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Physical Characteristics of Fractured Rock Controlling Flow and Transport

In fractured rock, individual rock fractures, blocks of rock matrix, and fracture infilling can each have discrete porosities that need to be understood to identify those features that control flow, storage, and transport. Characteristics of those features need to be well enough understood to discern what of the rock’s void space can be considered, from a hydrological engineering point of view, to be active or inactive. Matrix and fracture porosities contribute differently to contaminant fate and transport, and although the importance of multiple porosities has long been recognized within academe, much engineering and hydrogeologic work in general practice is dependent on tools that cannot accommodate multiple porosities. This chapter introduces the geologic, geomechanical, geochemical, geobiological, and hydrogeologic contexts to understand and effectively characterize rock fracture and matrix geometries and properties. This information provides the necessary basis for understanding fate and transport processes (described in Chapter 3), modeling (described in Chapter 4), characterization (described in Chapter 5), and remediation of fractured rock (described in Chapter 6).

This chapter begins with a discussion of qualitative aspects of rock fractures. The chapter then focuses on the quantitative description of rock fracture and matrix porosities, including for example, the concept of “sets” of similar fractures, descriptions of fracture orientation, extent, intensity (number of fractures in a given area), spatial pattern, and infillings. The final section focuses on how understanding fracture genesis and evolution over geologic time can assist understanding fate and transport in fractured rocks. The geologic, geomechanical, and geochemical processes that cause fractures to form and evolve allows the geometry and relevant characteristics of fractures and rock blocks to be predicted and quantified.

QUALITATIVE FRACTURE DESCRIPTION

From a rock mechanics viewpoint, a fractured rock mass is a system that includes intact blocks of rocks surrounded by fractures and other discrete features such as dikes, brecciated layers, and features related to karst (e.g., Hoek and Bray, 1981). Fractured rock systems, however, can vary greatly geologically, geomechanically, geochemically, geobiologically, and hydrogeologically. Thinking of fractured rock as a system is appropriate when considering fluid flow, storage, and transport in the rock. Characteristics of one part of the system can affect fluid behavior in other parts of the system, so it becomes necessary to identify and understand the discrete features that control behaviors of interest. Fracture flow and transport properties are sensitive to in situ stress through shear and normal displacements, producing changes in fracture aperture and roughness. They are also sensitive to geochemical conditions through sorption and precipitation processes. Such processes are discussed further in Chapter 3.

An individual fracture can be characterized as a separation of the rock (“face”). It consists of two or more surfaces with variable spacing. The space is described as a physical aperture and is generally filled with some combination of gases, liquids, and geologic materials. The geologic materials can consist of gouge, breccia, coatings, and minerals placed or formed in the fractures by chemical, advective, biological, or geomechanical processes. Fracture aperture is further discussed in Box 2.1. The surfaces themselves are rarely simple planes and need to be characterized in terms of their roughness, planarity, and undulation. Local roughness features are referred to in rock mechanics as asperities. Fracture persistence refers to continuity of the fracture—the percentage of the fracture surface that has a non-zero aperture (i.e., it does not include infilled fractures) (Ulusay and Hudson, 2007).¹ Individual fractures can be defined by the detailed three-dimensional coordinates of all of the faces. However, it can be more convenient to describe individual fractures by their location (center), orientation, size (extent in different directions), and physical aperture distribution.

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¹ The term “persistence” has conflicting usages in the rock mechanics literature, including uses to indicate fracture size, intensity, and continuity. This report refers to persistence in terms of fracture continuity, in a manner consistent with Ulusay and Hudson (2007).

Fracture channels form where portions of fractures are blocked preferentially (e.g., by mineralization or local asperity contacts) and where portions of fractures are locally more permeable (i.e., due to localized dissolution, mechanical processes at fracture intersections, or mismatches between asperity patterns on fracture faces). Fracture channels, as discussed below, can be important because they influence groundwater residence times and reactive surface areas. Multiple fractures within a rock mass can be described most effectively by grouping fractures with similar spatial patterns, extents, and orientations into sets. Physical characteristics such as roughness, infillings, physical apertures, and planarity can then be described for the fracture sets as variability distributions among the fracture population and correlations between properties (e.g., between orientation and extent, or between extent and physical aperture).²

QUANTITATIVE FRACTURE DESCRIPTION

The ability of fractures in rock aquifers to transmit groundwater will vary over many orders of magnitude (Freeze and Cherry, 1979), as confirmed in transmissivity measurements³ of individual or closely spaced fractures in boreholes in various rock types (Shapiro and Hsieh, 1998; Novakowski et al., 2000). Changes in transmissivity in fractured rock aquifers can also be abrupt, so hydraulic properties of fractures cannot be assumed to be uniform. For this reason, it is important to develop an appropriate geomechanical framework of fracturing for use as the basis to infer groundwater flow pathways. Transmissivity is further discussed in Chapter 3.

Fracture set spatial patterns can be quantified by defining the functional variation in fracture intensity and the interactions between fractures (e.g., Rogers et al., 2015). The intensity of a given set may be lower if near another set, or if the fracture orientation of a given set may be defined at a specific orientation relative to that of an adjacent fracture set. Fractures terminating against other fractures can control the fracture size. The mean spacing between fractures can be used to parameterize fracture intensity; however, for many practical and theoretical reasons, fracture intensity is better formulated as volumetric intensity (Dershowitz et al., 2000). The spatial pattern of fractures can also be parameterized in terms of termination percentages between fractures of different sets (Ulusay and Hudson, 2007) and by fracture intersections (Dershowitz, 1985).

Figure 2.1 illustrates a continuum of fractured rock intensity. At one extreme (see Figure 2.1a), the fractured rock can have very high fracture intensity. When characterizing such intensity in a site conceptual model (see Chapter 4), this type of system sometimes can be assigned a single porosity, as would a homogenous media or a soil. At the other extreme (see Figure 2.1c), there may be very few fractures of hydrogeologic significance, and those fractures would need to be included explicitly and deterministically in a site conceptualization, just as they would be in a conventional faulted porous media model. Both the fractures and matrix need to be adequately characterized in a site conceptualization for fractured rock environments between these two extremes (see Figure 2.1b). For such systems, there are numerous discrete fracture pathways to consider explicitly or stochastically, and there can be large differences between fracture and matrix flow velocities. Such interactions between fracture and matrix porosities affect transport. Box 2.2 lists fractured rock characteristics other than aperture (see Box 2.1) that affect contaminant fate and transport.

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² The extreme tails of the distributions can be important for flow and transport and may necessitate large sampling sizes to characterize properly.

³ Transmissivity is the volume of flow per unit time per unit width.

Description of Rock Matrix

Similar to fractures, rock matrix between fractures and other discontinuities are characterized by a combination of geometric (e.g., block size, shape, and spatial pattern), hydraulic, and transport

properties (e.g., hydraulic conductivity, porosity, storativity, tortuosity, minerology, and sorption characteristics). Matrix properties also vary spatially due to geologic, geomechanical, geochemical, and geobiological histories and current in situ conditions (Bibby, 1981; Berryman and Wang, 1995). Fracture properties also evolve (Tsang, 1984; Long and Witherspoon, 1985). Boxes 2.1 and 2.2 include rock matrix characteristics that may contribute to fate and transport in fractured rock. Igneous and metamorphic rocks have minimal matrix porosity that can range from less than 1 to 2 percent of the rock volume (e.g., Dietrich et al., 2005). Sedimentary rocks typically have higher matrix porosities, ranging as high as 30 to 45 percent of the rock volume. Table 2.1 includes one set of values derived for total and effective intact rock porosity and hydraulic conductivity. Other values are also used (e.g., those developed by the Argonne National Laboratory).⁴

Rock mass storage capacity can be underestimated if hydraulically inactive fracture porosity is not considered. From a transport standpoint, void space in rock can be divided into three types of porosity: mobile, immobile, and inaccessible. Mobile porosity is the percentage of the total volume of void space (matrix and fracture) that contributes to advective migration—this can be considered active void space. Immobile porosity is the combined volume of interconnected matrix porosity and hydraulically inactive fracture porosity divided by the total volume. Inaccessible porosity is the void volume of the rock not accessible by fluids or diffusing compounds, divided by the total volume. Hydraulically inactive fractures can become reservoirs for immobile fluids and contaminants. The increased storage can dominate contaminant transport in some rock types, particularly where the matrix porosity is low. The importance of immobile porosity in contaminant transport in fractured rock systems cannot be overstated. Continued diffusion from mobile to immobile porosities results in changes in the distribution of contaminant contained within a rock system. Contaminants may be contained initially in the mobile porosity, then migrate to be found primarily in the immobile porosity. Errors in travel time predictions, concentrations in hydraulically active fractures, and plume sizes can result from poor understanding of immobile porosity.

IMPORTANCE OF FRACTURE GENESIS

Fracture geometry and properties are determined by the combined geomechanical, geochemical, and geobiological processes that control fracture genesis and fracture evolution. It is therefore critical to understand the geologic setting to predict the location of fractures and their properties. NRC (1996) provides a good review of the relation between underlying geology and fracturing. Ways in which geology, geomechanics, and geochemistry influence fracture patterns relevant to fate and transport are described in Box 2.3.

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⁴ See http://web.ead.anl.gov/resrad/datacoll/porosity.htm (accessed August 21, 2015).