sediments has become of more and more interest. Three different methods to destabilize hydrates and release the CH 4 gas are discussed in principle: thermal stimulation, depressurization and chemical stimulation. This study focusses on the thermal stimulation using a counter-current heat-exchange reactor for the in situ combustion of CH 4 . The principle of in situ combustion as a method for thermal stimulation of hydrate bearing sediments has been introduced and discussed earlier [1,2]. In this study we present the first results of several tests performed in a pilot plant scale using a counter-current heat-exchange reactor. The heat of the flameless, catalytic oxidation of CH 4 was used for the decomposition of hydrates in sand within a LArge Reservoir Simulator (LARS). Different catalysts were tested, varying from diverse elements of the platinum group to a universal metal catalyst. The results show differences regarding the conversion rate of CH 4 to CO 2 . The promising results of the latest reactor test, for which LARS was filled with sand and ca. 80% of the pore space was saturated with CH 4 hydrate, are also presented in this study. The data analysis showed that about 15% of the CH 4 gas released from hydrates would have to be used for the successful dissociation of all hydrates in the sediment using thermal stimulation via in situ combustion. OPEN ACCESSEnergies 2013, 6 3003

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In this study we present results of our investigations on simple CH4-hydrate and mixed hydrates during the initial steps of the hydrate formation process. In situ Raman spectroscopy, microscopic observation and in situ X-ray diffraction were used in our systematic studies. Although these techniques give only a limited view on the molecular level the combined results from the experiments reported here indicate that the labile cluster hypothesis can describe the initial hydrate formation process. Specifically, the guest molecules dissolve in the aqueous phase before they are encaged into single hydrate cavities which agglomerate to a solid phase. Results from Raman spectroscopic measurements suggest that the initially formed solid phase can be characterized by an excess of pentagonal dodecahedrons, whereas the formation of tetrakaidecahedrons or hexakaidecahedrons occurs as a subsequent step. At the time the tetrakaidecahedrons or hexakaidecahedrons are observed with Raman spectroscopy, corresponding X-ray diffraction experiments indicate the formation of a crystalline hydrate phase. Therefore, we assume that the solid phase formed at the very first state is not a hydrate phase in terms of a crystalline structure but some kind of an amorphous hydrate which transforms subsequently into a crystalline hydrate phase. Furthermore, the results suggest that the formation process and the properties of the resulting hydrate phase strongly depend on the properties of guest molecules.

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The study reports on the differences between theoretically expected and effectively obtained volume fractions of THF hydrate depending on the THF-H2O ratio in the initial solution against the background of using it as a substitute for natural hydrate in laboratory simulations. Besides the stoichiometric solution, initial solutions with either H2O or THF as excess phase were prepared to define the wanted volume of hydrate in advance. In order to achieve a chemical equilibrium a complete conversion of H2O and THF into THF hydrate and the presence of a pure excess phase is impossible. Based on the specific enthalpy of hydrate-and ice melting gained from calorimetric measurements, considerably lower than expected hydrate volumes are concluded. For the stoichiometric solution, containing 19.1 Wt% THF, enthalpy recalculations and the occurrence of an ice melting endotherm indicate incomplete conversion with a residual of 4.3 Vol% unconverted THF-H2O solution. The deviations from expectations increase with decreasing amount of aspired THF hydrate saturation and are stronger when formed from H2O excess solutions with up to 25 Vol% less hydrate than projected for full conversion. THF-rich solutions form hydrate with melting enthalpies that recalculate for up to 15 Vol% hydrate less than theoretical assumptions.In samples with initial THF concentrations below 5 Wt% and above 82.7 Wt% no hydrate formation was evident.Based on the results we propose corrections to the initial solutions when defined THF hydrate volumes are required. Furthermore, THF excess and temperatures below zero assure stable conditions for hydrate-liquid setting at atmospheric pressure.2