Geochemical study of hydrate-bearing sediments : Impacts of the sediment on the formation mechanisms and the gas storage capacity, and application to the hydrate system of the Romanian sector of the Black Sea.

• Main Author: Agnissan, Art-Clarie Constant
• Other Authors: Institut des Sciences Moléculaires (ISM), Université Montesquieu - Bordeaux 4-Université Sciences et Technologies - Bordeaux 1 (UB)-École Nationale Supérieure de Chimie et de Physique de Bordeaux (ENSCPB)-Institut de Chimie - CNRS Chimie (INC-CNRS)-Centre National de la Recherche Scientifique (CNRS), Université de Bordeaux, Arnaud Desmedt
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2023
• Subjects: Methane hydrates; Sediment/Clays; High pressure experiments; Storage capacity; Geochemical analysis; Raman Spectroscopy; Black Sea; Hydrates de méthane; Sédiment/argiles; Expérimentations haute-pression; Capacité de stockage; Analyses géochimiques; Spectroscopie Raman; Mer Noire; [CHIM.OTHE]Chemical Sciences/Other; Noire; permafrost
• Online Access: https://theses.hal.science/tel-04245613; https://theses.hal.science/tel-04245613/document; https://theses.hal.science/tel-04245613/file/AGNISSAN_ART-CLARIE_CONSTANT_2023.pdf

Natural gas hydrates are widely distributed in both permafrost regions and marine sediments along the continental margins. They represent the largest methane reservoir on earth because of their high gas storage capacity. Societal and environmental concerns surrounding these natural compounds pertain to the amount of methane they may release and its fate. In-depth knowledge of hydrate formation processes in their natural sedimentary environment are essential to provide reliable answers to these questions. Like most of natural systems, hydrates deposits of the Romanian margin of the Black Sea are encountered in clayey sediments. However, the mechanisms of hydrate formation in such complex matrices are still poorly understood. Moreover, recent studies in this area showed that a progressive influx of salt water into the sediments triggered the destabilization of shallow gas hydrates, releasing methane in the water column.This PhD project proposes to improve our understanding of the distribution processes of gas hydrates in the clayey sediments of the Romanian sector, and to assess the amount of methane stored in these accumulations. It is based on a multidisciplinary and multiscale approach. Methane hydrates formation was carried out in laboratory by reproducing as well as possible natural conditions of the Black Sea (sediment mineralogy and water saturation). The results showed that clay content and clay mineralogy significantly affect the kinetics of hydrate formation as well as the hydrate morphology. Fine micro-scale analyses by Raman spectroscopy, of both synthetic and natural hydrate samples collected in the Black Sea, highlighted the influence of sediment mineralogy on the cage occupancy of hydrates, and thus on their gas storage capacity. In addition, geochemical analyses of pore waters collected from cores in the study area, have allowed the study of geochemical processes involving methane and their interplays with the distribution of gas hydrates. The synthesis of all these results led to a better ...