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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications Executive Summary All rocks in the earth's crust are fractured to some extent. Fractures form in response to stress. The origin of stress can be lithostatic (arising from the weight of the earth's crust), high fluid pressure, tectonic forces, or thermal loading. Fractures occur at a variety of scales, from microscopic to continental. Fractures are important in engineering, geotechnical, and hydrogeological practice. They can act as hydraulic conductors, providing pathways for fluid flow or barriers that prevent flow across them. Many petroleum, gas, geothermal, and water supply reservoirs form in fractured rocks. Fractures exert a strong influence on the formation of ore bodies because they act as conduits for ore-forming fluids. Fractures can control the transport of chemical contaminants into and through the subsurface. They also affect the stability of engineered structures and excavations. The application of fracture characterization and fluid flow analysis in engineering, geotechnical, and hydrogeological practice involves addressing three key questions: How can fractures that are significant hydraulic conductors or barriers be identified, located, and characterized? How do flow and transport occur in fracture systems? How can changes in fracture systems be predicted and controlled? These questions are discussed in turn below. 1. How can fractures that are significant hydraulic conductors or barriers be identified, located, and characterized? A fundamental step in understanding and predicting the behavior of fractures involves the identification and location of hydraulically significant fractures. Such
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications fractures are conduits for fluid flow and are connected to other hydraulically conductive fractures to form systems or networks. Conductive fracture networks may include a large number of interconnected hydraulically active features or may be limited to a very small proportion of the total fractures in the rock mass. Sometimes it is necessary to locate the hydraulically active fractures explicitly. In other cases it may be sufficient to determine the types of patterns (i.e., regularly repeating geometrical arrangements of fractures) that the fractures form or their statistical properties (e.g., orientation and density) or to locate only the major fractures explicitly. The requirements vary from site to site and from application to application. Chapters 2 through 5 address methods for locating and characterizing hydraulically conductive fractures through a combination of geological mapping, geomechanical analysis, and geophysical and hydrological measurements. It may be possible to predict the location of fractures at depth through an understanding of the processes that form fractures and the nature of the resulting fracture patterns. Chapter 2 addresses the development of fractures from a geological and geomechanical perspective and addresses the origin of fracture patterns as well as common fracture patterns found in various geological environments. Fractures represent concentrations of void space in rocks. The geometry of the void space affects both the flow properties and the physical properties of the rock mass, such as the elastic and electric properties. An understanding of how the void space geometry controls the fluid flow and geophysical properties of the rock forms the foundation of geophysical methods used to detect fractures in the subsurface. These hydraulic and geophysical properties of fractures are reviewed in Chapter 3. Geophysical techniques can be used to locate and measure the properties of fractures in the subsurface. There have been significant recent improvements in subsurface fracture detection techniques. Geophysical methods are particularly useful for identifying large individual fractures as well as groups of closely spaced and interconnected fractures or fracture zones. These methods can be used to image large fractures and fracture zones, as discussed in Chapter 4. For example, seismic propagation is slower through fractures than through intact rock. This fact can be used to locate regions that have higher fracture density and consequently regions that might also have higher hydraulic permeability. Because the relationship between geophysical properties of a rock mass and the hydrological parameters of fractures is not unique, it is usually necessary to make direct measurements. The flow properties of fracture systems can be tested and characterized directly with wells drilled into the subsurface, as discussed in Chapter 5. Well tests and tracer tests can be used to characterize flow system geometry, including the size, density, orientation, and interconnectedness of hydraulically conductive fractures. Flow systems developed in fractures can have quite complex geometries. Special techniques have evolved to interpret test data from these systems. These techniques are discussed in Chapter 5. These flow
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications properties are used in mathematical models of the flow system, as described in Chapter 6. 2. How do flow and transport occur in fracture systems? Numerical models are used to obtain quantitative estimates of flow and transport behavior in fracture systems. The first step in the development of a numerical model is the construction of an appropriate conceptual model of the fracture flow system. The conceptual model describes the main features of the geology and hydrology of the system that controls the flow and transport behavior of interest. A conceptual model incorporates an interpretation or schematization of reality that is the basis of mathematical calculations of behavior. Chapter 6 addresses the development of conceptual models from the information collected using the tools described in Chapters 2 through 5. Conceptual model development in fracture systems is an important but frequently undervalued step in numerical modeling. Particular difficulties arise in the determination of fracture geometry and flow physics because, as noted previously, fracture systems have complex geometries. In many cases more than one conceptual model can be constructed with available data. Indeed, most of the error involved in predicting flow behavior with a numerical model is usually due to deficiencies in the underlying conceptual models. A numerical model provides quantitative estimates of flow and transport behavior of the system described by the conceptual model. The mathematical formulation of the numerical model is determined by the flow geometry specified in the conceptual model. A wide variety of fluid flow models have been developed to address the prediction of flow and transport in fracture systems. All methods discretize the medium into conductive elements. The methods differ mainly in the way the conductive elements are defined (e.g., representing single fractures, groups of fractures, or equivalent continue) and in the way the models are parameterized (e.g., based on statistical analysis of small-scale data or on interpretation of large-scale hydraulic tests). Several classes of numerical models, their relationships to conceptual models, and their applications to fracture flow systems are described in Chapter 6. Emphasis is on models that deal with the heterogeneous nature of fracture systems. Model parameterization, that is, the estimation of unknown parameters in the numerical models (e.g., porosity and conductance) through field tests, also is addressed in Chapter 6. There is a strong dependence between field test design and model parameterization. For example, a numerical model of the distribution of fractures at a site requires the field collection of spatial statistics for parameters such as fracture density orientation, size, and conductivity. A numerical model that explicitly specifies hydrogeological units such as fracture zones requires well tests to measure the hydrological properties of those zones.
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications Numerical models are usually developed to aid in management and design decisions. Two basic questions must be addressed before these models can be used effectively as management or design tools. First, does the conceptual model provide an adequate characterization of the flow system? If not, it must be revised and reevaluated. Second, is the data base adequate to estimate the parameters in the numerical model with sufficient precision to produce reliable predictions for the intended application? If not, additional field data must be collected. Development of a numerical model should be an iterative process. It begins with the development of a conceptual model, which, in turn, is used to formulate a numerical model of the flow system. The parameters of the numerical model are determined through the collection and interpretation of field data, and the numerical model is used to predict the behavior of the flow system. The predicted behavior is compared to the actual behavior of the flow system as determined from field observations. If the predicted and observed behaviors do not match, the conceptual model is reevaluated and updated, and the process is repeated. 3. How can changes in fracture systems be predicted and controlled? The flow and transport behavior of fracture systems can be perturbed by natural processes or by the activities of people. For example, extraction of fluids from a fracture system can lower the fluid pressure, which will increase the effective stress in the rock, thereby causing the fractures to close. Temperature-induced stresses can have similar effects. Changes in fluid chemistry can lead to either mineral precipitation in fractures or mineral dissolution. In general, any change in the void geometry of a fracture system will alter the flow and transport behavior. Tools for predicting and controlling these changes are addressed in Chapter 7. In the final analysis, there is no prescribed procedure that can be followed to address flow and transport in fracture systems. This report describes tools that can be used in the process of characterization, model building, prediction, and refinement. These tools must be applied iteratively to determine if a flow and transport system has been characterized correctly. This iterative process must be designed to test improvements in the predictive capabilities of a numerical model. If the predictions improve with each iteration, confidence in the conceptual model as an accurate representation of the flow and transport system is enhanced. If the predictions do not improve, the conceptual model must be reexamined. Iterative procedures can be applied to many problems in the earth sciences. The tools associated with each step of this iterative process, as applied to fractures, are described in this report, but the report should not be viewed as a ''cookbook." Rather, the tools described here can be applied only in a manner appropriate to the specific geology and application under consideration. Case examples of the application of these tools are given in Chapter 8.
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications RECOMMENDATIONS The recommendations developed in the technical chapters of this report (Chapters 2 through 8) are presented in Chapter 9 (Technical Summary). These recommendations are summarized below, and their application to a variety of important practical problems is provided. The committee made no attempt to assign relative priorities to its recommendations. As a group, however, the recommendations represent the committee's assessment of high-priority research needs. The committee believes that concerned federal agencies as well as other research-sponsoring organizations should carefully consider the recommendations in this report. The following eight recommendations are cross-cutting; they do not fit neatly within the mission of any single agency or organization. The committee encourages those agencies and organizations concerned with the shallow subsurface of the earth to implement these recommendations through research partnerships and other joint-venture consortia. Among the federal agencies and other groups that would benefit from such research are the U.S. Army Corps of Engineers; Yucca Mountain Site Characterization Project Office; Department of Energy, Office of Conservation and Renewable Energy and Office of Fossil Energy; Environmental Protection Agency, Environmental Monitoring Systems Laboratory; U.S. Geological Survey; U.S. Department of the Interior, Bureau of Land Management, Bureau of Mines, and Bureau of Reclamation; Defense Nuclear Agency; and Nuclear Regulatory Commission, Office of Nuclear Regulatory Research. 1. Additional in situ research facilities should be developed in fractured rocks in a variety of geological environments. Advances in modeling methodologies require repeated sequences of characterization, prediction, comparison, and refinement. Research facilities provide the opportunity to move freely between these stages. The tools, approaches, and conceptualizations developed at these facilities can be applied to sites in similar geological environments. In situ research facilities are especially valuable for assessing the usefulness of various characterization tools and methodologies and for learning how to jointly interpret data obtained with different tools. Heavily characterized sites are ideal for experiments in fluid flow and chemical transport and for investigating the coupling between flow, stress, and temperature because the interpretation of such an experiment is much more certain when the site is well understood. Such experiments are needed because existing numerical models for predicting the effects of stress, flow, temperature, and multiple flow phases do not provide reliable long-term predictions of flow and transport. In situ experiments, particularly at regional scales, should be supported to develop reliable numerical models for predicting these effects.
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications In situ research facilities help in the development of successful testing strategies and provide critical information for extrapolating measurements from one time and one distance scale to others. Research programs at these facilities should be designed to address data requirements for evaluating various flow and transport modeling approaches. These programs should also address the practicality of collecting these data in routine hydrogeological practice. In situ facilities should be developed in a variety of rock types with different styles of fracturing. A number of excellent facilities already exist in crystalline rocks, but there is a dearth of in situ research facilities in bedded rocks, especially where more than one fluid phase is present. Consequently, less is known about how to effectively characterize flow and transport in bedded rocks. Research at facilities in bedded rocks would have a significant impact on understanding enhanced oil and gas recovery processes in fractured reservoirs. An example of such a facility is the Exploratory Studies Facility currently under construction at Yucca Mountain, Nevada, the site of a potential nuclear waste repository. The Yucca Mountain site is a bedded tuff that contains a variety of fracture types. The proposed repository is above the water table and, when filled with high-level nuclear waste, is expected to produce significant quantities of heat. A thorough characterization of this site is required before the behavior of fluid flow in the vicinity of the potential repository can be predicted. 2. The development of conceptual models for fluid flow and transport in fractured rock should be a focus of research. A conceptual model describes the main features of the geology and hydrology of the system that controls the flow and transport behavior of interest. A conceptual model incorporates an interpretation or schematization of reality that is the basis of mathematical calculations of behavior. Conceptualizations used for models of fluid flow in granular media (e.g., sand) are usually inadequate for models of fluid flow in fracture systems. For example, single fractures are commonly conceptualized as the space between two parallel plates separated by a constant aperture (distance), which is too simple for many applications, especially those involving mass transport. However, it is usually impractical to completely specify complex fracture geometries for single fractures, not to mention fully three-dimensional fracture network. Consequently, simple models should be developed to describe the salient features of fractures that account for the flow and transport behavior of interest. Numerical models developed from inappropriate conceptual models can have large uncertainties that are difficult to quantify. In many applications, errors in flow and transport predictions could be dangerous or costly. Research should be supported to develop realistic conceptual models for fracture systems. This research should identify commonly applicable relationships between rock type, stress, structure, fracture style, and flow and transport behavior. These studies
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications should be undertaken in a variety of geological environments and should address a variety of mass and heat transport phenomena. Theoretical investigations and experimental work should be supported to develop appropriate and practical conceptual models for transport. These are needed to predict the fate of contaminants underground and the behavior of complex petroleum recovery schemes. Theoretical investigations and experimental work should be supported to link hydrogeological and geochemical processes in transport models for reactive solutes that take account of the unique properties of fluid pathways in fractured rock. Research is needed to determine how the geometry of fractures determines the relative importance of preferential flow paths and the surface area affecting matrix diffusion and reactive transport. Development of realistic conceptual models for multiphase fluid flow (flow involving more than one fluid phase, e.g., water and steam) in fractures is an important research problem for nuclear waste repository siting, enhanced oil recovery, and subsurface contamination by nonaqueous phase liquids. New theoretical and laboratory work should be undertaken to relate multiphase flow in fractures to fracture geometry, rock matrix properties, and stress. Fracture network models involving two fluid phases can reveal how network geometry controls flow. Efforts to develop these models should be continued. There is a critical need for experimentation and observation of two-phase flow in natural systems. Laboratory and in situ experiments should be carried out to determine if the behavior of two-phase flow in fractures is chaotic and consequently difficult to predict in a deterministic sense. Confidence in a conceptual model is developed through the iterative process of characterization, model building, prediction, and refinement, as described in numerous places in this report. Such "confidence building" should become the standard approach for addressing regulatory requirements. A field investigation that has multiple repetitions of the same simple experiment may serve to build confidence better than one complex, all-encompassing experiment. 3. Research on the origin and development of fracture systems should be undertaken to provide an appropriate foundation for fluid flow studies. Understanding the origin and development of distinctive fracture patterns and void geometries in various geological environments and how these patterns affect fluid flow is extremely helpful in solving fracture flow problems. Numerical fluid flow models that account for the origin and development of fracture systems include more physical reality. This realism may provide a basis for extrapolating predictions of hydrological behavior to regions where there are few measurements. The origin and development of fracture systems should be studied in three dimensions because most natural flow problems are inherently three dimensional. The processes that determine how hydrologically important fractures form should be a priority for research. Understanding these processes will allow preliminary
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications conceptual models to be constructed for sites that have not yet been highly characterized. Such work will help in the exploration for resources such as petroleum, ore minerals, heat, and water and will aid in the design of optimal extraction or isolation schemes. For waste storage or contamination problems, this work will provide a physical basis for scaling up observed flow behavior to longer time and distance scales and will facilitate the design of schemes for in situ isolation of waste. A better understanding of fracture formation will also aid in the design of engineered structures in rock. 4. Efforts to develop and improve fracture detection methods should continue. Fracture detection methods and instrumentation have improved greatly in recent years. Several detection methods are now in common use, and there is a general understanding of their strengths and limitations. However, there is still a need for theoretical research on detection methods, new instrumentation, and improved algorithms for data interpretation. Geophysical imaging methods based on differencing concepts (i.e., based on measurements before and after a perturbation to the flow system) have great potential for imaging flow systems in fractured rock. These methods should be developed and improved. Techniques that combine different geophysical methods (e.g., seismic and electrical methods) also should be developed. Methods for identifying fracture zones based on multiple geological, geophysical, and hydrological measurements should be developed and validated at in situ research facilities. This effort should include research to relate various geophysical properties of fractures to flow properties. The effect of fractures on the propagation of shear waves (waves that cause particle motions perpendicular to the direction of wave propagation through the rock) is an important area of research for fracture detection, particularly for resource recovery. Acquisition and interpretation techniques should be developed to take advantage of recently discovered interactions between shear waves and fractures. Borehole shear-wave sources of significant strength should be developed for use in fracture imaging. There is a need to develop methods for detecting fractures some distance from a single borehole. For example, it would be useful to develop single-hole seismic reflection methods similar to the borehole radar reflection methods. Methods to produce three-dimensional subsurface images from seismic surveys conducted at the surface (3D seismic methods) are now relatively common in the petroleum industry. The technique is very expensive because it requires a large number of source-receiver pairs. Efforts should be made to reduce the cost of this method and to adapt it to shallower and smaller-scale environmental applications. Geophysical technologies should be promoted for routine use in the field. This effort should include both new technology and the transfer of technology
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications developed in the petroleum and nuclear waste disposal industries to other applications. In many cases the state of the art in geophysical methods is far more advanced than the state of practice. Programs should be developed to facilitate technology transfer to improve the state of practice. 5. Efforts to develop realistic and efficient numerical models of fluid flow and transport should continue. Research into efficient numerical models will facilitate practical applications of fracture flow and transport models and should be supported. These applications include resource extraction, contaminant transport and isolation, and structural integrity. Numerical models that provide simplified but adequate representations of complex fracture networks should be developed. Studies that identify appropriate ways to group features of the fracture network into simpler equivalent features are needed. Such models will expand the number of problems that can be treated numerically and will improve the treatment of problems where present data requirements are impractical. Simple but sufficient mathematical models of fracture networks are particularly important for inverse methods, which are used to estimate fracture system parameters from field observations of hydraulic behavior. Models that incorporate hierarchical relationships among fractures or account for the origin and development of fractures hold promise in this regard. These inverse methods should be developed further. Many inverse methods produce nonunique results owing to deficiencies in the quality or quantity of field data. Nevertheless, these methods can be used to obtain distributions of possible outcomes and to determine what constraints the data actually place on predicted behavior. 6. Research should be undertaken to better understand and predict the effects of coupling between stress and flow. Although much has been done to study the relationship between stress and flow in the laboratory, research is needed to evaluate the relationship for a range of practical field-scale problems. This research should identify threshold and scale effects and evaluate the influence of lithology and fracture history. This work should also relate the heterogeneity in stress to fracture permeability and flow. Research should be undertaken to relate temperature and effective stress changes to changes in flow properties through in situ experiments in well-characterized fracture systems. This work has important applications in geothermal reservoirs and nuclear waste repositories (e.g., the proposed Yucca Mountain repository), which are expected to generate significant quantities of heat. The effects of shear deformation on fracture permeability should be a focus for new research. This work has important implications for the stability of engi-
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Rock Fractures and Fluid Flow: Contemporary Understanding and Applications neered structures in fractured rock and for understanding the nature of fracture permeability in natural shear zones. Better methods are needed to predict the geometry of fractures produced by the hydrofracture process (which creates fractures by increasing fluid pressure in the rock). Research to predict fracture geometry in complex geologies, particularly in rocks that contain preexisting fractures, should be supported. An objective of this work should be to predict the effects of hydrofracturing on the local fluid flow regime. Programs should be developed to facilitate the transfer of hydrofracture technology from the petroleum industry to the water supply industry. 7. Research should be undertaken to understand and predict coupling between chemical processes and stress, flow, and temperature in rocks. Research to understand coupling between chemical processes and stress, flow, and temperature in rocks should be undertaken. The primary objective should be to develop techniques to predict coupling effects in engineered systems. These effects should be evaluated through field studies and augmented with laboratory tests. Part of the research effort should be directed toward understanding how to scale up the laboratory results in time. This scaling work has important applications in evaluating engineering projects with long design lives such as nuclear waste repositories and dams, where slow changes may be hard to predict based on short-term testing and evaluation schemes. It also has applications in engineering projects and petroleum and geothermal reservoirs, where extreme changes in pressure, temperature, and chemistry can lead to large changes in coupled behavior. 8. Additional work should be undertaken to develop and test waste isolation and in situ treatment technologies specifically for fractured rock. Given the large amount of waste in the subsurface and the extreme difficulty associated with removing it, in situ remediation and isolation technologies should be developed for fractured rock. Grouting (i.e., injection of cement slurries into fractures in the rock) as an isolation technique in fractured rock deserves much more attention. Research should couple in situ waste treatment and waste isolation techniques with characterization and flow modeling of the fracture system. Efficient, reliable, and economic methods should be developed to address contamination problems.
Representative terms from entire chapter: