MoFrac discrete fracture network (DFN) modeling software generates fracture network simulations with deterministic fractures constrained to known locations, and stochastic fractures conditioned to input data. A deterministic fracture network is generated through the modeling of a dataset that is representative of the lineaments typically found in a Canadian Shield environment. This model is used to constrain stochastic representations to observed fracture intensities and orientations. This study considers two- dimensional and three-dimensional length distributions and area distributions as constraints. Built-in metrics are used to analyze the size and orientation distributions of the stochastic models for comparison with the input data. Further calibration of constraints for these models is achieved by dividing fracture groups into subsets; this preprocessing task involves the definition of subsets of identified fracture groups based on orientation. The consistency and accuracy of the fracture network modeling are considered using three alternative conditioning methods. It was shown that generated fracture networks conform to the conditioning parameters for each method considered. Where multiple subsets were used to define fracture group parameters, resulting DFNs were more representative of the input data.
ABSTRACT: It has been observed that groundwater flow through fractures is channelized due to variations in fracture aperture, fracture roughness and different fracture intersections. Nevertheless, a common practice in DFN modelling is to model each fracture as hydraulically homogeneous, which is an obvious simplification of reality. In our study, we have evaluated whether a flow-calibrated DFN model, where each fracture is assumed hydraulically homogeneous, can reproduce the particle tracking behavior of a more realistic DFN model where each fracture is hydraulically heterogeneous. We used a concept of a synthetic reality where a model with heterogeneous hydraulic properties was used to produce simulated field data measurements to provide a benchmark for particle tracking analyses. Multiple synthetic reality models were used to reproduce a borehole pumping test mimicking a Posiva Flow Log (PFL) tool. Results from the pumping tests served as a calibration target for a model with the same fracture geometry, but homogeneous fracture properties. PEST ® was used as a tool for the calibration of homogeneous models so that they match the distribution of inflows obtained from the synthetic reality model. By using PEST®, it was possible to successfully calibrate a homogeneous models so that the distribution of inflows matched results from the synthetic reality model. A particle tracking analysis was then conducted on both heterogeneous and calibrated homogeneous models. Particle pathways were statistically analyzed in terms of particle travel distance, travel time and F-factor, which is a key parameter governing the transport of radionuclides within fractured rock. Results show that homogeneous calibrated models can produce non-conservative results in terms of the F-factors. In our study, we have demonstrated that DFN models successfully calibrated to boreholes inflow data could give a poor prediction in terms of particle tracking analysis.
ABSTRACT: A jointed rock mass is composed of intact rock matrix and joints. Joints play an important role in influencing the strength and deformation behaviors of jointed rock masses. Based on the concept of representative elementary volume (REV) and the synthetic rock mass (SRM) modeling technique, a conceptual DFN-DEM multi-scale modeling approach is proposed. Discrete fracture networks (DFNs) are generated using MoFrac – a newly developed DFN generation tool. For a given volume of jointed rock mass, multi-scale DFN models are constructed according to the hierarchical order of fracture size. Based on the DFN models of various scales, effective rock mass properties are obtained by the homogenization method using 3DEC SRM models. A tunnel excavation modeling using data from the Äspö TAS08 tunnel is conducted to demonstrate the applicability of the proposed multiscale modeling approach. The tunnel excavation modeling results show that it is very efficient to model rock mass mechanical response using the proposed approach and a reasonably good excavation response is captured. The proposed DFN-DEM multiscale modeling approach can be used as a virtual laboratory to conduct numerical experiments to capture mechanical behaviors of jointed rock mass in a more practical manner, which can be used for stability evaluations of slopes and underground excavations in rock engineering.
Junkin, W. (MIRARCO Mining Innovation) | Janeczek, D. (MIRARCO Mining Innovation) | Bastola, S. (MIRARCO Mining Innovation) | Wang, X. (MIRARCO Mining Innovation) | Cai, M. (MIRARCO Mining Innovation) | Fava, L. (MIRARCO Mining Innovation) | Sykes, E. (Nuclear Waste Management Organization) | Munier, R. (Swedish Nuclear Fuel and Waste Management Co.) | Srivastava, R. M. (FSS Canada Consultants Inc.)
ABSTRACT: This paper presents a validation study for a new software tool, MoFrac, which generates realistic discrete fracture network (DFN) models. MoFrac implements aspects of a unique geostatistical and rules-based methodology for DFN generation. Non-planar fractures are generated through a conditional simulation process that propagates deterministic fracture traces in three dimensions. Stochastic fractures are generated based on conditioning to statistics derived from field-mapped fracture traces and orientation parameters. Data mapped from the Äspö TAS08 tunnel close to Oskarshamn, Sweden were used for this validation study. The generated DFN model was analyzed for fracture orientation, size, intensity, and location. The modeling results demonstrate that MoFrac generates representative DFNs with a realistic appearance that conform to mapped fracture traces and statistics derived from the input data. MoFrac DFNs are suitable for integration into a variety of numerical models.
The dynamics of a rock mass are related to the occurrence of natural discontinuities which occur on different scales, with variable intensities, shapes, and distributions through space (Lei et al., 2017). Fracture truncation, branching, clustering and spacing, among other measurable attributes, define the geometry of a fracture network. Computational models that represent the geometry of fracture networks contribute to the understanding of the strength and deformation behavior of rock masses, rock fragmentation, slope stability, groundwater flow, and mass transport. A discrete fracture network (DFN) model represents the geometric arrangement and characteristics of fractures within a volume of rock.
Several means of fracture network generation are described in the literature. Placement methods are computationally efficient because fractures are modeled as simple planar shapes arranged within a domain (Long et al., 1985), but the realism and accuracy of resulting DFN models are often limited. Mechanical propagation methods can produce realistic DFNs by simulating the creation of fractures driven by complex geomechanical conditions. These models are generally limited to fracture propagation in two dimensions and are computationally intensive (Lei et al., 2014). Geometrical propagation methods (Srivastava, 2002) are employed by MoFrac and provide much of the realism of mechanical propagation methods, while remaining computationally efficient.
Discrete Fracture Network (DFN) modelling has increasingly being used in many geotechnical and mining engineering problems, the authors believe there is an ever greater demand to provide updated guidelines that address the collection of discontinuity data in the specific context of DFN models. In general terms, the generation of a DFN model requires collecting information on i) fracture orientation, ii) fracture intensity, iii) fracture length and iv) fracture terminations. The International Society of Rock Mechanics (ISRM) suggested methods for the quantitative description of discontinuities in rock masses directly include most of these parameters. However, there are important differences engineers should be aware. To what extent discontinuities can be sampled and which limitations are inherently introduced in the analysis by the sampling methods being adopted represent important aspects that should drive the collection of discontinuity data for DFN analysis; this will require the introduction of a more appropriate set of guidelines bridging the gap between the current, very practical, ISRM methods and the data requirements imposed by the use of new DFN technologies.
The last decade has seen a major increase in use of the Discrete Fracture Network (DFN) approach, both as a stand-alone tool or integrated within more complex geomechanical simulations (Rogers et al., 2014 and Elmo et al., 2014). In particular, the DFN approach offers the opportunity to maximise the use of fracture data collected from mapping of rock exposures and to construct synthetic rock mass (SRM) models. However, the potential of DFN based modelling is limited if there is insufficient care in collecting the necessary structural data at the required engineering scale. DFN models are also subject to the process of data calibration and validation (Hadjigeorgiou, 2012). With DFN modelling becoming an almost integral part of many geotechnical and mining engineering problems, the authors believe there is an ever greater demand to review the process of collecting fracture data in the specific context of DFN modelling.
The value of the DFN model depends directly on the quality and quantity of available field data. For example, fracture length (persistence) is an important parameter in DFN modelling; however, this parameter is either seldom available at the pre-feasibility stage due to a lack of exposures (man-made or natural), or engineers have access to limited length data collected along exploratory drifts. Characterisation of rock exposures should also take into account limits of window/scanline mapping techniques with respect to both trace length bias and the effect of cut-off assumptions inherent in the mapping methodology.
This paper presents a systematic numerical study to evaluate the effects of two different loading conditions, namely the axial stress and axial velocity, on testing compressive strength and deformability properties of fractured rocks. The UDEC code was used to perform a series of numerical tests on two-dimensional fracture network (DFN) models, in the similar ways for the uniaxial and biaxial laboratory testing on intact rock samples. The obtained stresses and strains from these numerical experiments were used to estimate equivalent directional Young’s modulus and fit the Mohr-Coulomb and Hoek-Brown failure criteria, represented by equivalent material properties defining these two criteria. The numerical results show that stress-strain behaviors changes by loading conditions with higher averaged axial stress under axial velocity condition than that under axial stress condition. Therefore, the effects of different loading conditions should be carefully considered for designing and interpretation of results for in-situ experiments with large volumes of fractured rocks.
Strength and deformation behavior of small rock samples obtained from standard laboratory testing are not representative of fractured rock masses containing large number of fractures. In-situ testing of larger volumes of fractured rocks has the difficulties of proper definition and control of initial and boundary loading conditions, besides representing fracture system geometry. Therefore, numerical models based on discrete elements method (DEM) can be used to investigate strength and deformability of fractured rocks considering the interactions between the intact rock matrix and fractures, when loading conditions of numerical experiments are well simulating to laboratory conditions.
In earlier studies about evaluating the strength and deformability of fractured rock masses, as reported in Amadei (1988), and Prudencio &Van Sint Jan (2007), regular fracture system geometry was assumed, which is often not good representation of rock realistic mass geometry. Sagong, et al. (2011) and Wang, et al. (2011) used finite element method (FEM) based on an overall continuum material assumption, which is not a proper method for simulating large displacements and rotations of individual rock blocks defined by fractures.
The axial velocity and axial stress loadings are the two commonly applied boundary/loading conditions for performing laboratory tests for intact rock material samples. How these two testing conditions may affect results when a large volume of fractured rock mass has not been investigated in conventional laboratory test environments, since practical difficulties make such physical tests not feasible.
This paper provides a brief summary of a continuous research programme by the authors since 2004, and highlights the research approach, achievements and outstanding issues for conceptual understanding, laboratory testing and mathematical modeling of the coupled stress-shear-fluid flow-solute transport processes of rock fractures. The focuses are put on stress and shear induced fluid flow anisotropy, transport pass channeling, and impact of considering different retardation mechanisms in single fractures of crystalline rocks, typically granites, due to its importance for the performance and safety assessments of geological radioactive waste disposal projects.
Rocks are natural geological materials containing fractures of different origins, sizes, mineral fillings, weathering degrees, orientations, termination patterns, thickness and shapes, and especially surface roughness features. In addition, rocks in-situ are under stress, caused by dynamicmovements in the upper crust of the Earth, such as tectonic plate movements, earthquakes, land uplifting/subsidence, glaciation cycles and tides, in addition to gravity. A rock mass is also a fractured porous medium containing fluids in either liquid or gas phases (e.g. water, oil, natural gases and air), under complex in-situ conditions of stresses, heating or cooling, freezing or thawing, fluid pressures, and complicated geochemical reactions, with connected fractures most often serve as the major energy and mass transport pathways and most active areas of geochemical reactions, especially for fractured hard crystalline rocks. This is the reason why the coupled thermal (T), hydraulic (H), mechanical (M) and chemical (C) processes is an issue of importance in the field of rock mechanics.
The terms “discontinuity” and “fracture” are used interchangeably in the rock mechanics literature. The term “fracture” is adopted throughout this paper as a collective term for all types of natural or artificial discontinuities such as faults, joints, dykes, fracture zones and other types of weakness surfaces or interfaces, unless specifically stated otherwise. The rock fractures are usually not just open voids with fresh and smooth surfaces. Their surfaces (or walls) are often rough, weathered and fully or partially filled with precipitated minerals, and their relative positions are often modified by geological history and loading conditions, such as opening, closing, faulting or shearing, with large or small relative displacements. The complexity in the surface topography makes understanding and quantitative representation of the physical-chemical behavior and rock fracture properties difficult issues.
This paper presents a review of a systematic research program for understanding scale and stress effects on transport behaviours of fractured crystalline rocks, using a hybrid discrete element and particle tracking approach. The motivation is the importance of understanding stress effects on behaviours of contaminant transport in fractured crystalline rocks, an important issue of rock mechanics for environmental safety assessments of many rock engineering projects. The study is divided into three steps. The first step is a basic study that established the mathematical platform for deriving the conditions, criteria, basic approaches and test case results for investigating stress and scale effects on hydraulic behavior of the fractured rock concerned. At the second step, based on outstanding issues drawn from the first step, the study was extended to consider effects of the correlation between the fracture aperture and size (represented by trace length) on the permeability of the fractured rock, and uncertainties in deriving equivalent continuum properties of fractured rocks. The third step added the particle/solute transport processes to the mathematical platform, including different retardation mechanisms, so that impact of stress on safety can be directly evaluated, even it can only be done conceptually. The obtained results show that stress, scale and inter-parameter correlations of the fracture system geometry are dominant issues for understanding and characterization of coupled hydro-mechanical processes of fractured rocks and play a significant role for understanding the mass transport behaviour in them, with direct impact on geo-environmental safety.
Ishibashi, Takuya (Tohoku University) | Watanabe, Noriaki (Tohoku University) | Hirano, Nobuo (Tohoku University) | Okamoto, Atsushi (Tohoku University) | Tsuchiya, Noriyosh (Tohoku University) | Tamagawa, Tetsuya (Japan Petroleum Exploration Company, Limited)
Coulter, A.W. (Dowell Division of The Dow Chemical Company) | Alderman, E.N. (Dowell Division of The Dow Chemical Company) | Cloud, J.E. (Dowell Division of The Dow Chemical Company) | Crowe, C.W. (Dowell Division of The Dow Chemical Company)
A new mathematical model has been developed which considers more of the variables encountered during fracture acidizing treatments than previous models. In particular the variables include, wellbore cooldown, temperature profile of fluid in the fracture, the fracture geometry created by both non-reactive and reactive fluids the spending of the leading edge of the acid, and the conductivity of the etched fracture faces. Productivity increases calculated by the new program correlate more closely with actual field results than those calculated by previous programs. This paper describes the previous programs. This paper describes the method of handling the variables in setting up the new model and presents the equations used to describe the reaction rate of the acid.
Fracture acidizing has been used for stimulating wells for over 25 years. The techniques used have developed more as an art, than a science, often based on intangible ideas, rather than on predictable facts. Although a mathematical model has been available since the early 1960's, little correlation has been observed between predicted and field results. One reason for this undoubtedly was due to the use of the acid as both the hydraulic fracturing fluid and as the reactive fluid. Another was the inadequacy of the model to describe the rheological and physical properties of the fluids in the fracture. properties of the fluids in the fracture. Only when treating techniques changed, in which better results were obtained by creating the fracture with a non-reactive pad fluid ahead of the acid, was serious effort directed toward describing the conditions or properties of the fluids in the fracture.
Equations were developed first to describe the cooldown of the wellbore area, as illustrated by Ramey's "Wellbore Heat Transmission" equations in the Appendix. Then, Whitsett and Dysart, and later Sinclair, proposed methods for describing the proposed methods for describing the temperature profile of fluids within a hydraulic fracture. Hall and Dollarhide provided basic equations for considering the fracture geometry created by more than one fluid within the fracture.