Rock Fractures and Fluid Flow Contemporary Understanding and Applications (1996) / Chapter Skim
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9 Technical Summary
9 Technical Summary
Pages 499-524
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(Chapters 2, 3, 4, and 5) · How do fluid flow and chemical transport occur in fracture systems?
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From page 500... ...
The geometry of the void space and the relationship between fracture permeability and matrix permeability determine how flow takes place. If the fractures are poorly interconnected and the matrix rock is relatively impermeable, an analysis of flow may need to account for the network of discrete fractures and could well ignore the matrix.
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Moreover, well tests may not be practical, especially on very large scales, when producing wells must be shut down, or in contaminant environments. Used together, these tools can provide an understanding of the likely locations and the hydrological properties of major flow paths.
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Examples of fracture patterns that result in hydraulically significant flow paths include: · Highly connected, regionally extensive networks of open fractures, such as cooling fractures that form polygonal networks. · Clusters of open joints or fracture zones.
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It is now possible to quantify the spatial relationships of asperity height and aperture distributions and to relate these geometric properties to a variety of fracture flow, mechanical, and geophysical properties. A number of recent studies have quantified the geometry of laboratory fracture samples as well as their hydrological and geophysical properties.
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Theoretical and laboratory studies of the correlation between hydrological and geophysical properties are useful, but it is essential to examine whether these relationships hold in situ. Two approaches hold promise: one is to make in situ measurements of elastic stiffness and electrical and hydraulic conductances of fractures and to use these measurements to develop empirical relationships.
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Work should be undertaken to develop borehole seismic rejection methods that can map fractures at an appreciable distance from a single borehole, similar to what can now be achieved with borehole radar. Tomography is practically limited to two dimensions, whereas hydrological flow paths in fractured rock are rarely confined to two dimensions.
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These interpretations will always be site specific. The primary difficulty is that interconnection of void space controls the hydrological-conductance properties of the rock, whereas interconnection has little effect on most geophysical properties.
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Packers set above and below the fracture zone in both the pumping and monitoring wells isolate the hydraulic response of the zone. The identification of hydraulically significant fracture zones in boreholes is not necessarily straightforward.
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Flow geometries in fractured rocks are usually more complex than this simple case; consequently, interpretation of hydraulic tests in fractured systems can be challenging. In addition, fluid flow is described by the diffusion equation, which means that the signal decays rapidly and may not be affected by heterogeneities.
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However, meaningful interpretation of the transport properties of the medium require that the geometry and boundary conditions of the flow system be extremely well understood. The only examples of successful tracer test interpretation are for simple fracture geometries, for instance, in cases in which there are dominant fracture zones.
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Additional in situ facilities should be developed in fractured rocks in a variety of geological environments in order to improve the ability to identify, locate, and characterize hydraulically conductive fractures.
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Laboratory studies are also the primary source of information to link geophysical and hydrological properties. Recent work has demonstrated that an appropriate conceptual model for flow in a single fracture is that of a two-dimensional porous medium that is sensitive
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In this case the matrix controls the flow and the fractures act as barriers. These two conceptual models give radically different values of travel time: the channelized flow model will conduct fluids much more quickly than the permeable matrix flow model.
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Development of appropriate conceptual models for multiphase flow in fractures is a critical research problem, with applications to many problems of societal interest, for example, the siting of nuclear waste repositories above or even below the water table, predicting the effects of corrosive gas releases from the corrosion of nuclear waste containers, for nonaqueous-phase liquid contamination, and for enhanced oil recovery. New theoretical and laboratory work should be undertaken to relate multiphase pow in fractures to aperture distribution, matrix properties, and stress.
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Conceptual models developed at such facilities will aid conceptual modeling process at other sites with similar geological conditions. Numerical Models Given an appropriate conceptual model, the analyst can choose from a large suite of numerical modeling tools.
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The subdomains may represent a single fracture, a channel in a fracture, or a fracture zone, for example. These models are suitable for calculating complex flow paths resulting from the geometry of the interconnected fracture network.
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These methods are particularly useful when the pattern of conductors determines the hydraulic behavior. For example, fracture systems can be modeled as simple, partially filled lattices that lie on planes representing major fracture zones.
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However, the development of appropriate conceptual models for transport in fractured rock is not yet complete. It is difficult to characterize solute distribution, especially when transport occurs in narrow pathways in fracture planes.
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Efforts should be made to focus model and field comparisons on predicted quantities that have small variances. Sublunary Conceptual modeling, numerical modeling, and in situ testing can be used to address the question: How do fluid flow and chemical transport occur in
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However, additional work should be undertaken to develop appropriate conceptual models for transport and multiphase flow. Conceptual models should have a strong physical basis to provide a reasonable rationale for extrapolation and scaling up.
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Our understanding of these phenomena is incomplete. A better understanding of the elect of shear on fracture permeability should be the focus for new research because of its significance for engineering projects, as well as for understanding the nature of fracture conductance in natural shear zones.
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It is especially important to understand the couplings between these processes for projects with long design lives because the elects of changes in fracture systems may be hard to predict based on short-term testing and evaluation schemes. There is considerable laboratory data to establish constitutive relationships between effective stress and fracture permeability for single fractures in rock cores (Chapter 3)
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Grouting programs are more likely to be successful if the flow paths are well characterized beforehand. Discrete fracture flow models developed for a site may be very useful in planning an isolation system, especially if the model can simulate the fluid properties of the grout.
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Field-scale experiments should be conducted to test models that calculate the deformation of a fractured rock mass during fluid injection or withdrawal. Efforts should be made to construct simplified hydromechanical models that predict changes in flow conditions from changes in stress.
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Numerical models that couple fracture flow, stress, and fracture filling are used to address the deformation of fractured rock masses. For example, deformation of a rock mass caused by well drawdown, or the effects of grouting and drainage on a fractured rock mass underlying a dam, can be calculated with these models.
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