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.

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 NO_x 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.

Density‐driven dissolution of carbon dioxide in water is a well‐known and much described mechanism in geological sequestration of this greenhouse gas. It is remarkable that such enhanced dissolution does not receive attention in karst hydrology and speleology. Models and hypotheses on karst development are complex and consider many different processes. We focus here on the influence of CO₂ partial gas pressures at the interface between atmosphere and karst water on the dynamics of dissolved CO₂ concentrations below the water table. Seasonal variations of microbial soil activity and root respiration or barometric‐pressure changes cause fluctuations in CO₂ partial pressures. Dependent on the existence and strength of a karst‐water background flow, fingering regimes might be triggered causing enhanced dissolution of CO₂. This allows replenishment of CO₂, and, thus, dissolutional power even deep in the water body without the need for percolating water to transport dissolved CO₂. We present and discuss simplified and generic experimental and computational scenarios to strengthen our claim, and we try to give answers to: how much? and under which circumstances? The applied numerical model solves the Navier‐Stokes equation with water density dependent on CO₂ concentration and temperature. We show that calculated CO₂ mass fluxes into the water bodies are dependent on the ratio of Péclet to Rayleigh numbers (Pe/Ra) and show a local minimum around Pe/Ra = 1, i.e. when natural and forced convection are about equal. Concluding, we claim that there is sufficient reason to consider density‐driven dissolution as a process of relevance in karstification if circumstances are given.

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.