Characterization, Modeling, Monitoring, and Remediation of Fractured Rock (2020) / Chapter Skim
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3 Flow and Transport: Underlying Processes
3 Flow and Transport: Underlying Processes
Pages 26-45
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From page 26... ...
. Project objectives and fracture size characteristics relative to the physical dimensions of the phenomenon or site of interest may require, however, that effective media properties be used to represent fluid flow and contaminant transport in fractured rock (see Chapter 6)
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From page 27... ...
. Structural Controls on Hydrodynamic Dispersion Dispersion due to mechanical effects in flow pathways results from mixing at fracture intersections, fracture tortuosity (the length of the actual flow path relative to the linear distance between two points)
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is an inherent and critical characteristic in fractured rock masses and has a pronounced effect on all flow-related problems, and equivalent continuum soil-derived approaches need to be used with caution and appropriate qualifications. Dispersion and Transport in Cyclic Flow Another transport mechanism results from the combination of hydrodynamic dispersion and cyclic fluid flow whereby inflow-outflow cycles (with zero remnant invasion at the end of the cycle)
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From page 29... ...
CHEMICAL PROCESSES: DIFFUSION AND REACTION Although fractures dominate advective transport in most fractured rock, the volume of void space in the rock matrix is, in most cases, orders of magnitude greater than the volume of void space in the fractures. This skewed volume ratio results in the matrix accounting for the majority of the contaminant storage in fractured rock settings.
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From page 30... ...
Given the geothermal gradient, analyses need to incorporate increasing diffusion rates and decreasing fluid viscosities with increasing temperatures. Both diffusive and advective processes are also affected by the tendency of deep aquifer rocks to have lower matrix porosities and narrower fracture apertures.
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A constant concentration of contaminant enters the left side of a fracture (distance = 0 m) in an infinite rock block (perpendicular to the plane of the fracture)
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relevant to fractures and fracture networks. Geochemical reactions, advective contaminant transport, and diffusive transport rates control the evolving topology of dissolution (Steefel and Lichtner, 1994; Szymczak and Ladd, 2013)
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Eventually, large-scale instabilities may follow. BOX 3.3 Relationship Between Chemical Reaction Rates and Advective and Diffusive Transport Rates The relationship between chemical reactions and advective and diffusive contaminant transport rates can be captured in two dimensionless numbers: Peclet's number, defined in Box 3.2, and the Damköhler number αl Reaction rate Da = = ν Advective transport rate where v is flow velocity (in meters/second)
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From page 34... ...
Within these environmental niches, they form complex communities that can act as catalysts for a wide range of chemical reactions, including nucleation of gas bubbles that change water saturation; precipitation of minerals that cement fractures; dissolution of minerals that widen fractures; degradation of toxic organic contaminants; and formation of biofilms that hinder fluid flow and lead to clogging (Baveye and Valocchi, 1989; Kindred and Celia, 1989; Taylor and Jaffe, 1990a,b; Taylor et al., 1990; Clement et al., 1996; Ehrlich, 1999; Ross et al., 2001; Wagner et al., 2013)
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From page 35... ...
Although biodegradation within the primary porosity of the rock matrix has been hypothesized as a natural attenuation mechanism that may reduce back diffusion loading to fractures, little is known about the presence, health, or activity of microbial communities in rock matrices. Common Bio-Mediated Geochemical Cycles Microbial communities can participate in the biogeochemical cycling of many elements, including carbon, nitrogen, sulfur, and numerous metals (Taylor and Jaffe, 1990a; Clement et al., 1996; Krumholz et al., 1997; Ehrlich, 1999; Geller et al., 2000; Ross et al., 2001; Wagner et al., 2013)
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From page 36... ...
High methane concentrations can result in the formation of bubbles that may also affect fluid flow. Consequently, when groundwater transports compounds such as iron, manganese, sulfur, and CO2 from suboxic to oxic environments or vice versa, chemolithotrophic organisms1 can flourish, oxidizing or reducing dissolved forms to generate less soluble forms that can become clogging precipitates, dissolving insoluble forms to widen fractures, and generating gases that become bubbles.
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From page 37... ...
Thus, even if TCE is not found as a separate phase in the rock matrix, it may be present in the rock matrix at concentrations that are a risk to human and ecological health. The relationships presented in Box 3.4 lead to the following general observations related to mixed fluid conditions in fractured rock masses: • Capillary forces o Affect contaminant invasion; variations in fracture aperture and matrix pore size result in a complex contaminant distribution in fractured rocks.
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From page 38... ...
Experimental evidence (Fourar et al., 1993; Persoff and Pruess, 1995; Longino and Kueper, 1999; Akin, 2001; Rangel-German et al., 2006; Richardson et al., 2013) and complementary numerical studies relevant to fractures and fracture networks (Pruess and Tsang, 1990; Riaz and Tchelepi, 2006)
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From page 39... ...
During stage 3 (see Figure 3.7c) all NAPL mass has dissolved and diffused and is found in aqueous or sorbed phases in the fracture and the matrix; contaminant concentrations may decrease as clean water flows through the fracture, resulting in concentration gradients into (back)
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From page 40... ...
Matrix porosity = 0.3, fracture aperture = 35 µm, and groundwater velocity through the fracture = 0.64 m/day. Concentrations exiting the fracture are above the regulatory limits for hundreds of years longer than the pure phase (NAPL)
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From page 41... ...
For example, consideration of diffusive loss is critical in lightly to moderately fractured sandstones, but is likely to be less critical in highly fractured granite. FINES MIGRATION AND ENTRAPMENT: EMERGENT TRANSPORT PROCESSES Advective flow can mobilize and transport suspended fine mineral grains.
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From page 42... ...
Reactive fluid flow, mixed-fluid flow, and fines releasemigration-entrapment may be concurrent and coupled in fracture networks, such as the release of clay fines from carbonates during acid flow for oil recovery (e.g., Sakar and Sharma, 1990; Weisbrod et al., 2002; Zhang et al., 2012)
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From page 43... ...
as well as for thermal remediation strategies. Thermal Rock Mass Properties Thermal properties relevant to processes in fractured rock include latent heat, specific heat, thermal conductivity, and thermal diffusivity.
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Solid-to-solid con duction becomes increasingly more important in deep fractured rock masses. Heat conduction by advective fluid flow can be estimated from the fluid flow velocity v [m/s]
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From page 45... ...
is anticipated for most field situations. Advective fluid flow along fractures combines with liquid-solid and solid-liquid conduction in and out of rock blocks to sustain effective heat transport during cyclic fluid flow with zero fluid mass flux at the end of the cycle (experimental evidence in Yun et al., 2011; path 8 in Figure 3.11)
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