Methodology and results obtained from a hydraulic characterization campaign in granite rocks are described. The study is included in a research project developed for the Geological Survey of Spain (IGME, Spanish acronym).
The investigation area is constituted by a surface postcinematic granite pluton. It is located in the western sector of the Spanish Central System, at about 100 km from Madrid City (Spain), and includes two main granitic facies: biotitic monzogranites with megacristals and biotitic leucogranites. The surface fracturation is not very significant. Systematic measuring shows a main frequency of fractures with direction N 90° E and subvertical dipping. Reconnaissance boreholes have found two zones associated to tardihercinic or alpine faults located at depths of 66.6 m and 93 m respectively.
The outcropping granite is very homogenous and is being exploited as an ornamental rock. It is extracted from block quarries under different commercial names. In one of these quarries the IGME has drilled an investigation borehole, 200 m depth, 116 mm diameter, in order to carry out the tests.
Hydraulic tests have been provided by means of the Hydrogeological Mobile Unit of low permeability formations (UMH), developed by the IGME. It allows the performance of different types of hydraulic tests, with a range of flow rates for injection tests between 0.0005 L•min-1 to 40.3 L•min-1. Tests are able to do with single or double packer device, up to 1200 m depth. The interpretation of the tests was done by means of the Hytool code.
The methodology applied comprises two phases: the overview phase consists in carrying out injection tests in a quasi-steady-state throughout the length of the borehole, in sections measuring approximately 10% of its length, with a double packer device, and one transient-state injection test of the whole column. In the detailed reconnaissance phase, test section length is shorter, normally between 1 and 6 m, depending on the fracture analysis. General test sequence begins with a constant-head transient-state injection test and the subsequent pressure fall-off period.
A total amount of 40 hydraulic tests have been done: 10 quasi-steady-state injection, 13 constant-pressure transient-state injection, 1 constant-rate-of-flow transient-state injection, 5 pressure fall-off, 2 slug and 9 pulse.
Hydraulic conductivity values obtained varies from 3.24•10-7 m s-1, for the upper weathered layer, to 8.20•10-13 m s-1, for the test section 97.36 to 116.15 m depth. Also, it was observed a slight increase in the hydraulic conductivity between 63 m and 96 m, where two faults are intercepted by the borehole. As a result, values of permeability are lower than data obtained from studies in other granitic basements. The most abundant value was N•10-10 m s-1 to N•10-11 m s-1. 60% of the results were included in the mentioned interval; due to the fact that the test site was selected for the small deformation history and the lower amount of fractures.
