Mechanical behaviour of gas-hydrate-bearing sediments – HYDRE

The behaviour of gas hydrates in the presence of mineral (e.g., quartzitic) substrates and in confined conditions representative of soil pores will be first examined at pore scale (few micrometers) – Task 1. As quartz is the main solid component of soils, the hydrate growth and dissociation mechanisms that occur on quartzitic substrates will be characterised, mainly by optical microscopy.

Second, the behaviour of the materials at the (typically, millimetric) scale of a representative volume element (RVE) will be investigated in the laboratory – Task 2. Methods for synthetizing gas hydrates in soils will be first considered. In order to observe the creation and the dissociation of gas hydrates in soil, non-destructive methods (Resonance Magnetic Imagery and X-ray microtomography) will be used. The mechanical behaviour of GHBS will be investigated using a temperature-controlled triaxial cell.

The mechanical behaviour at the RVE scale will be equally studied by numerical modelling methods in Task 3. First, the link between the behaviour at pore scale and RVE scale will be assessed using discrete element (DEM) numerical simulations. Second, the link between the behaviour at the RVE scale and at the site scale will be considered by Finite Elements Method. The effect of gas hydrates on soil mechanical behaviour will be considered to elaborate constitutive laws. These laws will be then implemented in a finite element numerical code in order to predict the ground behaviour during the industrial exploitation of methane gas.

Finally, macroscopic data of GHBS soils will be collected in situ by piezocone, and suitably analysed at this (metric) scale – Task 4. Such measurements have been carried out by IFREMER, using a piezocone and a sonic fork, to obtain mechanical and acoustic properties, respectively. This dataset is completed by profiles of physical properties, mineralogical composition and geochemical parameters measured on sedimentary core samples.

The following scientific benefits are:
- Acquisition of knowledge on the formation/dissociation processes of gas hydrates at the pore scale;
- Characterization of the saturation, distribution and morphology of hydrates on the mechanical behaviour of GHBS by using various experimental and numerical approaches;
- Mastery of techniques for the detection and quantification of gas hydrates from marine sediments;
- Assessment of geo-hazards related to natural gas production from GHBS;

The economical, environmental and societal benefits are mainly related to the methane and carbon dioxide gases:
- The scientific and technical results of the project contribute to the progress in the domain of natural gas production as an alternative to the conventional energy sources;
- Knowledge on the formation of CO2 hydrate has important impact to the geological storage of CO2; actually, formation of CO2 hydrate can contribute to the sealing of leakage in this storage;
- Observation on the methane hydrate dissociation have to improve the prediction related to the emission of green-house gas due to global warming;
- Finally, other technological domains related to hydrates will also get profits from the project: CO2 sequestration, secondary refrigeration, desalinisation of seawater, treatment of sewage, etc. In general, the results obtained are useful for all hydrate-related processes where solid phase (particles, etc.) is present.

The results of the present proposal would help engineers to better deal with the geomechanics related hazards and ensure safe and efficient gas production from hydrate deposits, minimizing environmental impacts. One potential production scheme involves injecting carbon dioxide in the GHBS deposits. This could minimize geohazards such as soil destabilisation and sand production while reducing the environmental impacts of natural gas consumption by geologically storing carbon dioxide.

To be updated

The present project aims at studying the mechanical behaviour of gas-hydrate-bearing- sediments (GHBS) which is essential to provide the information required to assess and minimize the environmental impacts of any future exploitation of deep-sea methane hydrates. The GHBS will be studied by a multi-scale approach and the project is organised in four tasks.

The behaviour of gas hydrates in the presence of mineral (e.g., quartzitic) substrates and in confined conditions representative of soil pores will be first examined at pore scale (few micrometers) – Task 1. As quartz is the main solid component of soils, the hydrate growth and dissociation mechanisms that occur on quartzitic substrates will be characterised, mainly by optical microscopy. An attempt will be made to characterize the micromechanical properties attached to hydrate habits (e.g., grain cementing). Experiments will be first carried out with cyclopentane, which forms hydrates representative of natural gas hydrates at ambient pressures, before moving to natural gas hydrates.

Second, the behaviour of the materials at the (typically, millimetric) scale of a representative volume element (RVE) will be investigated in the laboratory – Task 2. Methods for synthetizing gas hydrates in particulate soils will be first considered. The study will then be extended to fine-grained soils. In order to observe the creation and the dissociation of gas hydrates in soil, non-destructive methods (Resonance Magnetic Imagery and X-ray microtomography) will be used. The mechanical behaviour of GHBS will be investigated using a temperature-controlled triaxial cell.

The mechanical behaviour at the RVE scale will be equally studied by numerical modelling methods in Task 3. First, the link between the behaviour at pore scale and RVE scale will be assessed using discrete element (DEM) numerical simulations. Second, the link between the behaviour at the RVE scale and at the site scale will be considered by Finite Elements Method. The effect of gas hydrates on soil mechanical behaviour will be considered to elaborate constitutive laws. A thermodynamically consistent poro-mechanical model will be derived and calibrated using the laboratory tests and DEM analyses. These laws will be then implemented in a finite element numerical code in order to predict the ground behaviour during the industrial exploitation of methane gas. The instability of continental slope due to the presence of gas hydrates will be also considered.

Finally, macroscopic data of GHBS soils will be collected in situ by piezocone, and suitably analysed at this (metric) scale – Task 4. Such measurements have been carried out by IFREMER, using a piezocone and a sonic fork, to obtain mechanical and acoustic properties, respectively. This dataset is completed by profiles of physical properties, mineralogical composition and geochemical parameters measured on sedimentary core samples. All these data will be analysed in details in the first stage. The insight gained through this approach will be invaluable for the planning and the execution of laboratory tests on synthetic samples attempting to emulate field conditions.

The results of the present proposal would help engineers to better deal with the geomechanics related hazards and ensure safe and efficient gas production from hydrate deposits, minimizing environmental impacts. One potential production scheme involves injecting carbon dioxide in the GHBS deposits. This could minimize geohazards such as soil destabilisation and sand production while reducing the environmental impacts of natural gas consumption by geologically storing carbon dioxide.