Gaz de schiste

Résumé

Le gaz de schiste, également appelé gaz de roche-mère (ou plus rarement au Québec « gaz de shale »), est un gaz naturel contenu dans des roches marneuses ou argileuses riches en matières organiques, roches qui peuvent avoir une structure litée de schiste. Contrairement au gaz naturel conventionnel qui est retenu dans une roche perméable permettant une exploitation facile, le gaz de schiste est piégé dans les porosités d'une roche rendue imperméable par l'argile qu'elle contient. L'extraction du gaz de schiste, particulièrement difficile, nécessite le recours systématique aux techniques combinées du forage dirigé et de la fracturation hydraulique à grands volumes particulièrement coûteuses. Les roches-réservoir contenant du gaz de schiste peuvent aussi contenir de l'huile de schiste (pétrole non conventionnel), mais dans des proportions beaucoup plus faibles. L'exploitation à grande échelle du gaz de schiste a démarré au cours des années 2000 lorsque le prix des hydrocarbures de roche

Source officielle

https://fr.wikipedia.org/wiki/Gaz_de_schiste

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In the last decade, the exponential development of shale gas activities in United States resulted in a real revolution both in terms of gas prices and energy security. However, next to these positive effects, many risks and impacts for the environment and for human health related to shale gas activities have been identified. Risks of water and soil contamination or air pollution issues related to shale gas activities have been revealed in different recent scientific studies. During the last years, and because of the growing concern of local populations, shale gas became very debated in the media. In the scientific community as well, shale gas is a debated issue but in terms of greenhouse gases (GHG) emissions. This study is a scientific assessment of the environmental impacts related to shale gas extraction trying to synthesize information available from different scientific studies and providing potential mitigation measures. As Poland currently plays the most active role in European shale gas exploration, it has been chosen has the reference location in order to evaluate the different impacts. The results are that potential impacts on both human health and the environment can potentially be important but that an important lack of reliable information exists in order to fully assess most of the potential risks related to shale gas activities. Due to the high sensitivity of this issue among scientists, shale gas GHG emissions have been calculated in this study using a Monte Carlo simulation in order to define potential results based on variable parameters instead of using fixed values. The results show that shale gas GHG emissions could be 28% larger than coal over a 20 year timeframe and identical to coal over a 100 year timeframe. Mitigation measures can however reduce shale gas emission and most of the other impacts. The largest problem in order to currently asses and adequately reduce the risks related to shale gas activities is the current paucity of reliable information. An important uncertainty indeed exists in the evaluation of most of the risks. Many more scientific studies are therefore urgently needed in order to fully assess the potential impacts of shale gas activities and in order to ensure that the development of such activities will be done in Poland in the best possible conditions. In the meantime, the biggest risk would be to not accept our current ignorance and a precautionary approach should therefore be adopted.

2012

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Numerical investigation on water exchange of shale samples

Alessio Ferrari, Sara Incollingo

Interest in the hydraulic and mechanical characterization of shales has grown in recent years, because of their application in the context of energy geotechnics. In the frame of nuclear waste disposal shales are considered as host formations for the placements of nuclear waste at high depths. In the frame of hydrocarbon production they are considered as unconventional reservoirs, from which extracting natural gas. Understanding how fluids flow through shales is then a key aspect for both fields of application. This paper focuses on the analysis of the transport of water vapour through laboratory samples. After reviewing the balance and flow laws that govern the transport of fluid in unsaturated porous media, a simplified model is put forward. The model was implemented in a commercial finite element code, and it was used to reproduce the results of a literature study on wetting and drying of Opalinus Clay shale samples, imposed through the vapour equilibrium technique. Back analysis of the water content and volume strains of these specimens suggests that existing models underestimate the actual flow rate of water vapour which takes place at low suctions. The current interpretation also seems to be consistent with microstructural investigations on the interconnection between large pores of this material.

E D P SCIENCES2020

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Multiphysical behaviour of shales from Northern Switzerland

Valentina Favero

Shales are involved in energy-related applications such as the extraction of shale gas, the CO2 storage and the nuclear waste disposal. In these contexts, fundamental issues come along with the multiphysical conditions in which shales are found, where mechanics, temperature, chemistry and unsaturated conditions play a relevant role. This thesis analyses, theoretically and experimentally, the multiphysical behaviour of Swiss shales: retention capacity, hydro-mechanical behaviour, thermo-mechanical response and chemo-mechanical processes are investigated. An advanced experimental methodology to analyse the retention behaviour of shales is presented. The existence of main wetting, drying and scanning paths is investigated, together with the hysteresis domain, the volume change induced by suction variations, and the dependency of the air entry value on porosity. The generation of excess pore water pressure due to changes in stress is a concern during tunnel excavations and drilling operations, therefore, the consolidation of shales must be analysed. This thesis describes an apparatus designed to perform high-pressure oedometric tests in multiphysical conditions, and an analytical model which allows to gather information on the coefficient of consolidation, stiffness, poroelastic properties, secondary compression and permeability. Remoulded shales behaviour is compared to the intact shales response, in order to analyse the impact of diagenesis. The results highlight greater stiffness, lower porosity, lower swelling sensitivity and less creep for the intact shale with respect to the remoulded one: this response is considered to be the result of diagenesis. The thermo-mechanical behaviour of Opalinus Clay is investigated at different overconsolidation ratio (OCR) values. Thermal expansion is found when heating is carried out at high OCR, while irreversible thermal compaction is observed when heat is applied at low OCR. A decrease of the yield threshold with increasing temperature is observed. Compressibility and swelling indexes are not significantly influenced by thermal changes, whereas consolidation processes occur faster at high temperature. The research on chemo-mechanical processes in Opalinus Clay highlights that the volume change of the material upon wetting is related to the osmotic suction of the pore fluid. Moreover, an increase in osmotic suction at constant stress leads to development of irreversible settlements, whose magnitude depends on the stress history. The compressibility and consolidation coefficient of Opalinus Clay are not significantly affected by osmotic suction. An elasto-plastic chemo-mechanical framework is formulated in order to model the chemo-mechanical behaviour of Opalinus Clay. The strength and stiffness of Opalinus Clay are investigated through the performance of triaxial tests. The results allow to gain information on the anisotropic and stress-dependent behaviour of the material. Based on the experimental evidences, a constitutive framework is formulated. Transversely isotropic elasticity is considered in order to reproduce the anisotropic elastic response of the material. Moreover, the elastic parameters are defined in order to account for stress dependency. The possibility to consider anisotropic strength response is also included in the model. The constitutive approach includes the introduction of the disturbed state concept (DSC) in order to reproduce softening.

EPFL2017

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Multiphysical behaviour of shales from Northern Switzerland

Valentina Favero

EPFL2017

2012