The Driving Forces of Guest Substitution in Gas Hydrates—A Laser Raman Study on CH4-CO2 Exchange in the Presence of Impurities

The recovery of CH4 gas from natural hydrate formations by injection of industrially emitted CO2 is considered to be a promising solution to simultaneously access an unconventional fossil fuel reserve and counteract atmospheric CO2 increase. CO2 obtained from industrial processes may contain traces of impurities such as SO2 or NOx and natural gas hydrates may contain higher hydrocarbons such as C2H6 and C3H8. These additions have an influence on the properties of the resulting hydrate phase and the conversion process of CH4-rich hydrates to CO2-rich hydrates. Here we show results of a microscopic and laser Raman in situ study investigating the effects of SO2-polluted CO2 and mixed CH4-C2H6 hydrate on the exchange process. Our study shows that the key driving force of the exchange processes is the establishment of the chemical equilibrium between hydrate phase and the surrounding phases. The exchange rate is also influenced by the guest-to-cavity ratio as well as the thermodynamic stability in terms of p-T conditions of the original and resulting hydrate phase. The most effective molecule exchange is related to structural changes (sI-sII) which indicates that hydrate decomposition and reformation processes are the occurring processes

Evaluation of CH4 Gas Permeation Rates through Silicone Membranes and Its Possible Use as CH4-Extractor in Gas Hydrate Deposits

This preliminary study discusses the option of using silicone (PDMS) tubes for the extraction and monitoring of CH4 gas from hydrate deposits during decomposition. For this, the gas flow of gaseous and dissolved CH4 through hand-manufactured silicone tube membranes has been tested on a small lab scale. The permeability of pure CH4 gas at ambient pressure and 295K is in good agreement with literature reports. The permeability rate of CH4 derived from dissolved methane is expectedly lower and decreases by more than half. Theoretical assumptions and lab results are not in straightforward correlation. This leads to the conclusion that experimental constraints influence the permeability. These are mainly an increase of the gas stream resistivity due to the membrane-filling and membrane-sediment contact points as well as the effect of degassing dissolved CH4 on the tube surface due to pressure gradients. Therefore, the use as a monitoring tool needs to be individually tested before the respective application. Due to the robust nature of the membranes, their application as a tool for capturing gaseous CH4 during hydrate decomposition is a feasible goal

A Counter-Current Heat-Exchange Reactor for the Thermal Stimulation of Hydrate-Bearing Sediments

Since huge amounts of CH4 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 CH4 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 CH4. 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 CH4 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 CH4 to CO2. 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 CH4 hydrate, are also presented in this study. The data analysis showed that about 15% of the CH4 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