Physico-Chemical investigations of sedimentary gas hydrates reproducing their natural geo-environment. – HYDRACLAY

Natural methane hydrates (NGH)– formed from the reaction of pore fluids and gases within sediments - occur at cold seeps on continental margins and in the permafrost of the Polar Regions. They represent the largest methane reservoir on Earth, and fuel the development of a large variety of chemosynthetic communities living in such areas. In such environments, methane hydrates are intimately linked to the sediment and ensure the seafloor stability. The estimated amount of methane trapped in the overall deposits ranges between 600 to 10000 billion tons, and this large range clearly shows that there is a need to reduce the uncertainty on the estimate of methane bound in NGH. Achieving this goal will allow a better understanding of their role in the deep carbon cycle, and the consequences of a sustained release of methane from its decomposition due to temperature increase caused by climate change. In nature, 90% of hydrate deposits are characterized by fine-grained clay-rich sediments, and several studies have recognized the importance of sediment minerals on the nucleation time, growth rate, morphology, and stability field of natural gas hydrates. Nevertheless, we are far from having reached an in-depth understanding of the critical mechanisms that control the kinetics of hydrate formation and the resulting cage occupancy, especially in confined media like clay minerals.
A better understanding of the influence of minerals on the mechanism of hydrate formation and accumulation in geological matrices represents a major step forward in improving the inventories of natural gas hydrates, and quantitatively appreciating their potential in terms of geohazard and their sensitivity to climate-change-related issues. The HYDRACLAY project is based on the reproduction of their natural conditions of formation in terms of thermodynamics, geochemistry and mineralogy and the scientific objectives are centered around three physical-chemistry challenges: (i) determine the preferential location for hydrate formation in clays at the nanometric scale, (ii) investigate the potential role of interlayer cations (when present) and (iii) investigate on the influence of gas flow rate on cage occupancy rates.
A research program (gathering physical-chemists and geoscientists) is implemented for investigating synthetic and natural gas hydrate-bearing sediments from the Black Sea area. The very recent scientific cruise in the Black Sea (sept. 2021) has provided the HYDRACLAY consortium with a unique set of samples of sediment, pore water and natural gas hydrates. The consortium will adopt a multiscale approach combining nano- to microscale investigations based on advanced physical-chemistry techniques (spectroscopy, diffraction and tomography) to observe and describe how hydrates invade clay-rich matrices, study the growth kinetics and determine cage occupancy, with macro- to geological scale hydrate distribution in clay-rich sediments.
Thus, the HYDRACLAY project will tackle a “final product” objective, aiming at bridging the physical-chemistry results on the mechanisms of hydrate formation and their accumulation in natural geological matrices - as required for developing reliable method to evaluate the methane amount in marine gas hydrates.