Discrete Fracture Network Approach Research Articles

Fluid flow and solute transport simulations in tight geologic formations: Discrete fracture network and continuous time random walk analyses

Understanding the effect of fracture network heterogeneity on flow and transport in tight formations with low permeability values is of great importance in a wide variety of applications, such as groundwater pollution and storage as well as water production. Although numerous studies have investigated solute transport in fractured media, investigation about scale effects on flow and transport in low-permeability aquifers is limited. More specifically, we are not aware of any study that has addressed how the continuous time random walk (CTRW) model parameters vary with scale and fracture network properties. To this end, we performed a set of numerical simulations in sparse fracture networks of sizes L = 20, 35, and 50 m under two different volumetric fracture counts (p30 = 0.05 and 0.1) using the discrete fracture network approach. The fractures in the networks were elliptical in shape, and their radii followed a truncated power-law distribution with exponents α = 1.5, 2, and 2.5. We simulated fluid flow using the Reynolds equation, and solute transport was simulated using particle tracking. The solute transport behavior was quantified by fitting the solution of the CTRW transport equation to the arrival time distributions. Results showed that as the truncated power law exponent α increased, the permeability of the networks decreased. In all the fracture networks studied, we found non-Fickian solute transport behavior deduced from small values of β(0.69≤β≤1.25), the exponent in the truncated power-law arrival time distribution in the CTRW model. In our fracture networks, the aperture size spanned less than one order of magnitude meaning that the fracture permeability distribution was relatively narrow. Therefore, the observed non-Fickian behavior should be attributed to sparse fracture networks and low connectivity. We found that the effective permeability, keff, at p30 = 0.05 was more scale-dependent compared to that at p30 = 0.1 because the fracture networks with p30 = 0.05 were less connected and, thus, more heterogeneous. Our simulation also disclosed the scale dependence of the CTRW model parameters. We conducted multiple linear regression analysis to link the CTRW model parameters to the network size as well as the geometrical and topological properties of the network. Results of regression analysis showed that all the CTRW model parameters except the average solute velocity were significantly dependent on the system size L (p-value < 0.05). Further investigations using a broader range of fracture network properties, such as broader aperture size distributions, smaller and larger network sizes and volumetric fracture counts are still required.

Effect of fracture geometry, topology and connectivity on energy recovery from enhanced geothermal systems

A combination of hydraulic fracturing and horizontal wells is now being used to tap geothermal energy from naturally fractured reservoirs. Fully grid-based numerical models are currently used to simulate heat recovery from enhanced geothermal systems (EGS). Such models require a fine unstructured mesh and are computationally expensive. In this paper we present a computationally efficient model that allows us to accurately simulate fracture propagation, fluid flow, and heat transfer in networks of natural fractures that may be created in naturally fractured geothermal reservoirs.The integrated simulator is developed by combining the displacement discontinuity method (DDM) for fracture propagation in naturally fractured reservoirs with a general Green's function solution for fluid and heat flow from the matrix to the fracture. This eliminates the need to discretize the matrix domain resulting in a very computationally efficient solution. A discrete fracture network (DFN) approach is used to represent the pre-existing natural fractures.The model is first validated against an analytical solution for fluid flow and heat transfer in a rock matrix with a single fracture. The computation time with and without discretizing the rock matrix shows a 100-fold reduction in computation cost with very little loss in accuracy. Parametric studies are conducted to investigate the effect of the distribution of natural fracture density, length, and orientation. The results show that the efficiency of tapping geothermal energy is impacted by geometrical and topological complexities of the fracture network and in particular the connectivity of backbone fractures. It is, therefore, important to optimize (not maximize) the connectivity and complexity of the backbone fracture network. The computationally efficient model presented here provides a practical tool for optimizing operational parameters for efficient geothermal production.

Characterization of a fractured rock mass using geological strength index: A discrete fracture network approach

Rock mass contains siginificant heterogeneity due to the presence of structural discontinuities. The failure in the rock mass may cause a serious concern to road and rail transportation. Rock mass characterization is a first step towards preliminary investigation for road or tunnel excavations. Different field-based methods have been developed to characterize the rock mass conditions. The Geological Strength Index (GSI) is a simple and commonly adopted method with wide applicability in rock engineering. Traditional approaches are limited to 2D exposures for mapping purposes, but block formation or joint intersection is a 3D parameter. The advancement in computational techniques led to significant involvement of numerical modeling techniques such as those backed with discrete fracture network (DFN). The remote sensing techniques render the data with high precession potential not accessible with conventional methods. The stochastic DFNs generated based upon the statistical distribution of the input parameters can represent the natural fracture system in 3D. The developed synthetic fracture network can be used to examine the rock mass characteristics. This work addresses the incorporation of the Discrete Fracture Network (DFN) in the estimation of the Geological Strength Index (GSI) of the rock mass. The work compares the results of DFN generated using aggregate and disaggregate approaches in block size distribution (BSD) and rock quality designation (RQD) measurements for a fractured slope. The calculated BSD and RQD using DFN and field-estimated joint condition parameter are used to estimate GSI of the rock mass. A machine learning based python GUI tool was developed to find GSI from block volume and joint condition parameters. The prediction of GSI from input parameters using machine learning has led to systematically digitizing the standard GSI chart.

Capability for Hydrogeochemical Modelling within Discrete Fracture Networks

A new method for simulating solute transport and geochemical interactions within fractured rock is presented. This will be an important capability for assessing the safety of radioactive waste disposal facilities that are located within fractured crystalline bedrock. Specifically, the discrete fracture network (DFN) module within the ConnectFlow groundwater flow and transport software has been updated to: (i) simulate the advection and diffusion of more than one solute species (with the flow and transport equations coupled by the evolving density and viscosity); (ii) model the diffusion of solutes into the rock matrix between fractures; and (iii) utilise the iPhreeqc library to model chemical reactions involving solutes, minerals on fracture/pore surfaces and rock minerals. The performance of ConnectFlow’s DFN module has also been significantly improved via parallelisation which allows more complex calculations to be attempted. These developments are significant because hydrogeochemistry within fractured rock is more accurately represented in an explicit DFN, rather than using more approximate equivalent continuous porous medium (ECPM) methods. Illustrative calculations have been completed for the disposal facility for spent nuclear fuel at Olkiluoto in Finland, and the former candidate site for spent fuel disposal, Laxemar, in Sweden. These calculations show that DFN simulations provide results that are qualitatively similar to results from ECPM calculations. However, because the ECPM is a less direct approach, notable differences exist when compared to the DFN approach.

Impact of fracture normal and shear stiffnesses on the scattering attenuation of P and S waves in a naturally fractured rock

In this paper, numerical simulations are conducted to study elastic wave transport, scattering, and attenuation in a naturally fractured rock associated with length-correlated fracture normal and shear stiffnesses. The model represents the pattern of a real fracture outcrop in an explicit fashion based on the discrete fracture network approach and computes the dynamical interaction between waves and fractures based on the displacement discontinuity method. A broad spectrum of geologically relevant fracture stiffness values are explored to analyse the impact of fracture normal and shear stiffness components on the wavefield evolution. It is observed that when the fracture normal and shear stiffnesses are both high, the wavefield is a propagative mode dominated by a forward ballistic transport. With the reduction of fracture normal and/or shear stiffnesses, the wavefield becomes diffusive characterised by the emergence and dominance of coda waves. If the fracture stiffnesses are very low, waves become trapped entering the so-called localisation regime associated with an absence of effective transport as well as a profound attenuation. Our results show that the scattering attenuation of S waves tends to be greater than that of P waves in the propagation and diffusion regimes, but becomes similar in the localisation regime. The research findings of this paper have important implications for understanding and predicting the seismic wave attenuation behaviour in naturally fractured rocks for various geophysical applications.

Finite Element Model to Simulate Two-Phase Fluid Flow in Naturally Fractured Oil Reservoirs: Part I.

Naturally fractured reservoirs host more than 20% of the world’s total oil and gas reserves. To produce from such reservoirs efficiently, a good understanding of the reservoir behavior at various conditions is essential. This allows us to predict the reservoir performance in advance and assess its economic feasibility. However, the production from such reservoirs is challenging due to (a) uncertainty associated with the fracture map, (b) complex physics phenomena of fluid and rock interaction, and (c) lack of comprehensive knowledge of the extent, orientation, and permeability sensitivity of the fracture network. This paper addresses the abovementioned challenges by presenting a three-dimenisonal (3-D) two-phase fluid flow model in a poroelastic environment. The model is based on a hybrid methodology by combining single continuum and discrete fracture network approaches. Also, the capillary pressure effect, saturation, and relative permeability variations are considered. A mathematical formulation for three-dimensional, two immiscible fluid flows including rock deformation for the fracture network and the rock matrix is presented. A standard Galerkin-based finite element method is applied to discretize the poroelastic governing equations in space and time. The characteristic Galerkin discretization method is used to stabilize the solution of the convection equation in a finite element approach. The 3-D model is validated against IMEX commercial software and finite element package to test its robustness. The results show that the developed model has the ability to predict the two-phase flow behavior precisely, which can be used to assess the production performance of naturally fractured reservoirs. Numerical results of fluid flow profiles for single and multiple fractures with different orientations that match experimental oil drainage tests and field case studies are presented that proves the reliability of the developed multiphase flow model. Results of simulated well production data show an excellent match with the field production data.

Reliability and sensitivity analyses of discrete fracture network based contaminant transport model in fractured rocks

Understanding the flow and transport of contaminants in fractured rocks play a fundamental role in geo-environmental problems since subsurface contamination poses a serious threat to human health and the environment. Therefore, it is imperative to develop predictive models and understand the potential impact of contaminant movement in the subsurface environment (i.e., fractured rocks). In this paper, a two-dimensional numerical model is developed to study the groundwater flow and contaminant transport behavior in fractured rock mass based on the discrete fracture network approach. The effect of aperture variation along the fractures is also incorporated as an additional feature in the model. The contaminant transport through heterogeneous fracture network and the intact rock matrix is simulated, the contaminant concentration evolving over spatial and temporal scales is computed, and the influence of various transport mechanisms on the contaminant migration process is also discussed. Uncertainty analysis forms an intrinsic part of predictive modelling. Hence, the stochasticity in the fracture pattern generation, uncertainties in the geological properties of the medium, transport properties of the contaminant are considered, and efficient probabilistic techniques are employed to quantify the effect of these uncertainties. The probability of contaminant concentration exceeding the permissible limit (known as probability of failure) is estimated by implementing computationally effective subset simulation method. The influence of the coefficient of variation in the uncertain input parameters of the model, and the stochasticity in the fracture network on the probability of failure are also studied. Further, the critical parameters affecting the concentration of the contaminant are estimated by performing sensitivity analysis. Thus, the probabilistic framework presented in this study highlights the impact of various uncertainties on the contaminant migration process. It also provides a technical basis for the engineering practitioners in identifying the key areas that require utmost attention while planning contaminant site remediation or designing waste disposal system.

Modeling radial groundwater flow in fractured media using fracture continuum approach

Two modeling approaches are commonly utilized for simulating flow in fractured formations: the discrete fracture network (DFN) approach and the stochastic continuum (SC) approach. Although the DFN approach is the most accurate, it has computational and memory constraints. The SC approach ensures fast processing but results in system over-homogenization. The fracture continuum (FC) approach arises as an integrated technique that incorporates the merits of both approaches. The main objective of this research is to develop a computationally efficient technique based on the FC approach to simulate the radial groundwater flow towards wells through two-dimensional fractured media under both steady and transient conditions. A stochastic generation of the DFN is performed in a Monte Carlo framework taking into account wells positioning. The DFN flow system is solved by applying the mass balance equation at fracture intersections. Fracture segments are mapped onto grids of 1 × 1 m and 5 × 5 m resolution as conductivity and specific storage cells. The grid flow problem is solved via MODFLOW. Flow and head discrepancies between the proposed technique and the DFN approach (reference solution) are assessed in steady and transient conditions. A grid-conductivity correction is needed to preserve the DFN flow in the presence of wells. A porosity estimation is proposed to identify the grid-pressure transient response. Promising flow and head results are observed for fine and coarse grid models. Some of the studied cases show large discrepancies in the maximum drawdown obtained on the coarse grid model. Accordingly, a new technique is proposed to handle such discrepancies and is found efficient in transient simulations (e.g., 11% and 26.12% discrepancies are minimized to − 0.93% and − 1.03% for two studied cases). The adopted mapping technique is found efficient when the interest is to estimate the average drawdown over an aquifer as correlation coefficients of 0.89 and 0.97 are found for the coarse and fine grid models, respectively when compared to the DFN model. However, the technique has limitations in estimating the drawdown at locations of wells.

Interpretation of a network-scale tracer experiment in fractured rock conducted using open wells

The presence of fractures in bedrock allows for rapid aqueous contaminant transport through complex pathways and for diffusion of solutes between the fractures and the matrix. To better understand transport in these settings, tracer experiments are a commonly used tool. The need for expensive multi-level wells to obtain depth-specific concentrations, however, significantly limits the cost efficiency. The primary objective of this study is to develop a method whereby a discrete fracture network approach can be used to simulate the results of a divergent tracer experiment conducted using open observation boreholes in a well-characterized dolostone over distances of 55 m to 242 m. The experiment was conducted using a fluorescent tracer which allowed for continuous concentration measurement with depth in each open observation well. Two numerical models were employed in the interpretation of the experiment. The first was a 1-D finite difference model focused on flow and transport in the observation wells and the second was a 3-D control-volume finite element model capable of simulating the entire fracture network. Through fitting the experimental data to simulations, the most important fractures for transport in the system were identified. The number of fractures that participated in transport was few relative to the number of fractures observed in core and in constant head test results. Heterogeneous distribution of the fracture apertures was determined to be the likely cause of the highly tortuous transport observed at the site. This study demonstrates that tracer experiments conducted using open observation boreholes and a downhole fluorometer can improve our understanding of large-scale transport in fractured rock, especially when analysed with multiple models, and compared to other measured properties such as matrix porosity, hydraulic aperture, and fracture orientation.

Numerical study of fluid injection-induced deformation and seismicity in a mature fault zone with a low-permeability fault core bounded by a densely fractured damage zone

We apply a fully-coupled hydro-mechanical simulation tool to study fluid injection-induced pressure diffusion, poroelastic response, fracture slip, damage growth, and microseismicity occurrence in a complex fault zone. The fault zone has a low-permeability fault core bounded by a high-permeability damage zone, which is characterised by a fracture network with the fracture density exponentially decaying away from the fault core. We employ the discrete fracture network approach to explicitly represent the distribution and behaviour of these fractures in the fault zone. We use the finite element method to solve the coupled governing equations of fluid flow, solid deformation, and damage evolution in the fractured porous rock. We explore a number of scenarios with different orientations for the in-situ stress field and different locations for a constant rate fluid injection. We report an interactive control of fracture density, fracture orientations, in-situ stress state, and injection point position on the seismo-hydro-mechanical behaviour of the fault zone subject to fluid injection. Insight from our research findings may have important implications for injection-related geoengineering activities such as the development of enhanced geothermal systems and unconventional hydrocarbon reservoirs.

Upscaling of fractured rock mass properties – An example comparing Discrete Fracture Network (DFN) modeling and empirical relations based on engineering rock mass classifications

In this study a comparison of Discrete Fracture Network (DFN) modeling and empirical relationships based on conventional engineering rock mass classifications is carried out in order to determine the elastic properties of fractured rock masses. Uncertainties are considered by a stochastic description of the fracture network geometry and a probabilistic approach, utilizing Monte Carlo simulation techniques, for the various other input parameters. The proposed workflow is applied to two sandstone outcrops. Geometric information on the fracture systems is obtained from terrestrial laser scanning providing the input parameters (orientation, size and intensity) for each fracture set including probability density functions required for DFN modeling. After adding the mechanical properties of intact rock and fractures obtained by laboratory tests, the DFN approach allows to calculate values for rock mass deformation modulus and Poisson's ratio assuming either isotropy or vertical-transverse isotropy. These estimates are compared to values derived from empirical relationships based on engineering rock mass classifications. The comparison of the two approaches and, in particular, of the different empirical relationships reveals a significant scatter in the estimated rock mass properties. Only those empirical equations which incorporate intact rock properties in addition to the classification score yield plausible results, i.e., a pronounced weakening due to the fractures, and are in agreement with the estimates of the DFN approach. The additional effort of generating a DFN model shows its benefits if an anisotropic rock mass and/or spatially varying properties have to be considered, thus, providing a comprehensive hydromechanical characterization of anisotropic fractured rock masses.

Impact of fracture shear dilation on long-term heat extraction in Enhanced Geothermal Systems: Insights from a fully-coupled thermo-hydro-mechanical simulation

Shear dilation of fractures has been recognized as a main mechanism of permeability enhancement by hydraulic stimulation in Enhanced Geothermal Systems (EGSs); however, the interactive role of fracture shear dilation and thermo-hydro-mechanical (THM) coupling processes in long-term heat extraction performance of EGSs remains unclear. In this study, we develop a novel THM coupling model based on the discrete fracture network approach, which can realistically capture important processes including hybrid normal-shear deformation of fractures, thermal expansion of rocks, fluid flow in both fractures and rocks, and heat convection/conduction as well as local thermal non-equilibrium effect and changes in physical parameters of fluid. We quantitatively investigate the effects of fracture network geometries and geomechanical boundary constraints on fracture shear dilatancy, and the resulting heat transfer characteristics of EGSs. Numerical results reveal that shear dilation of fractures can be triggered by transient pore pressurization and thermal stress under anisotropic in-situ stress condition, and would severely engender flow channeling as well as anisotropic heat transfer, which strongly impact the heat extraction performance. The production temperature tends to be overestimated while the thermal production rate may be underestimated, if the shear dilatational behavior is not incorporated. Increased in-situ stress ratio and injection/production pressure would magnify the effects of shear dilation, and lead to considerable enhancement of fracture permeability, eventually resulting in much earlier and quicker temperature drop. Excessive increase of fracture density and the location of injection-production wells parallel to potential channelized flow paths, formed by intersected fractures preferentially oriented for shear sliding, tend to form short circulating flow paths and reduce the heat extraction performance. Our study demonstrates the importance of considering fracture shear dilation and fully-coupled THM behaviors when evaluating the long-term performance and efficiency of heat extraction in EGSs.

Transport and Localization of Elastic Waves in Two‐Dimensional Fractured Media: Consequences on Scattering Attenuation

AbstractWe present numerical simulations of elastic wave transport in two‐dimensional fractured media. Natural fracture systems, following a power‐law length scaling, are modeled by the discrete fracture network approach for geometrically representing fracture distributions and the displacement discontinuity method for mechanically computing fracture‐wave interactions. The model is validated against analytical solutions for wave reflection, transmission, and scattering by single fractures, and then applied to solve the wavefield evolution in synthetic fracture networks. We find that the dimensionless angular frequency ῶ = ωZ/κ plays a crucial role in governing wave transport, where ω, Z, and κ are the angular frequency, seismic impedance, and fracture stiffness, respectively. When ῶ is smaller than the critical frequency ῶc (≈5), waves are in the extended mode, either propagating (for small ῶ) or diffusing by multiple scattering (for intermediate ῶ); as ῶ exceeds ῶc, waves become trapped, entering either the Anderson localization regime (kl* ≈ 1) in well‐connected fracture systems or the weak localization regime (kl* &gt; 1) in poorly‐connected fracture systems, where k is the incident wavenumber and l* is the mean free path length. Consequently, the inverse quality factor Q−1 scales with ῶ obeying a two‐branch power‐law dependence, showing significant frequency dependence when ῶ &lt; ῶc and almost frequency independence when ῶ &gt; ῶc. Furthermore, when ῶ &lt; ῶc, the wavefield exhibits a weak dependence on fracture network geometry, whereas when ῶ &gt; ῶc, the fracture network connectivity has an important impact on the wavefield such that strong attenuation occurs in well‐connected fracture systems.

The Impact of Fracture Persistence and Intact Rock Bridge Failure on the In Situ Block Area Distribution

The in situ block size distribution is an essential characteristic of fractured rock masses and impacts the assessment of rockfall hazards and other fields of rock mechanics. The block size distribution can be estimated rather easily for fully persistent fractures, but it is a challenge to determine this parameter when non-persistent fractures in a rock mass should be considered. In many approaches, the block size distribution is estimated by assuming that the fractures are fully persistent, resulting in an underestimation of the block sizes for many fracture geometries. In addition, the block size distribution is influenced by intact rock bridge failure, especially in rock masses with non-persistent fractures, either in a short-term perspective during a slope failure event when the rock mass increasingly disintegrates or in a long-term view when the rock mass progressively weakens. The quantification of intact rock bridge failure in a rock mass is highly complex, comprising fracture coalescence and crack growth driven by time-dependent changes of the in situ stresses due to thermal, freezing-thawing, and pore water pressure fluctuations. This contribution presents stochastic analyses of the two-dimensional in situ block area distribution and the mean block area of non-persistent fracture networks. The applied 2D discrete fracture network approach takes into account the potential failure of intact rock bridges based on a pre-defined threshold length and relies on input parameters that can be easily measured in the field by classical discontinuity mapping methods (e.g., scanline mapping). In addition, on the basis of these discrete fracture network analyses, an empirical relationship was determined between (i) the mean block area for persistent fractures, (ii) the mean block area for non-persistent fractures, and (iii) the mean interconnectivity factor. The further adaptation of this 2D approach to 3D block geometries is discussed on the basis of general considerations. The calculations carried out in this contribution highlight the large impact of non-persistent fractures and intact rock bridge failure for rock mass characterization, e.g., rockfall assessment.

Improvement of the concept of the blockiness level of rock masses

The concept of blockiness level, which enables the measurement of the jointing degree of rock mass in three dimensions, is limited due to some significant shortcomings. In this study, to tackle the limitations, the guidelines for the applications of the concept were formulated according to the calculation basis of blockiness level. Then, a more reasonable computational method of the blockiness level was proposed, the ratings for the blockiness levels and corresponding jointing degrees of rock mass were modified, and the improved concept of the blockiness level was validated and supported through the artificial data. Finally, the improved concept was applied to two real cases, i.e., the Wudongde Hydropower Project (China) and the underground powerhouse of the Three Gorges Project (China). Different discontinuity network models, which were generated by deterministic discontinuities and following the Discrete Fracture Network approach, respectively, were presented in the two case studies, and the results show that the blockiness levels of the two adits (i.e., PD49-1 and PD 4) in Wudongde Hydropower Project are 12.847% and 10.168%, respectively, both belonging to the relatively integrated category, and the blockiness level of the underground powerhouse of the Three Gorges Project is 3.6‰, falling into the integrated category. The improved concept of the blockiness level was found to be acceptable and practicable.

The concept of representative elementary length (REL) as an effective tool to study scale effects in rock engineering problems

In this paper, a discrete fracture network approach (DFN) is used to study scale effects on rock quality designation (RQD) measurements. RQD is a parameter that describes rock mass quality and represents a fundamental component of several rock mass classification systems. The results demonstrate that it is possible to define a representative elementary length (REL), above which RQD measurements represent an average indicator of rock mass quality. However, the directional bias of RQD measurements is such that the choice of REL is itself a function of the orientation of the sampling line used to estimate RQD. By considering multiple sampling directions, this paper introduces the concept of a REL Ellipsoid, whereby the normalized value of the REL along three sampling directions indicates the degree of homogeneity and isotropy of the rock mass with increase in problem scale. In the authors’ opinion, the REL Ellipsoid concept allows to better capture the nature of the 3D representative elementary volume (REV) for both isotropic and anisotropic rock masses Mapping data from a room-and-pillar mine are used in the initial validation of the proposed REL Ellipsoid concept.

Role of interaction between hydraulic and natural fractures on production

One of the main purposes of hydraulic fracturing in unconventional resources such as shale is to improve the connectivity between existing natural fractures and the production well. Since the inherent permeability of shale is extremely low, this newly formed fracture connectivity provides faster paths for the hydrocarbon stored in preexisting natural fractures. We performed numerical simulations of the free gas production at large scale fractured reservoir with different pre-existing natural fracture network intensities and with variable hydraulic fracture sizes and staging. We explored production characteristics as a function of the properties of both, natural fractures (intensity) and hydraulic fractures (size and staging). Our hypothesis is that once maximum fracture connectivity is achieved, additional stimulation does not increase production significantly. In this numerical study we use an advanced modeling tool to study fractures connectivity, Discrete Fracture Network (DFN) approach, which can represent fracture networks similar to those observed on individual reservoir sites. In our model the horizontal production well and vertical hydraulic fractures were deterministically defined and surrounded by stochastically generated natural fracture networks. The performed numerical experiments show how fracture connectivity depends on hydraulic fracture settings in fractured reservoir with different preexisting fracture densities. We numerically showed that cumulative free gas production is not affected significantly by hydraulic fractures size (25 m–150 m) or spacing (15 m–45 m) for dense natural fracture networks. However, the hydraulic fracture size is important in reservoirs with sparse natural fractures, although spacing (number of stages) does not play a significant role.

Transient hydraulic tomography approach to characterize main flowpaths and their connectivity in fractured media

In sparse fracture systems flow and transport patterns are often dominated by main flowpaths and characterized by a strong complexity induced by the fractures’ surface roughness, hierarchical arrangement in networks, and from the interaction of the fractures with the surrounding rocks. The accurate characterization of the hydraulic properties and connectivity of major fracture zones is essential to model flow, solute and heat transport and fluid pressure propagation in fractured media. In this study, we present a novel deterministic inverse modeling method for imaging the connectivity and spatial variability of transmissivity and storativity of a network of fractures. The method is based on a numerical model that simulates fluid flow in a simple three-dimensional (3-D) discrete fracture network (DFN) with a fixed fracture network structure. The forward model is coupled to an inverse algorithm to match observed pressure transients obtained from sequential hydraulic cross-hole tests. This method has been successfully tested for constant rate injection tests carried out at the Grimsel Test Site (GTS) in Switzerland. Cross-hole injection tests were conducted in a tomographic configuration, with hydraulic responses monitored at four observation intervals at various depths in two different boreholes. A discrete fracture network approach with a simple parametrization is developed to estimate log-transformed transmissivity (T) and storativity (S) values of hydraulically active fractures between boreholes by inverting the pressure transients, under the hypothesis that T and S are independent variables. We identified several permeable fractures and their connectivity without attempting to represent explicitly the true fracture network geometrical properties (length, orientation, dip), focusing instead on the calibration of a simple model capturing the observed pressure main behavior. The identified inter-borehole connectivity structure agrees well with independent information, including additional data from a cross-borehole step rate injection and hydrogeophysical data. Hence, the proposed tomography approach appears to be a promising approach for characterizing the connectivity structure and hydraulic properties of the main flowpaths in sparsely fractured rock.

An automated fracture trace detection technique using the complex shearlet transform

Abstract. Representing fractures explicitly using a discrete fracture network (DFN) approach is often necessary to model the complex physics that govern thermo-hydro-mechanical–chemical processes (THMC) in porous media. DFNs find applications in modelling geothermal heat recovery, hydrocarbon exploitation, and groundwater flow. It is advantageous to construct DFNs from the photogrammetry of fractured outcrop analogues as the DFNs would capture realistic, fracture network properties. Recent advances in drone photogrammetry have greatly simplified the process of acquiring outcrop images, and there is a remarkable increase in the volume of image data that can be routinely generated. However, manually digitizing fracture traces is time-consuming and inevitably subject to interpreter bias. Additionally, variations in interpretation style can result in different fracture network geometries, which, may then influence modelling results depending on the use case of the fracture study. In this paper, an automated fracture trace detection technique is introduced. The method consists of ridge detection using the complex shearlet transform coupled with post-processing algorithms that threshold, skeletonize, and vectorize fracture traces. The technique is applied to the task of automatic trace extraction at varying scales of rock discontinuities, ranging from 100 to 102 m. We present automatic trace extraction results from three different fractured outcrop settings. The results indicate that the automated approach enables the extraction of fracture patterns at a volume beyond what is manually feasible. Comparative analysis of automatically extracted results with manual interpretations demonstrates that the method can eliminate the subjectivity that is typically associated with manual interpretation. The proposed method augments the process of characterizing rock fractures from outcrops.

Modelling transport in fractured media using the fracture continuum approach

Two of the commonly used approaches in modelling flow and transport in fractured geological media are the discrete fracture network (DFN) approach and the stochastic continuum (SC) approach. The former approach is computationally demanding and requires large input parameters, whereas the latter approach is computationally efficient and requires fewer parameters but at the expense of not preserving fracture network details and properties. The fracture continuum (FC) approach combines the advantages of both the DFN and SC approaches. The main objective of this research is to develop a mapping technique utilizing the FC approach to preserve transport characteristics between DFN and the mapped continuum. A two-dimensional particle tracking model is developed to simulate conservative contaminant transport through DFN. The fracture network is randomly generated in a stochastic Monte Carlo framework, and the flow problem is solved using mass conservation at the fracture junctions. The transport problem is then solved via the developed particle tracking model on the DFN. The obtained transport solution is used as a reference solution. The fracture network is mapped onto a finite difference grid at four different grid cell sizes: 1 m × 1 m, 2 m × 2 m, 5 m × 5 m, and 10 m × 10 m, and the flow problem is solved via MODFLOW. The transport problem is solved on the grid using a particle tracking method. Comparisons between the transport characteristics for both approaches (DFN and FC) are performed, and the percentage error in each case is quantified. It is found that a new correction factor is needed to preserve conservative transport characteristics on the grid. The developed correction factor improves the ability of the FC technique to preserve transport on the grid for the case of conservative contaminant transport.