1.1 What is a Fractured Aquifer
Fractured aquifers are geological materials of low primary permeability in which groundwater flow takes place mainly through voids bounded by fracture faces (Domenico & Schwartz, 1990; Freeze & Cherry, 1979; Singhal & Gupta, 2010). These voids, which are much better connected than matrix pores, are commonly called fracture porosity and can constitute an effective conduit for groundwater flow.
Medium- to high-grade metamorphic and intrusive igneous rocks are geological materials in which groundwater flow takes place almost exclusively through fracture porosity. Groundwater flow in fractures is also significant in fine-grained sedimentary rocks (Figure 2), where fracture flow often constitutes the most important flow conduit. In these lithologies, such as shale, some limestones and well-cemented sandstones, the intergranular system of the rock matrix is poorly connected. Sedimentary rocks usually bear both fracture and intergranular permeability.
Figure 2 – Fracture flow may be significant in sedimentary rocks, mainly the ones with fine texture (silt, clay) such as mudstone (a), siltstones (b), some limestones (c), as well as hard sandstones (d). In poorly cemented sandstones (e,f), fractures may be absent or appear only in hardened portions of the formation. In (e) the dense fractures in the basalt (top) do not propagate into the sandstone (bottom). In (f) the fractures in the sandstone have propagated only to a limited depth. On the other hand, the fractures in the hard fine sandstone (d) are dense and persistent (photographs a, b, d, e, f: Amélia Fernandes; photograph c: Alain Rouleau).
As shown in Figure 3, small fracture apertures, ranging from 0.01 to 0.05 mm, are much more frequent (Van Golf Racht, 1982, in Domenico & Schwartz, 1990). Fracture aperture values typically show a skewed distribution, as most of them are indeed small. However, the few large-aperture fractures are significant for providing the overall transmissivity and preferential pathways through the rock mass. A density of one fracture per meter, considering the most frequent aperture range of 0.01 to 0.05 mm, renders hydraulic conductivity (K) values from ~10−8 to ~10−6 m/s.
Figure 3 – Hydraulic conductivity of an interval of a rock mass containing a number of idealized fractures. a) Vertical fractures in a limestone. b) Representation of vertical fractures in a block diagram of the rock mass. c) Small apertures, ranging from 0.01 to 0.05 mm, are the most frequent (Van Golf Racht, 1982, in Domenico & Schwartz, 1990). d) Cubic law equation (Snow, 1968), which is used to calculate the flow through a section of rock mass perpendicular to the fractures. Considering the density (N) of just one fracture/meter, the most frequent apertures (e) provide hydraulic conductivities (K) ranging from ~10-8 to ~10-6 m/s.
As shown in Figure 4, K values for fractured igneous and metamorphic rocks vary by five orders of magnitude, from 10-8 to 10-4 m/s, the greater being a response to the presence of the less frequent but larger apertures. The values are similar to those obtained in field tests by many authors (e.g., Rouleau et al., 1996; Shapiro et al., 2007; Parker et al., 2018). Most aperture determinations come from hydraulic tests in wells, where transmissivity is measured and the cubic law is used to calculate the so-called hydraulic aperture. The hydraulic aperture is the opening width of an ideal fracture between smooth and parallel walls that would produce the measured transmissivity. Thus, the value approaches the effective mechanical aperture. Unfractured crystalline rocks have extremely low K values comparable to those of shale and clay (Freeze & Cherry, 1979) as shown in Figure 4a. Rock cores used in laboratory tests are small samples usually devoid of the fractures observed on rock exposures. Thus, in those tests, the calculated K values are similar to the ones of unfractured crystalline rocks (Figure 4b).
Figure 4 – a) Values of permeability (k) and hydraulic conductivity (K) for different geological materials. K values for the fractured igneous and metamorphic rocks (between the blue lines) vary by five orders of magnitude. Unfractured crystalline rocks have extremely low K values comparable to those of shale and clay (modified from Freeze & Cherry, 1979). b) Hydraulic conductivity of a fractured orthoquartzite formation, estimated at laboratory and field scales (modified from Rouleau et al., 1996).