Abstract:The presence of natural gas hydrates at all active and passive continental margins has been proven. Their global occurrence as well as the fact that huge amounts of methane and other lighter hydrocarbons are stored in natural gas hydrates has led to the idea of using hydrate bearing sediments as an energy resource. However, natural gas hydrates remain stable as long as they are in mechanical, thermal and chemical equilibrium with their environment. Thus, for the production of gas from hydrate bearing sediments, at least one of these equilibrium states must be disturbed by depressurization, heating or addition of chemicals such as CO 2 . Depressurization, thermal or chemical stimulation may be used alone or in combination, but the idea of producing hydrocarbons from hydrate bearing sediments by CO 2 injection suggests the potential of an almost emission free use of this unconventional natural gas resource. However, up to now there are still open questions regarding all three production principles. Within the framework of the German national research project SUGAR the thermal stimulation method by use of in situ combustion was developed and tested on a pilot plant scale and the CH 4 -CO 2 swapping process in gas hydrates studied on a molecular level. Microscopy, confocal Raman spectroscopy and X-ray diffraction were used for in situ investigations of the CO 2 -hydrocarbon exchange process in gas hydrates and its driving forces. For the thermal stimulation a heat exchange reactor was designed and tested for the exothermal catalytic oxidation of methane. OPEN ACCESSEnergies 2011, 4 152 Furthermore, a large scale reservoir simulator was realized to synthesize hydrates in sediments under conditions similar to nature and to test the efficiency of the reactor. Thermocouples placed in the reservoir simulator with a total volume of 425 L collect data regarding the propagation of the heat front. In addition, CH 4 sensors are placed in the water saturated sediment to detect the distribution of CH 4 in the sample. These data are used for numerical simulations for up-scaling from laboratory to field conditions. This study presents the experimental set up of the large scale reservoir simulator and the reactor design. Preliminary results indicate that the catalytic oxidation of CH 4 operated as a temperature controlled, autothermal reaction in a countercurrent heat exchange reactor is a safe and promising tool for the thermal stimulation of hydrates. In addition, preliminary results from the laboratory studies on the CO 2 -hydrocarbon swapping process in simple and mixed gas hydrates are presented.

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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

In 2004 three seismic surface sources (VIBSIST, accelerated weight drop and MiniVib) were tested in a pilot study at the Ketzin test site, Germany, a study site for geological storage of CO 2 (EU project CO 2 SINK). The main objectives of this pilot study were to (1) evaluate the response of the Ketzin site to reflection seismics, especially at the planned injection depth, (2) test different acquisition parameters and (3) use the results to guide the planning of the 3D survey. As part of these objectives, we emphasize the source performance comparison in this study. The sources were tested along two perpendicular lines of 2.4 km length each. Data were acquired by shooting at all stations (source and receiver spacing of 20 m) on both lines, allowing CMP stacked sections to be produced. Sources signal characteristics based on signal-to-noise ratio, signal penetration and frequency content of raw shot records were analyzed and stacked sections were compared. The results show that all three surface sources are suitable for reflection seismic studies down to a depth of about 1 km and provide enough bandwidth for resolving the geological targets at the site, i.e., the Weser and Stuttgart Formations. Near surface conditions, especially a thick weathering layer present in this particular area, strongly influence the data quality, as indicated by the difference in reflectivity and signal-to-noise ratio of the two CMP lines. The stacked sections of the MiniVib source show the highest frequency signals down to about 500 ms traveltime (approx. 500 m depth), but also the shallowest signal penetration depth. The VIBSIST source generates signals with the highest signal-to-noise ratio and greatest signal penetration depth of the tested sources. In particular, reflections below 900 ms (approx. 1 km depth) are best imaged by the VIBSIST source. The weight drop performance lies in between these 2 two sources and might be recommended as an appropriate source for a 3D survey at this site because of the shorter production time compared to the VIBSIST and MiniVib sources.

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