Study of the mechanical behaviour of hydrate-bearing clayey sediments based on in situ measurements

• Main Author: Taleb, Farah
• Other Authors: Laboratoire Aléas géologiques et Dynamique sédimentaire (LAD), Unité de recherche Géosciences Marines (Ifremer) (GM), Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Université de Bretagne occidentale - Brest, Nabil Sultan
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2019
• Subjects: In-situ measurements; Gulf of Guinea; Gas hydrates; Mechanical properties; Clayey sediments; Hydrate de gaz; Propriétés mécaniques; Milieu argileux; Mesures in-situ; Golf de Guinée; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; permafrost
• Online Access: https://tel.archives-ouvertes.fr/tel-02612639; https://tel.archives-ouvertes.fr/tel-02612639/document; https://tel.archives-ouvertes.fr/tel-02612639/file/These-2019-SML-Geosciences_marines-TALEB_Farah.pdf

Gas hydrates (GH) are composed of gas molecules, often methane, trapped in a lattice of hydrogen-bonded water molecule. They are found in sediments of continental margins and permafrost, lakes and inland seas, where their stability conditions (high pressure and low temperature) are met. GH are considered as a potential energy resource but furthermore as a potential submarine geohazard and source of greenhouse gases. It is essential to understand the consequences of the presence of these metastable components on the geo-mechanical properties of their host sediment (GHBS). Ifremer has conducted several oceanographic campaigns aiming to assess such geohazard in an area of the deep-water Niger Delta, characterised by hydrates which formed in high gas flux environments in clayey sediment. The database is composed of in-situ acoustic, geotechnical, pore-pressure dissipation measurements, as well as cores and seismic profiles.The PhD work exploited the dataset with the aim of understanding the effect of GH content, morphology and distribution on the mechanical and hydraulic properties of the GHBS. This rare field study revealed that marine GH-bearing clays have a contractive behaviour upon shearing, which contrasts with the dilative behaviour of sandy GHBS often discussed in litterature. Alternatively, different morphologies of GH have been linked with different features of the mechanical behaviour of GHBS. For high GH saturtion (> 10%), the hydraulic diffusivity of the GHBS was observed to increase with increasing GH content, which was linked to fractures or decrease in compressibility. A new simple constitutive model based on “equivalent skeleton void ratio” was used in order to simulate the mechanical behaviour of GHBS.Preliminary results show that only one additional parameter related to the morphology of hydrate is necessary to correctly simulate the mechanical behaviour of GHBS. Les hydrates de gaz (GH) sont composés de molécules de gaz, souvent du méthane, piégées dans des cages d’eau. Ils se trouvent ...