Description: Microscopy, confocal Raman spectroscopy and powder X-ray diffraction (PXRD) were used for in situ investigations of the CO2-hydrocarbon exchange process in gas hydrates and its driving forces. The study comprises the exposure of simple structure I CH4 hydrate and mixed structure II CH4–C2H6 and CH4–C3H8 hydrates to gaseous CO2 as well as the reverse reaction, i.e., the conversion of CO2-rich structure I hydrate into structure II mixed hydrate. In the case of CH4–C3H8 hydrates, a conversion in the presence of gaseous CO2 from a supposedly more stable structure II hydrate to a less stable structure I CO2-rich hydrate was observed. PXRD data show that the reverse process requires longer initiation times, and structural changes seem to be less complete. Generally, the exchange process can be described as a decomposition and reformation process, in terms of a rearrangement of molecules, and is primarily induced by the chemical potential gradient between hydrate phase and the provided gas phase. The results show furthermore the dependency of the conversion rate on the surface area of the hydrate phase, the thermodynamic stability of the original and resulting hydrate phase, as well as the mobility of guest molecules and formation kinetics of the resulting hydrate phase.

Description: Gases encountered in different salt beds from evacuated and packer-sealed borehole sections in a potash mine were sampled and characterized for their chemical and isotopic composition so as to conclude on their origin and evolution in the salt rocks. These gases were either generated autochthonally or originate from fluid influx from the surrounding rocks outside the salt formation. Fixation in the salt rocks can take place laminar on mineral grain boundaries, disrupter and fracture zones or trapped in inclusions inside or between mineral grains. In situ flow tests with pure argon between several boreholes at distances ranging from decimeter to meter suggest that formation gas is stripped from the intermediate salt packet. This gas must have been trapped on grain boundaries along the pathways of the flowing argon. The stripped formation gas comprises mainly CO2 with traces of CH4 and H2. The CO2 isotopic composition matches well with gases originating from a mantle source, whereas CH4 is classified to be of thermogenic origin formed in a marine environment. Plausible explanations for the H2 generation are the radiolysis of water, reaction of FeII with water or microbial processes. We conclude that these trapped gases are of allochthonous origin migrating from the surrounding rocks into the salt formation where they were fixated mainly along fracture surfaces and fissures.

Description: The exchange of hydrate-bond CH4 with CO2 is one possible method for the production of CH4 from hydrate-bearing sediment which was investigated on different scales in the SUGAR project. Tubular polydimethylsiloxane (PDMS) membranes were utilized to monitor the spatial and temporal gas distribution in a large-scale experimental simulation on CO2–CH4 gas hydrate exchange. The suitability of PDMS membranes for the measurement of gaseous and dissolved CO2 and CH4 concentrations in pure and mixed gas systems was evaluated in lab-scale experiments. The results reveal a strong interacting mutual influence of CO2 and CH4 in CO2–CH4 mixed feed composition and in the presence of water. The competitive absorption between CO2 and H2O as well as membrane plasticization, which increases CH4 permeability and reduces CO2 permeability, makes a direct correlation of mixed systems to pure systems and a quantification of the gas concentration in the feed reservoir impossible. The successful run of five tubular PDMS membranes, employed in a large test reservoir during an experimental simulation of CO2-driven CH4 hydrate decomposition, demonstrates the high stability of the material in harsh conditions. Also, a time-resolved observation of the progressing CO2 front is possible and makes membrane incorporation a valuable addition to conventional ex situ gas measurements in reservoir tests of various dimensions. The monitoring technique can significantly contribute to a comprehensive process understanding with respect to the spatial distribution of hydrate formation, dissociation and reformation in the presence of CO2 and CH4.

Description: In eastern Hesse and western Thuringia, Germany hosts significant potassium-bearing salt deposits industrially excavated in the Werra-Fulda mining district. The salt belongs to the upper Permian (Zechstein) and was deposited around 258 to 252.5 Ma ago. In the Werra-Fulda mining district, the halite rocks (Werra-Rocksalt, z1NA) contain two minable potash seams, potash Seam Thüringen (z1KTh) and potash Seam Hessen (z1KHe), with an average thickness between 2 m and 3 m (Figure 1). To investigate the chemical and isotopic composition of the gas phase of Seam Hessen, gaseous samples were collected from five, 2-meter deep, horizontal boreholes drilled in the potash horizon which is mined at a depth of 540 m. About 4 weeks in advance of the gas sampling, the packer-closed boreholes were evacuated to about 3kPa and the pressure gradient inside the holes was continuously monitored in the boreholes D1 to D5. Selected gas samples were analyzed for their noble gas isotopic composition and the δ13C values of CO2 and CH4. The noble gas isotopic compositions were determined using a VG 5400 noble gas mass spectrometer after purification in a preparation line. The carbon isotope compositions were analysed with a GC-IRMS, comprising a GC 6890N connected to a GC-C/TC III combustion device and coupled to a MAT 253 mass spectrometer. The standard deviation of the δ13C values (in ‰ vs. VPDB) is ± 0.5 ‰.

Description: The “guest exchange” of methane (CH4) by carbon dioxide (CO2) in naturally occurring gas hydrates is seen as a possibility to concurrently produce CH4 and sequester CO2. Presently, process evaluation is based on CH4−CO2 exchange yields of small- or mediumscale laboratory experiments, mostly neglecting mass and heat transfer processes. This work investigates process efficiencies in two large-scale experiments (210 L sample volume) using fully water-saturated, natural reservoir conditions and a gas hydrate saturation of 50%. After injecting 50 kg of heated CO2 discontinuously (E1) and continuously (E2) and a subsequent soaking period, the reservoir was depressurized discontinuously. It was monitored using electrical resistivity, temperature and pressure sensors, and fluid flow and gas composition measurements. Phase and component inventories were analyzed based on mass and volume balances. The total CH4 production during CO2 injection was only 5% of the initial CH4 inventory. Prior to CO2 breakthrough, the produced CH4 roughly equaled dissolved CH4 in the produced pore water, which balanced the volume of the injected CO2. After CO2 breakthrough, CH4 ratios in the released CO2 quickly dropped to 2.0−0.5 vol %. The total CO2 retention was the highest just before the CO2 breakthrough and higher in E1 where discontinuous injection improved the distribution of injected CO2 and subsequent mixed hydrate formation. The processes were improved by the succession of CO2 injection by controlled degassing at stability limits below that of the pure CH4 hydrate, particularly in experiment E2. Here, a more heterogeneous distribution of liquid CO2 and larger availability of free water led to smaller initial degassing of liquid CO2. This allowed for quick re-formation of mixed gas hydrates and CH4 ratios of 50% in the produced gases. The experiments demonstrate the importance of fluid migration patterns, heat transport, sample inhomogeneity, and secondary gas hydrate formation in watersaturated sediments.