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.

Introduction

In the last decades, hydrogeological research has increasingly focused on rock environment with low permeability, especially related to the storage of waste or energy in geological formations.1, 2, 3, 4, 5, 6 The study of hydraulic properties in the vicinity of underground constructions including deep geological repository of radioactive waste is a prerequisite for safety analysis.

Hydrogeological research of crystalline rocks is in many cases focused on identification of preferential pathways of groundwater flow in open fractures and fault zones.7, 8, 9, 10 Conductive fractures constitute only a small part of the total volume of rocks. Fracture porosity of solid rocks ranges from 0.1 vol % to less than 0.0001 vol %, depending on the degree of fracturing and the depth of the rock below the surface,^11,12 as well as on the geometry of the fracture system and the characteristics of single fractures.^13,14 Drill logging at depths up to 120 m in granitic rocks of the Bohemian Massif at study sites shows that the proportion of significant open and permeable fractures ranges from 2.5 to 10 % of the total number of fissures. A similar proportion was also found at other locations.¹⁵ Bear et al.¹⁶ estimated a portion of permeable fractures up to 20% and according to Samaniego and Priest¹⁷ only 10–20% of fractures form an interconnected network. Generally, geological environment is formed by hard rocks with rare macrofractures and mostly closed or filled cracks of local importance are present. Intact solid rock (rock matrix) has hydraulic conductivity by several orders of magnitude lower than open brittle faults. Still, the rock matrix is not completely impermeable. A system of interconnected microfissures exists even in the intact solid rock matrix. It leads to spreading of groundwater and dissolved substances by diffusion or slow dispersion.18, 19, 20 The rock matrix acts as a retention part of rock environment due to its large volume and it significantly reduces the speed of migration of pollutants into biosphere.

Field measurements of hydraulic properties of the rock matrix and low permeable rocks require precise and sensitive measuring devices and the use of special measurement methods. The standard pumping tests and measuring equipment are not suitable for this purpose.

Standard WPTs in situ usually can test rocks with hydraulic conductivity in a range from 10 to 11 to 10-6 m·s^-1. The lower limit of applicability of standard WPTs is determined by the sensitivity of flowmeters in measuring devices, generally with a limit close to 1 mL min^-1. The hydraulic conductivity corresponding such limit is in order 10^–11 to 10^-9 m·s^-1 depending on the test pressure and on duration of the test. For example, the measuring device PSS3 (SKB downhole pipe string system) used for measurements in deep boreholes by SKB in Sweden has flow measurement limit 1 mL min^-1.²¹ A similar measurement unit UMH (Hydrogeological Mobile Unit) used by the Spanish Geological Survey IGME is able to measure a flow as low as 0.5 mL min^-1.⁴ The unit used by Czech Geological Survey has flow rate limit of 1 mL min^-1 too. Although there exist flowmeters with higher sensitivity lower than 1 mL min^-1, their use in field measuring devices is limited by narrow measuring range and/or difficulty in handling such devices under field conditions.

In deep boreholes with very low permeable rocks, tests below the detection limit of the standard WPT devices represent an important number of the measurements. For example, Walker et al.²² report 10% and 40% of the tests bellow the equipment detection limit for two different experimental sites in Sweden. This percentage can be even higher for individual boreholes. For example, 63% of tests were below or close to the measurement limit in the borehole of KFM01D at Forsmark site.²¹

Pulse tests specially developed for rocks with very low hydraulic conductivity are usually used in nearly impermeable environment. Using a different kind of tests causes problems in the evaluation and particularly in comparison of results provided by various methods. The coefficients of hydraulic conductivity obtained by various test methods in the same borehole interval can vary by several orders of magnitude.^4,23 The deviations are caused not only by different types of the tests but also by using different evaluation equations and testing different volumes of rocks.

Hydraulic conductivity of intact rocks has been mostly measured under the laboratory conditions on samples obtained from drill cores.24, 25, 26, 27 These samples are affected by strain and manipulations during the mining and processing.

New research works are more focused on the application of 2D numerical and 3D hydrogeological models that extend the small scale to a wide area.^{6,28,29,30,31} The problem of “scale effect” of solid rocks with fracture permeability was investigated by many authors in the past.³² The majority of reports deal with a comparison of results of hydrodynamic tests in boreholes with different length of test sections, a comparison of different types of tests and they summarized data from different types of hydrodynamic tests from different locations.^8,33,34

The Czech Geological Survey has developed a device which allows testing of the hydraulic properties of low permeability rocks in situ. It enables measurements of the hydraulic conductivity of the rock matrix in situ to be compared with laboratory measurements of hydraulic conductivity performed on borehole core samples. Measurements were carried out in eight types of granitic rocks in the Bohemian Massif. The survey was accompanied by a geological assessment of the regional fracture network at the studied sites and by the study of microcrack networks on thin sections of those rocks.

The main aim of this article is the presentation of new equipment for measuring very low hydraulic conductivity of rocks in situ, and the comparison of results of the measurements in granites with different rock structures and degree of fracturing at eight studied sites.