Characterizing flow and transport in fractured geological media: A review

A review of techniques, advances and outstanding issues in numerical modelling for rock mechanics and rock engineering

Among the current problems that hydrogeologists face, perhaps there is none as challenging as the characterization of fractured rock (Faybishenko and Benson 2000). This paper discusses issues associated with the quantification of flow and transport through fractured rocks on scales not exceeding those typically associated with single- and multi-well pressure (or flow) and tracer tests. As much of the corresponding literature has focused on fractured crystalline rocks and hard sedimentary rocks such as sandstones, limestones (karst is excluded) and chalk, so by default does this paper. Direct quantification of flow and transport in such rocks is commonly done on the basis of fracture geometric data coupled with pressure (or flow) and tracer tests, which therefore form the main focus. Geological, geophysical and geochemical (including isotope) data are critical for the qualitative conceptualization of flow and transport in fractured rocks, and are being gradually incorporated in quantitative flow and transport models, in ways that this paper unfortunately cannot describe but in passing. The hydrogeology of fractured aquifers and other earth science aspects of fractured rock hydrology merit separate treatments. All evidence suggests that rarely can one model flow and transport in a fractured rock consistently by treating it as a uniform or mildly nonuniform isotropic continuum. Instead, one must generally account for the highly erratic heterogeneity, directional dependence, dual or multicomponent nature and multiscale behavior of fractured rocks. One way is to depict the rock as a network of discrete fractures (with permeable or impermeable matrix blocks) and another as a nonuniform (single, dual or multiple) continuum. A third way is to combine these into a hybrid model of a nonuniform continuum containing a relatively small number of discrete dominant features. In either case the description can be deterministic or stochastic. The paper contains a brief assessment of these trends in light of recent experimental and theoretical findings, ending with a short list of prospects and challenges for the future.

Trends, prospects and challenges in quantifying flow and transport through fractured rocks

A large-scale investigation of fracture flow was recently conducted in a granite uranium mine at Fanay-Augeres, France. Its aim was to develop a methodology for the investigation of possible nuclear waste repository sites in crystalline environments, and thus to determine what measurements to make and what models to use in order to predict the flow and transport properties of the medium, i.e., their average behaviors and spatial variabilities at different scales. Four types of data were collected: (1) geometry of the fracture network; (2) local hydraulic properties measured by injection tests in boreholes; (3) global hydraulic behavior from flow rate and piezometric head distribution at a 106 m3 scale; and (4) tracer tests performed at a scale of up to 40 m. A stochastic fracture network model assuming negligible matrix permeability was developed and calibrated essentially on data 1 and 2 above; this was then used to predict data 3 and 4 in an attempt to validate both the parameters and the structure of the model. In this first part, only the flow problem (data 1) is discussed.

Modeling fracture flow with a stochastic discrete fracture network: calibration and validation: 1. The flow model

Karst regions represent 7–12% of the Earth's continental area, and about one quarter of the global population is completely or partially dependent on drinking water from karst aquifers. Climate simulations project a strong increase in temperature and a decrease of precipitation in many karst regions in the world over the next decades. Despite this potentially bleak future, few studies specifically quantify the impact of climate change on karst water resources. This review provides an introduction to karst, its evolution, and its particular hydrological processes. We explore different conceptual models of karst systems and how they can be translated into numerical models of varying complexity and therefore varying data requirements and depths of process representation. We discuss limitations of current karst models and show that at the present state, we face a challenge in terms of data availability and information content of the available data. We conclude by providing new research directions to develop and evaluate better prediction models to address the most challenging problems of karst water resources management, including opportunities for data collection and for karst model applications at so far unprecedented scales.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2013RG000443

Karst water resources in a changing world: Review of hydrological modeling approaches

As part of the development of a methodology for investigating flow and transport in fractured rocks, a large-scale experiment was recently performed at Fanay-Augeres, France. In a companion paper (Cacas et al., this issue) (paper 1) the results of the flow measurements were analyzed. In this paper, the results of the tracer experiments are interpreted. A particle following is coupled to the flow model, described in paper 1. Microscopic dispersion in the fractures and retardation effects due to unevenness of the flow paths are taken into account. The transport model is calibrated on in situ tracer tests, whereas the parameters of the hydraulic model were initially fitted on structural and hydraulic measurements (paper 1). The dispersive properties of the model are reasonably comparable to those of the real site. It tends to confirm the validity of the preliminary hydraulic calibration of the model and thus to validate further the approach used to simulate hydraulic and transport phenomena.

Modeling fracture flow with a stochastic discrete fracture network: Calibration and validation: 2. The transport model

Modeling fracture flow with a stochastic discrete fracture network: Calibration and validation - 1

Because of the multiscaled character of fracture networks and their high degree of heterogeneity, characterization and modelling of fractured reservoirs requires different techniques to the well-established geostatistical methods derived for modelling rock heterogeneity. We have developed a method to improve the geological model used as an input of fractured reservoir fluid flow simulators, either in single or dual permeability simulations, as well as new specific procedures. The method is based on three nested models. The first, the ‘global geo-cellular fracture model’, considers fracture average property distribution at the scale of the reservoir. Location of the major faults as well as properties, such as horizon curvature, are taken into account. ‘The global discrete model’ considers the fault system at the same scale and algorithms generate realistic synthetic fault patterns using an object-orientated approach. The third model, the ‘local discrete model’, creates realistic synthetic fracture patterns at the decametre scale with another object-orientated procedure. These models are all constrained by hard data acquired from seismic, well imaging and logs. They can be constrained by output from deterministic approaches like structural evolution analyses and geomechanical modelling. Interrelationships between these models enable account to be taken of the complex interrelationships between fractures at different scales and rock material heterogeneity. Finally, this modelling approach makes the geological model used as an input of fractured reservoir fluid flow simulators more satisfying than conventional approaches.

Nested geological modelling of naturally fractured reservoirs

Both characterization and dynamic simulation of naturally-fractured reservoirs have benefited from major advances in recent years. However, the reservoir engineer is still faced with the difficulty of parameterizing the dual-porosity model used to represent such reservoirs. In particular, the equivalent fracture permeabilities and the equivalent matrix block dimensions of such a model cannot be easily derived from observation of the complex images of natural fracture networks. This paper describes a novel and systematic methodology to compute these equivalent parameters. The results of its implementation with specially-designed software demonstrate its validity and efficiency in dealing with field situations. A tensor of equivalent fracture permeability is derived from single-phase steady-state flow computations on the actual fracture network using a 3D resistor network method and specific boundary conditions. The equivalent block dimensions in each layer are derived from the rapid identification of a geometrical function based on capillary imbibition. The methodology was validated against fine-grid reference simulations with a conventional reservoir simulator. Then, a complex outcrop image of a sandstone formation was processed for demonstration purposes. This innovative tool enables the reservoir engineer to build a dual-porosity model which best fits the hydraulic behavior of the actual fractured medium.

/pdf/a-rapid-and-efficient-methodology-to-convert-fractured-mtjebnnwbr.pdf

M. C. Cacas

Papers

Modeling fracture flow with a stochastic discrete fracture network: calibration and validation: 1. The flow model

Modeling fracture flow with a stochastic discrete fracture network: Calibration and validation: 2. The transport model

Modeling fracture flow with a stochastic discrete fracture network: Calibration and validation - 1

Nested geological modelling of naturally fractured reservoirs

A Rapid and Efficient Methodology to Convert Fractured Reservoir Images Into a Dual-Porosity Model