Borehole Geophysics, Hydraulic Characteristics and Chemistry of Groundwater Flow in Fractured Granite With Very Low Permeability

Three test boreholes 10.9 and 10.5 meters apart were drilled in a compact granite at locality Podles' in the Bohemian Massif of central Europe. The depths of the drill boreholes were 349, 300 and 296 m. The location of the boreholes form a triangle. The main goal of this study was to determine the degree to which the very compact granite is fractured and what is the hydraulic conductivity of the fracture system. A combination of neutron log, electrical resistivity logs, gama gama log, fluid-resistivity log, and acoustic log was used to test the function of the fracture system. The hydraulic connection among the boreholes was determined from the fluid-resistivity log using injected salt as a tracer. The pressure communication through fractures among boreholes was investigated by a set of water pressure tests (WPT) in one borehole with simultaneous monitoring of responses in the other two holes equipped by a multipacker system. The connection of selected permeable fracture systems was verified by a combination of hydraulic stress tests in one borehole and simultaneous fluid-resistivity logging in other two boreholes. Indication of salt in water in adjoining boreholes was registered as anomalies on curves of the fluid-resistivity record. Several communications between boreholes were along a horizontal level, however, some connections were through combination of both horizontal and vertical fractures. The hydraulic connection between two of the boreholes was fast and straightforward. The connection with the third borehole was inexpressive. This difference was due to the position of the boreholes with regard to the direction of main fractures and the direction of natural groundwater flow. All fractures were identified using acoustic tele-viewer and inspection of core. The density of the fractures varied with depth. The density was 3 fractures per meter near surface, the density dropped to 0.9 fractures per meter at the depth of 80 m. Between 80 and 300 m, the density increased with depth to the maximum of 8.8 fractures per meter. The hydraulic conductivity of the fractured granite was investigated with WPT and slug tests. We measured the water loss at pressures 2.7x10-1 and 4.5x10-1 MPa. The water loss was tested in sections separated with the packers. The thickness of the sections varied from 1 to 50 meters. The water loss below 150 m was extremely low from 5x10-5 to 4x10-4 liters per minute per meter. Above 100 m depth the water loss varied from 3x10-3 to 7x10-1 L/min/m. The weathered zone to a depth of 25 m had a permeability of 2x10-6 m/s. The zone of opened fractures reached to a depth of 95 m with permeability from 2x10-9 to 2x10-7 m/s. Granite below 95 m had a dense net of fractures, however, the fractures were closed so that the permeability was very low from 2x10-12 to 4x10-9 m/s. After the logging and hydraulic tests, samplers of water were installed in four sections of one of the drill holes. Very small quantities of water are periodically pumped out and analyzed. Electric conductivity and chemical composition of groundwater from the granite returned to a natural composition not affected by injection of salt after several months. Water from the deepest section (230-296 m) reflects the influence of local stream water while water from the shallower sections reflects a prolonged or more intensive interaction with granite. This indicates also the complexity of the fracture system. The data are used to build a specific simulation model to represent groundwater flow and water-rock interaction in fractured granite.