Comparison of hydraulic conductivity of rock matrix and fractured blocks of granitic rocks

Abstract

Field measurements of hydraulic properties of the rock matrix and in low permeability hard rocks generally require highly accurate and sensitive technical equipment and special methods of measurement. Hydraulic conductivity of intact rocks is mostly measured in the laboratory conditions; field measurements are less common. The Czech geological survey developed equipment, which allows measurements of very low consumption of water during WPT (water pressure tests). This device enables to implement the same testing methodology for all sections of the borehole and subsequently compare the obtained results from low-permeability as well as more permeable sections. Hydraulic conductivity of eight different granitoids was studied in the Bohemian Massif in the Czech Republic. Mean values of hydraulic conductivity of fractured rock are on the order of 10-8 to 10-7 m·s-1, while hydraulic conductivity of rock matrix was most frequently on the order of 10–11 to10-12 m·s-1. The difference of values measured in laboratory and field is caused by scale effect. The significance of scale effect in rock matrix is determined mainly by the connectivity of a network of microfissures. Length and aperture of microfissures govern mainly the connectivity of fissure net in rock matrix. Coarse-grained granitoids have greater length and aperture of microfissures, higher connectivity and hydraulic conductivity, and the scale effect is less significant. The fracture network appearance and hydraulic conductivity of rock matrix is similar and characteristic for different types of granitoid rocks.