Rock permeability is an essential parameter for the long-term safety performance of a repository of radioactive high-level waste (HLW). To characterise hydraulic properties of fractured rock and to determine this important parameter, hydraulic tests with a single or multiple packer system in the deep borehole were carried out in Beishan, Gansu Province, NW China—the most potential site for China's geological repository for high level radioactive waste. In the granitic rock, fractures do often exist in form of a network. If water was injected into the packed-off interval, where fracture network was observed, the water demonstrates complex flow regime due not only to the single fracture properties (aperture, roughness, permeability) but also to the fracture furcation. Within a Chinese-German co-operation program between BRIUG and BGR, fracture data from deep geological borehole from Beishan site has been analyzed. These data are from the core mapping and borehole scanning. On the basis of the geological analysis, a fracture network model is therefore built. 3D numerical model including fracture network, borehole interval, and packer is built to be used to interpret the data from the water injection test. Different flow regime in a fracture network has been analyzed.
The permeability of rock plays an essential role regarding the barrier effect of the potential formation for the disposal of radioactive wastes. The knowledge of the permeability and their spatial distribution in the rock and especially around the underground excavation is a basic element for the safety analysis of a final waste repository.
The Federal Institute for Geosciences and Natural Resources (BGR) carries out the in-situ permeability measurement in salt rock (Gorleben site and Morsleben site), in granite (Grimsel Test Site and Hard Rock Laboratory Äspö) and in clay stone (underground rock laboratories Mont Terri and Bure). As hydraulic borehole test, different methods can be used for the determination of formation permeability. The principle of the different test methods is the increase in pressure in the pore area of the formation by injection of a test medium and the measurement of the pressure changes resulting from it. We follow the classical borehole test method using movable packer system for the in-situ permeability measurement. For the expected low permeability of the undisturbed zone of the formation, pulse test using gas (e.g. Nitrogen) as test medium is best suitable (Zaretzki et al., 2013). In the fractured and disturbed zones (Himmelsbach et al., 2003, Marschall et al., 1999), where high permeability is to be expected, constant pressure or constant rate injection tests with the measurement of the flow rates can be accomplished. In the water saturated or partially saturated formation, a stepwise increasing of gas pressure injection tests (SGPT) may be introduced to be performed in order to determine the gas entry pressure (Shao & Lege, 2004). In order to obtain more precise value of rock permeability, a test methodology has been developed including total system calibration and three-dimensional numerical evaluation.
