Abstract In the underground laboratory of Meuse/Haute-Marne (Bure) in France, different fracture types have been intensively investigated. Within a co-operative project between BGR and ANDRA, geophysical measurements and borehole gas tests were conducted in three well-designed boreholes in the gallery GRM to characterize the fracture structure and to determine the gas tracer velocity within fracture networks. On the basis of seismic measurements and nitrogen gas interference tests, helium was injected into an interval of borehole OHZ3003, where an unloading joint has been identified. The injection took place in the form of a 10 min ‘pulse’ with an injection pressure of 2 bar. The other two boreholes, OHZ3002, where an upper part of the shear-mode fractures (‘chevron’ pattern) dominate, and OHZ3001, where shear-mode fractures (subvertical ‘oblique’ fractures and ‘chevron’ pattern) exist, served as observation holes. To maintain the pressure gradient between the injection hole and the observation holes, nitrogen gas was subsequently flushed into the injection hole. Two breakthrough curves of helium concentration and pressure developments in the two observation holes were continuously monitored using two helium leakage detectors and pressure gauges. To interpret the measured pressure and concentration data, numerical models were constructed. A 3D model was used to simulate nitrogen gas flow and 2D models were applied to simulate a helium transport process. The real volume of the injection interval was considered in the model and the experimental process was simulated. Using the calibrated transport parameter data for a helium tracer from previous studies in the Mont Terri Rock Laboratory, the calculated breakthrough curves agreed well with those obtained from measuring the variation in permeability. The permeability derived from the helium tracer test agrees well with the estimation obtained from the nitrogen gas tests.

Abstract Permeability and its spatial distribution around an underground opening in a geological formation are important for the interpretation of thermal, hydraulic and mechanical findings from an in situ demonstration experiment. Within the site characterization programme of the Full-scale Emplacement (FE) experiment, permeability measurements with nitrogen gas have been conducted from six short boreholes. Four of them were located in a section without shotcrete support and two in a section with a three-layer-shotcrete lining. As expected, the extension of the zone with an increased permeability was larger (up to 2 m) in the area without shotcrete support than that in the section with a shotcrete lining (less than 1.5 m). The water content in the sections with or without shotcrete linings also showed different behaviour over long-term monitoring. The water content in the deep borehole section in the area with a shotcrete lining stayed almost constant, while the water content in the deep borehole section in the area without shotcrete tended to continuously decrease. In general, the water content close to the tunnel is influenced by the seasonal change in the temperature and relative humidity within the tunnel, especially in the section without a shotcrete lining. Analysis of the abovementioned observations/findings was done by performing FEM (finite-element method) calculations with OpenGeoSys (OGS) software using a coupled hydromechanical model. Owing to the high stiffness of shotcrete, the displacement in the section with a shotcrete lining was smaller. This, in turn, results in a smaller extension in the excavation damaged zone (EDZ). However, shotcrete has a relatively high suction capacity and high initial water content: thus, the interface between the shotcrete and the Opalinus Clay becomes more saturated. Therefore, the excavation-induced fractures in the Opalinus Clay close to the shotcrete can be sealed by swelling. The water content decreases continuously, as a result of desaturation occurring during the operational phase and the associated change in porewater pressure.

Abstract To investigate gas-migration processes in saturated low-permeability argillaceous rocks, gas-injection tests under different injection pressures were carried out at different scales: on core samples at the laboratory scale; in the packed-off section of boreholes at the borehole scale (HG-B); and in the sealed microtunnel at the tunnel scale (HG-A) – a 1:2 scale experiment at the Mont Terri Rock Laboratory, Switzerland. All three tests at the Mont Terri Rock Laboratory involved Opalinus Clay. A fully coupled hydromechanical model has been developed that takes account of elastic and plastic anisotropies, anisotropic two-phase flow based on the van Genuchten function, and permeability changes when evaluating the experimental data. Two different flow regimes were studied: two-phase flow under low gas-injection pressure and dilatancy-controlled gas flow under high gas-injection pressure above the confining pressure in the laboratory experiment or the minimal principal stress in situ . When dealing with the dilatancy-controlled gas-flow regime, special consideration was made by applying two permeability approaches in which (i) the permeability change was pore-gas-pressure dependent and (ii) where the permeability change was deformation dependent. Using the parameter values determined by laboratory data, the in situ borehole tests obtained under well-defined hydromechanical conditions could be analysed accordingly. The gas-flow regime in large-scale experiments, as in the case of HG-A, is mainly governed by experimental circumstances: in this case, the excavation-induced fractures around an opening with a permeability four order of magnitude higher than that in the undisturbed rock mass.