Influence of creep on water pressure measured from borehole tests in the Meuse/Haute-Marne Callovo-Oxfordian argillites

International audiencePorewater pressure is an important parameter for use in safety assessment of underground waste disposal. In order to measure porewater pressure in the Callovo-Oxfordian argillites, Andra has performed several in-situ tests, which consist in measuring time-evolution of water pressure in an almost closed chamber. This was accomplished using an in-place pressure sensor coupled with an electromagnetic transmission device (called an EPG probe). The measured values show a small hydraulic overpressure (0.1 MPa) compared with the estimated value at the corresponding depth. In the framework of a scientific cooperation agreement between Andra and Ineris, a study was undertaken to examine whether all or a part of this overpressure could be attributed to the hydro-mechanical coupled processes linked with the creep of argillites and the stress relaxation in the experimental measuring chamber-nearby. In the whole, the numerical results were in good agreement with the measured results. Poroviscoplasticity can explain the increasing pressure in the borehole. The measured overpressure can be reached with adequate viscoplastic model parameters

Hydro-mechanical behaviour of compacted bentonite-sand mixture used as sealing materials in radioactive waste disposal

In order to verify of the geological high-level radioactive waste disposal, IRSN has undertaken the SEALEX research project to control the long-term performance of swelling clay-based sealing systems. Compacted bentonite-sand mixtures are one of the most appropriate sealing materials studied in this project because of its low permeability and good swelling capacity. Once installed, this material will be in contact with the host-rock pore water and start swelling to close all the gaps in the system (internal pores, rock fractures and technological voids) and then, swelling pressure develops. In parallel with the in-situ SEALEX project, laboratory experiments are performed to investigate the sealing properties under this complex hydro-mechanical condition taking into consideration the effect of technological voids. In this work, two approaches for this study are presented : one studies the sealing properties of the buffer as a homogeneous material, while the second distinguishes two states of the material : the compacted central part and the loose gel formation in the technological void following swelling

Microstructure and anisotropic swelling behaviour of compacted bentonite/sand mixture

AbstractPre-compacted elements (disks, torus) of bentonite/sand mixture are candidate materials for sealing plugs of radioactive waste disposal. Choice of this material is mainly based on its swelling capacity allowing all gaps in the system to be sealed, and on its low permeability. When emplaced in the gallery, these elements will start to absorb water from the host rock and swell. Thereby, a swelling pressure will develop in the radial direction against the host rock and in the axial direction against the support structure. In this work, the swelling pressure of a small scale compacted disk of bentonite and sand was experimentally studied in both radial and axial directions. Different swelling kinetics were identified for different dry densities and along different directions. As a rule, the swelling pressure starts increasing quickly, reaches a peak value, decreases a little and finally stabilises. For some dry densities, higher peaks were observed in the radial direction than in the axial direction. The presence of peaks is related to the microstructure change and to the collapse of macro-pores. In parallel to the mechanical tests, microstructure investigation at the sample scale was conducted using microfocus X-ray computed tomography (μCT). Image observation showed a denser structure in the centre and a looser one in the border, which was also confirmed by image analysis. This structure heterogeneity in the radial direction and the occurrence of macro-pores close to the radial boundary of the sample can explain the large peaks observed in the radial swelling pressure evolution. Another interesting result is the higher anisotropy found at lower bentonite dry densities, which was also analysed by means of μCT observation of a sample at low bentonite dry density after the end of test. It was found that the macro-pores, especially those between sand grains, were not filled by swelled bentonite, which preserved the anisotropic microstructure caused by uniaxial compression due to the absence of microstructure collapse

Impact of a 70°C temperature on an ordinary Portland cement paste/claystone interface: An in situ experiment

International audienceRadioactive wastes in future underground disposal sites will induce a temperature increase at the interface between the cementitious materials and the host rock. To understand the evolution of Portland cement in this environment, an in situ specific device was developed in the Underground Research Laboratory in Tournemire (France). OPC cement paste was put into contact with clayey rock under water-saturated conditions at 70°C. The initial temperature increase led to ettringite dissolution and siliceous katoite precipitation, without monosulfoaluminate formation. After one year of interaction, partial decalcification and diffuse carbonation (calcite precipitation) was observed over 800 μm in the cement paste. At the interface, a layer constituted of phillipsite (zeolite), tobermorite (well-crystallised C-S-H), and C-(A)-S-H had formed. Globally, porosity decreased at both sides of the interface. Geochemical modelling supports the experimental results, especially the coexistence of tobermorite and phillipsite at 70°C, minerals never observed before in concrete/clay interface experiments

Finite Element Modelling in Structural and Petroleum Geology

This thesis is dedicated to the study of structural and petroleum geology problems. To this purpose, a frictional elastoplastic law based on the Van Eekelen criterion is formulated, which avoids the classical drawbacks of the Drücker Prager criterion. Also, a 2D automatic adaptive re-meshing algorithm is developed for complex multidomains configurations, in order to overcome the limitation of the Lagrangian mesh. Details of the hydromechanical formulation implemented in the LAGAMINE FE code in a large strain context are presented.Application cases (reproduction of sandbox simulation, study of a hydrocarbon trap) concentrate on the study of the strain localisation and potential fracturation using different criteria. In the first case, re-meshing technique allowed to reproduce successfully analogue experiment of thrusting propagation. In the second case, a detailed study based on different initial conditions has brought new insight to the reactivated origin of some faults and has allowed to obtain information on the potential fracturing of the hydrocarbon reservoir unit. As an academic case, the study of anorthosite diapirism is carried out, which confirms the validity of the petrological model of diapirism. Eventually, the hydromechanical coupling effects between a layered porous medium and a fault are illustrated on a simple case

SEALEX in-situ experiments-performance tests of repository seals: experimental observations and modelling

The paper describes observations and numerical analysis of SEALEX performance tests installed in Tournemire Underground Research Laboratory (URL). One of the objectives of the large scale in-situ tests is to investigate the impact of technological gaps on the long term performance of bentonite based seals. The swelling cores consist of pre-compacted blocks of a natural sodic Wyoming bentonite (MX80 type) mixed with quartz sand in a ratio of 70/30 (in dry mass) with different geometries (monolithic disks or four jointed disks). Several technological gaps exist within the in situ tests: Gaps between the blocks and annular gap with variable width between the bentonite-based core and the host rock. All the tests are extensively instrumented for monitoring the main Hydro-Mechanical (HM) variables. Comparison of the experimental results showed that the presence of technological gaps constituted new hydration sources (annular gaps) and flow paths (gaps between the blocks) that changed the saturation kinetics. A coupled HM formulation that incorporates the relevant processes involved in the problem under consideration has been adopted to analyse the effect of the annular technological gap on dry density homogenization of the bentonite based core as hydration progresses. Technological gaps were demonstrated to have an impact on dry density distribution

Understanding oxidizing transient conditions in clayey rocks

International audienceLarge amounts of carbon steels are foreseen in most deep geological disposals of high-level radioactive waste. The disposal cells will be initially aerated and the ventilation will renew oxygen at their front edge for years to decades. Excavation damaged zones (Edz) along underground works (tunnels, drifts) may promote oxygen migration. The diffusion and reactivity of oxygen within such zones is still poorly known despite that steel corrosion is significantly enhanced in the presence of oxygen. With this respect, four tests were implemented in the Tournemire Underground Research Laboratory, France. In each test, the oxygen partial pressure and the total pressure within a chamber isolated from the atmosphere by a packer were recorded over 90 to 150 days. Oxygen was never completely depleted in the chambers, likely due to oxygen diffusion through the Edz, whereas pyrite oxidation within the Edz was themain oxygen sink. Four in situ tests have been carried out, with differences in terms of geometries (volume and surface of chambers) and drilling modes (compressed air flushing vs. pressurized argon flushing). A sequential global optimization of the relevant parameters (diffusion and reaction) has been developed, aiming at reproducing as closely as possible the measured four in situ tests data by modelling. The key controlling parameters are shown to be the Edz thickness and the ratio De/kre (effective diffusion coefficient over oxygen consumption first-order effective rate), whose optimum was found between 23 m 2 and 45 m 2 . The Edz extent active for oxygen diffusion ranges between 0.6 and 1.0 borehole radius, with mean water saturation of 0.7, a mean D e of 6×10 −7 m 2 .s −1 and a mean k re of 2×10 −8 s −1 (or k r = 6×10 −7 s −1 per equivalent pore volume). With respect to the safety assessment of radioactive waste disposals, any plug system that would not intercept the Edz could not ultimately prevent oxygen entry into the structure, which could have an im- pact on the dimensioning of carbon steel components and disposal concepts. The proposed optimization methodology could be used to quantify diffusion and reaction kinetics in other contexts where pyrite reactivity and/or oxygen diffusion in water unsaturated media play a key role, such as the leaching of mining ores and acid mine drainage

Gravity induced tectonics in the Rogaland anorthositic province, Southwest Norway

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