The influence of SO2 and NO2 impurities on CO2 gas hydrate formation and stability

Unraveling the Role of Natural Sediments in sII Mixed Gas Hydrate Formation: An Experimental Study

Considering the ever-increasing interests in natural gas hydrates, a better and more precise knowledge of how host sediments interact with hydrates and affect the formation process is crucial. Yet less is reported for the effects of sediments on structure II hydrate formation with complex guest compositions. In this study, experimental simulations were performed based on the natural reservoir in Qilian Mountain permafrost in China (QMP) due to its unique properties. Mixed gas hydrates containing CH4, C2H6, C3H8, and CO2 were synthesized with the presence of natural sediments from QMP, with quartz sands, and without sediments under identical p-T conditions. The promoting effects of sediments regardless of the grain size and species were confirmed on hydrate formation kinetics. The ice-to-hydrate conversion rate with quartz sand and natural QMP sediments increased by 23.5% and 32.7%, respectively. The compositions of the initial hydrate phase varied, but the difference became smaller in the resulting hydrate phases, having reached a steady state. Beside the structure II hydrate phase, another coexisting solid phase, neither ice nor structure I hydrate, was observed in the system with QMP sediments, which was inferred as an amorphous hydrate phase. These findings are essential to understand the mixed gas hydrates in QMP and may shed light on other natural hydrate reservoirs with complex gas compositions.

Influence of Gas Supply Changes on the Formation Process of Complex Mixed Gas Hydrates

Natural gas hydrate occurrences contain predominantly methane; however, there are increasing reports of complex mixed gas hydrates and coexisting hydrate phases. Changes in the feed gas composition due to the preferred incorporation of certain components into the hydrate phase and an inadequate gas supply is often assumed to be the cause of coexisting hydrate phases. This could also be the case for the gas hydrate system in Qilian Mountain permafrost (QMP), which is mainly controlled by pores and fractures with complex gas compositions. This study is dedicated to the experimental investigations on the formation process of mixed gas hydrates based on the reservoir conditions in QMP. Hydrates were synthesized from water and a gas mixture under different gas supply conditions to study the effects on the hydrate formation process. In situ Raman spectroscopic measurements and microscopic observations were applied to record changes in both gas and hydrate phase over the whole formation process. The results demonstrated the effects of gas flow on the composition of the resulting hydrate phase, indicating a competitive enclathration of guest molecules into the hydrate lattice depending on their properties. Another observation was that despite significant changes in the gas composition, no coexisting hydrate phases were formed.

New Insights on a µm-Scale into the Transformation Process of CH4 Hydrates to CO2-Rich Mixed Hydrates

The global occurrences of natural gas hydrates lead to the conclusion that tremendous amounts of hydrocarbons are bonded in these hydrate-bearing sediments, serving as a potential energy resource. For the release of the hydrate-bonded CH4 from these reservoirs, different production methods have been developed during the last decades. Among them, the chemical stimulation via injection of CO2 is considered as carbon neutral on the basis of the assumption that the hydrate-bonded CH4 is replaced by CO2. For the investigation of the replacement process of hydrate-bonded CH4 with CO2 on a µm-scale, we performed time-resolved in situ Raman spectroscopic measurements combined with microscopic observations, exposing the CH4 hydrates to a CO2 gas phase at 3.2 MPa and 274 K. Single-point Raman measurements, line scans and Raman maps were taken from the hydrate phase. Measurements were performed continuously at defined depths from the surface into the core of several hydrate crystals. Additionally, the changes in composition in the gas phase were recorded. The results clearly indicated the incorporation of CO2 into the hydrate phase with a concentration gradient from the surface to the core of the hydrate particle, supporting the shrinking core model. Microscopic observations, however, indicated that all the crystals changed their surface morphology when exposed to the CO2 gas. Some crystals of the initial CH4 hydrate phase grew or were maintained while at the same time other crystals decreased in sizes and even disappeared over time. This observation suggested a reformation process similar to Ostwald ripening rather than an exchange of molecules in already existing hydrate structures. The experimental results from this work are presented and discussed in consideration of the existing models, providing new insights on a µm-scale into the transformation process of CH4 hydrates to CO2-rich mixed hydrates.

A new high-pressure cell for systematic in situ investigations of micro-scale processes in gas hydrates using confocal micro-Raman spectroscopy

Natural gas hydrates are ice-like solids composed of gas and water molecules. They are found worldwide at all continental margins as well as in permafrost regions. Depending on the source of the enclathrated gas molecules, natural gas hydrates may occur as coexisting phases with different structures containing predominantly CH₄, but also a variety of hydrocarbons, CO₂ or H₂S. For a better understanding of these complex hydrate formation processes on a micrometer level, an experimental setup with a new high-pressure cell was developed, which can be used in a pressure range between 0.1 MPa and 10.0 MPa. Peltier elements ensure precise cooling of the cell in a temperature range between 243 K and 300 K. The selected temperature and pressure ranges in which the cell can be used make it possible to simulate the formation of gas hydrates in their natural environment, e.g., on continental margins or in permafrost areas at a depth of up to 1000 m. The cell body is made of Hastelloy, which generally also allows the use of corrosive gases, such as H₂S, in the experiments. The inner sample space has a volume of about 550 µl. A quartz window allows for microscopic observations and the systematic and continuous in situ Raman spectroscopic investigations of the forming hydrate phase mimicking natural conditions. Single point measurements, line scans, and area maps provide information on spatial heterogeneities regarding compositions and cage occupancies. The pressure cell can be operated as a closed system or as an open system with a defined continuous gas flow. The use of a continuous gas flow also allows for the in situ investigation of transformation processes induced by changes of the feed gas composition. In this paper, all details of the new experimental setup as well as preliminary results of our investigations on the formation of complex mixed hydrate systems both in the open and closed systems as well as the CH₄-CO₂ transformation process are presented.

Gas hydrates in sustainable chemistry

This review includes the current state of the art understanding and advances in technical developments about various fields of gas hydrates, which are combined with expert perspectives and analyses.

Cage occupancy and structural changes during hydrate formation from initial stages to resulting hydrate phase

Hydrate formation processes and kinetics are still not sufficiently understood on a molecular level based on experimental data. In particular, the cavity formation and occupancy during the initial formation and growth processes of mixed gas hydrates are rarely investigated. In this study, we present the results of our time-depending Raman spectroscopic measurements during the formation of hydrates from ice and gases or gas mixtures such as CH4, CH4-CO2, CH4-H2S, CH4-C3H8, CH4-iso-C4H10, and CH4-neo-C5H12 at constant pressure and temperature conditions and constant composition of the feed gas phase. All investigated systems in this study show the incorporation of CH4 into the 5(12) cavities as first step in the initial stages of hydrate formation. Furthermore, the results imply that the initial hydrate phases differ from the resulting hydrate phase having reached a steady state regarding the occupancy and ratio of the small and large cavities of the hydrate.