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.