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Modélisation du comportement des sédiments riches en hydrates de gaz via l'homogénéisation des propriétés micro-mécaniques

Abstract : Gas hydrates represent an important potential energy resource, but also a risk of instability for the environment (landslides, global warming) that it is essential to control. The study of gas hydrate bearing soils, most often located on the ocean floor or in permafrost, is therefore a major challenge. The formation and dissociation of hydrates in these soils modifies the microstructure and with it the physical properties of the material. The objective of the thesis was to develop a model that could predict the behaviour of soils containing gas hydrates, initially on the scale of the conventional laboratory sample. Several multi-physical computational models applied to gas hydrate-enriched soils have already been published, but the mechanical part is still relatively underdeveloped due to the lack of experimental data and the relatively late interest shown by the mechanics' community in the subject.Based on this observation, we first focused our analysis on mechanical behaviour. The results of tests on sediments rich in methane hydrates available in the literature have been used as a basis for analyzing the effect of hydrates on the mechanical properties of a soil. In particular, the relationship between the elastic moduli of a soil and the volume fraction of hydrates was determined using an analytical homogenization calculation. However, sediments containing gas hydrate inclusions exhibit macroscopic behaviour that is far from linearly elastic. The latter is strongly related to the different physical and morphological characteristics of both matrix sediments and hydrates formed in the pore space.These observations led to the application of a numerical homogenization method based on Fast Fourier Transforms (FFTs). This method allows for the use of elastoplastic laws and complex geometries to define the microstructure components of the material to be homogenized. The results can therefore be used to determine a non-linear constitutive macroscopic model adapted to the type of sediment/hydrate composite to be simulated.The previous developments were then integrated into a finite element computation code first at the scale of the assumed homogeneous laboratory sample. Hydraulic couplings via pressures and conventional fluid flow models could therefore be integrated, as well as the solubility of methane in the aqueous phase and phase changes through a kinetic law. The thermodynamic aspect was also included. Mechanical behaviour could be defined either by analytical homogenization laws or by multi-scale calculations. The numerical homogenization calculation by FFT is carried out at the microstructure scale at Gauss integration points.These calculations were compared with laboratory test results for volume fractions of constant hydrates or for hydrate dissociation tests in soil samples. Finally, data from an exploration site were obtained from the literature and used to conduct a reservoir-scale calculation.Translated with
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Submitted on : Monday, November 23, 2020 - 5:17:56 PM
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Axelle Alavoine. Modélisation du comportement des sédiments riches en hydrates de gaz via l'homogénéisation des propriétés micro-mécaniques. Mécanique des matériaux [physics.class-ph]. Université Paris-Est, 2020. Français. ⟨NNT : 2020PESC1008⟩. ⟨tel-03020159⟩



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