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ERE6.3

Fractures are discontinuities in rocks that are present in almost all geological settings and at any scale. They may represent small-scale fissures or build up large scale faults. Fractures are extreme forms of heterogeneities, often with a small extension but huge impact.
The presence of fractures modifies the bulk physical properties of the original media by many orders of magnitudes, and they often introduce a strongly nonlinear behavior. This refers in particular to the mechanical properties via reduction of strength and stiffness. Fractures also provide the main flow and transport pathways in hard rock aquifers, dominating over the permeability of the rock matrix, as well as creating anisotropic flow fields and transport. Understanding their hydraulic and mechanical properties of fractures and fracture networks thus are crucial for predicting the movement of any fluid such as of water, air, hydrocarbons, or CO2. Consequently, fractures are of great importance in various disciplines such as hydrogeology, hydrocarbon reservoir management, and geothermal reservoir engineering.
The geologist toolbox to explore and model fractured rocks is getting more and more extended. This session is dedicated to novel ideas and concepts on treating the challenges related to the generic understanding, the characterization and the modelling of fractured geological media.
Contributions are welcome from the following topics
• Exploration methods for mechanical and/or hydraulic characterization of fractured media
• Structural construction of fractured media by deterministic or stochastic approaches,
• Representation of static hydraulic and/or mechanical characteristics of fractured media involving continuous or discontinuous methods,
• Simulation of dynamic processes and the hydraulic and/or mechanical behavior of fractured media,
• Theoretical studies and field applications in fractured geological formations,
• Concepts of accounting for fractured properties specifically in groundwater, petroleum or geothermal management applications.

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Co-organized by EMRP1/TS3
Convener: Márk SomogyváriECSECS | Co-conveners: Florian Amann, Peter Bayer, Mohammadreza Jalali
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| Fri, 08 May, 16:15–18:00 (CEST)

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Chat time: Friday, 8 May 2020, 16:15–18:00

D779 |
EGU2020-13670
| solicited
Philipp Blum, Sina Hale, Chaojie Cheng, Tobias Kling, Frank Wendler, Lars Pastewka, and Harald Milsch

In various reservoirs such as geothermal reservoirs or host rocks for nuclear waste, fractures and in particular fracture apertures play a crucial role in acting as conduits or even barriers, and therefore control fluid flow and solute transport in such reservoirs or host rocks. Often such reservoirs are simulated by discrete fracture network (DFN) models, whose performance however rely strongly on reliable input parameters such as fracture apertures under different conditions. Hence, in this study we examine various novel field and numerical methods, which are able to determine hydraulic, mechanical and even chemical apertures of natural fractures. First, we compare three different methods, (1) syringe air permeameter, (2) microscope camera and (3) laser scanner for determining hydraulic fracture apertures. Our results prove that the air permeameter allows direct and reliable measurements of hydraulic apertures in the laboratory and also in the field. Additionally, the novel air permeameter could be successfully validated by flow through experiments using various types of fractured core samples. In contrast, microscope camera and laser scanner only provide reliable mechanical apertures. In order to also simulate fracture closure under normal stresses, an innovative contact mechanical approach is introduced and validated using a granodiorite fracture. The simulations indicate the best performance for an elastic–plastic (EP) model, which fits almost perfectly the experimentally derived normal closure data. Finally, a phase-field model (PFM) for hydro­thermally induced quartz growth is used to understand the effect of sealing fractures on the flow behaviour. Our results demonstrate that flow behaviour and hydraulic properties of such chemically altered fractures, i.e. chemical fractures, significantly depend on the evolving crystal geometries. Consequently, a novel equation to estimate hydraulic apertures is derived, which includes a geometry factor α for dissimilar crystal geometries (α = 2.5 for needle quartz and α = 1.0 for compact quartz). Finally, the outcome of our studies clearly demonstrate that nowadays novel experimental and numerical methods exist to precisely determine various fracture apertures improving our understanding of coupled processes on the fluid flow behaviour in fractured media.

Acknowledgements to Florian Amann, Christoph Butscher, Frieder Enzmann, Christoph Naab, Jens Oliver Schwarz and Daniel Vogler

How to cite: Blum, P., Hale, S., Cheng, C., Kling, T., Wendler, F., Pastewka, L., and Milsch, H.: Determination of hydraulic, mechanical and chemical fracture apertures , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13670, https://doi.org/10.5194/egusphere-egu2020-13670, 2020

D780 |
EGU2020-2937
A model for off-fault plastic poroelastic deformation and its effects on permeability
(withdrawn)
Bora Yalcin, Olaf Zielke, and P. Martin Mai
D781 |
EGU2020-6920
Cyrill von Planta, Maria G.C. Nestola, Daniel Vogler, Patrick Zulian, Nasibeh Hassanjanikhoshkroud, Xiaoqing Chen, Martin O. Saar, and Rolf Krause

Fictituous domain methods provide an promising way for simulating fluid structure interaction in fractures with complex geometries. The main characteristic of the method is that the solid and the fluid problem are simulated on different, non-matching meshes, with the solid being immersed into the fluid. The problems are coupled by L- projections, which transfer physical variables between the two computational domains and either the penalty, augmented Lagrangian or Lagrange multiplier method to represent the solid in the fluid. We show the evolution of our framework in the last three years, starting with benchmark problems such as Poiseulle flow, with successive extension to contact, fracture intersections and thermal coupling.

How to cite: von Planta, C., Nestola, M. G. C., Vogler, D., Zulian, P., Hassanjanikhoshkroud, N., Chen, X., Saar, M. O., and Krause, R.: Fictitious Domain methods for simulating thermo-hydro-mechanical processes in fractures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6920, https://doi.org/10.5194/egusphere-egu2020-6920, 2020

D782 |
EGU2020-7914
Farzad Basirat, Chin-Fu Tsang, Alexandru Tatomir, Yves Guglielmi, Patrick Dobson, Paul Cook, Chris Juhlin, and Auli Niemi

Characterization of the coupled hydro-mechanical properties of rock fractures has become an increasingly important field of geosciences research, relevant for a number of key applications. Examples include analysis of enhanced geothermal systems, hydraulic fracturing operations, CO2 geological storage, nuclear waste disposal and mining operations. A newly developed technology that allows conducting advanced experimentation of the coupled HM processes in the field is the step-rate injection method for fracture in-situ properties (SIMFIP) by Guglielmi et al. (2014). The SIMFIP method is unique in that it measures simultaneously the time evolution of flow rate, pressure and 3D deformation of a packed off borehole interval.  

During June 2019 a field campaign was carried out in Åre, Sweden, where the SIMFIP was applied in the COSC-1 scientific borehole to estimate the fracturing and fracture propagation behavior during hydraulic stimulation in some previously well-characterized rock sections. Three intervals were investigated: an unfractured section (intact rock) at 485.2 m depth, a non-conductive steeply dipping fracture at 515.1 m depth, and a section with a gently dipping hydraulically conductive fracture at 504.5 m depth (Niemi et al., in prep.). 

As a first step for analyzing the results, this work aims to develop a simple hydrologic model for the interpretation of the collected pressure and flow data during different stages of the experiments. Modeling has been used to estimate the key parameters of the induced and propagated fractures such as the length, aperture and geometry, based on the pressure response during the water injection and abstraction steps. A numerical model based on COMSOL Multiphysics combining the fluid flow within the fracture and rock domains was developed and the permeability of fractures was defined by the well-known cubic law function of the local fracture aperture. The initial low injection-pressure data for the test interval without any fracture were used to find the parameters of the packed off borehole interval. Consequently, these parameters were used in the analysis of the case with a conducting fracture, as well as the case with a non-conducting fracture. Models in agreement with the observed pressures and injection flow rates could be defined for all the three cases, allowing parameters to be estimated for the length and aperture of the induced fractures in each case.

 

Guglielmi Y, Cappa F, Lançon H, Janowczyk JB, Rutqvist J, Tsang CF and Wang JSY. (2014) ISRM Suggested Method for Step-Rate Injection Method for Fracture In-Situ Properties (SIMFIP): Using a 3-Components Borehole Deformation Sensor. Rock Mech Rock Eng 47:303–311. https://doi.org/10.1007/s00603-013-0517-1

Niemi, Auli, Yves Guglielmi, Patrick Dobson, Paul Cook, Chris Juhlin, Chin-Fu Tsang, Benoit Dessirier, Alexandru Tatomir, Henning Lorenz, Farzad Basirat, Bjarne Almqvist, Emil Lundberg and Jan-Erik Rosberg 'Coupled hydro-mechanical experiments on fractures in deep crystalline rock at COSC-1 – Field test procedures and first results’. Manuscript under preparation, to be submitted to Hydrogeology Journal.

 

 

How to cite: Basirat, F., Tsang, C.-F., Tatomir, A., Guglielmi, Y., Dobson, P., Cook, P., Juhlin, C., and Niemi, A.: Analysis of flow and pressure data for the estimation of fracture generation and propagation – first model results from coupled hydromechanical experiments in COSC-1 borehole in deep crystalline rock, Åre, Sweden, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7914, https://doi.org/10.5194/egusphere-egu2020-7914, 2020

D783 |
EGU2020-7786
Shahin Jamali, Volker Wittig, and Rolf Bracke

Acoustic Emission (AE) based systems have been under development and used in this research at Fraunhofer – IEG to monitor, evaluate, and control conventional and novel drilling processes and their pertinent equipment used in geothermal applications. Moreover, new stimulation and high pressure (radial) jetting and drilling operations in deep geothermal reservoirs do heavily rely on such new technologies in order to be able to control them properly and thus, to generate an optimal connection between the main wellbore and the reservoir. As Service intervals and lifetime of machines have long been predicted and monitored via Acoustic Emission (AE) systems, and it is becoming a standard in numerous other industrial operations, AE is known as being a promising technique to be used for such monitoring purposes. AE monitoring is based on the detection and conversion of elastic waves into electrical signals, which are typically associated with a rapid release of localized stress-energy propagating within a given material. Thus, it is passive testing, logging, and analysis method to evaluate changes in the properties and behavior of machines and also mineral type materials such as rocks during operations. Such changes may be induced by drilling, jetting, or other drilling methods and being recorded, located, and evaluated via an AE system. This is the core of Fraunhofer – IEG’S new development, the AE based, so-called Multi-Sensor acoustic parameter analysis (MoUSE) as the primary control and monitoring mechanism during rock breaking, drilling, jetting, and stimulation. AE signals generated during jetting or bit-rock interaction are being monitored and analyzed extensively using novel numerical methods, based on sound analysis and engineering applications. The objective of this paper is to present an alternative approach for QA and QC during drilling, jetting, and stimulation operations based on AE waveforms generated during such continuous processes, including jetting and thermal drilling processes. Initial results of rock breaking tests, including mechanical, and non-contact drilling or jetting, will be presented.

How to cite: Jamali, S., Wittig, V., and Bracke, R.: Multi-Sensor Acoustic Parameter Analysis System for Monitoring and Evaluation of Deep Drilling, Jetting, and Stimulation Operations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7786, https://doi.org/10.5194/egusphere-egu2020-7786, 2020

D784 |
EGU2020-8987
Mohammad Javad Afshari Moein

Enhanced Geothermal System (EGS) development requires an accurate fracture network characterization. The knowledge on the fracture network is fundamental for setting up numerical models to simulate the activated processes in hydraulic stimulation experiments. However, direct measurement of fracture network properties at great depth is limited to the data from exploration wells. Geophysical logging techniques and continuous coring, if available, provide the location and orientation of fractures that intersect the wellbore. The statistical parameters derived from borehole datasets (either from image logs or cores) constrain stochastic realizations of the rock mass, known as Discrete Fracture Network (DFN) models. However, accurate parametrization of DFN models requires sufficient knowledge on the depth-dependent spatial distribution of fractures in the earth’s crust.

This analysis includes a unique collection of fracture datasets from six deep (i.e. 2-5 km depth) boreholes drilled into crystalline basement rocks at the same tectonic settings. All the wells were drilled in the Upper Rhine Graben in Soultz-sous-Forêts Enhanced Geothermal System, France, except the well that was drilled in Basel geothermal project, Switzerland. The datasets included both borehole image logs and core samples, which have a higher resolution. Two-point correlation function was selected to characterize the power-law scaling of fracture patterns. The correlation dimension of spatial patterns showed no systematic variations with depth at one standard deviation level of uncertainty in moving windows of sufficient number of fractures along any of the boreholes. This implies that a single correlation dimension is sufficient to address the global scaling properties of the fractures in crystalline rocks. One could also anticipate the spatial distribution of deeper reservoir conditions from shallower datasets. On the contrary, the fracture density showed some variations with depth that are sometimes consistent with changes in lithology and geological settings at the time of fracture formation.

How to cite: Afshari Moein, M. J.: Depth-dependent analysis of fracture patterns inferred from image logs and cores in crystalline rocks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8987, https://doi.org/10.5194/egusphere-egu2020-8987, 2020

D785 |
EGU2020-20127
Alexis Shakas, Hannes Krietsch, Marian Hertrich, Nima Gholizadeh, Katrin Plenkers, Hansruedi Maurer, Matthias Meier, Simon Loew, Morteza Nejati, Rebecca Hochreutener, Xiaodong Ma, Stefan Wiemer, Thomas Driesner, Domenico Giardini, Francisco Serbeto, Raymi Castilla, and Peter Meier

Engineered Geothermal Systems (EGS) are gaining increasing popularity as a source of renewable energy without significant CO2 emissions. Fractured crystalline rock masses offer a promising environment for exploitation of geothermal energy. In such a setting, fractures and faults are the main conduits for fluid flow and heat transport. In-situ fracture permeabilities are usually too low at depths where rock mass temperatures are sufficiently high for geothermal energy production. Therefore, a suitable heat exchanger needs to be engineered by hydraulic stimulations. A proper in-situ characterization of the fracture geometry and hydro-mechanical properties is of primary importance for the design of the stimulation operations. This is often the most challenging task, since the majority of the fractures in the reservoir are usually inaccessible for direct characterization.

 

The Bedretto Underground Laboratory for Geosciences (BULG) provides a novel and unique environment to study EGS-related processes, such as seismo-hydro-mechanical fault zone response during hydraulic stimulation and subsequent fluid circulation experiments. The laboratory is hosted in an access tunnel from the Bedretto Valley in the Southern Swiss Alps to a railway tunnel from the Matterhorn-Gotthard-Bahn. The overburden of more than 1000 m above the BULG provides conditions that are approaching those of realistic EGS systems. For the rock mass characterization, three boreholes were drilled perpendicular to tunnel axis with lengths ranging from 190 m to 300 m.

 

We present first data sets from a variety of methodologies, ranging from hydrological tests to geophysical borehole- and remote-imaging. The complementary nature of these data sets allows us to construct a preliminary three dimensional geological model. Notably, the individual measurements yielded information over a multitude of scales, ranging from millimeter-scale core-log information to decameter scale low-frequency Ground Penetrating Radar measurements. Such a wide range of scales is critical for the characterization of EGS reservoirs. The most prominent feature found is a large-scale fracture zone that extends across the entire investigation volume. This fracture zone will be the target for upcoming stimulation experiments.

How to cite: Shakas, A., Krietsch, H., Hertrich, M., Gholizadeh, N., Plenkers, K., Maurer, H., Meier, M., Loew, S., Nejati, M., Hochreutener, R., Ma, X., Wiemer, S., Driesner, T., Giardini, D., Serbeto, F., Castilla, R., and Meier, P.: Preliminary results from interdisciplinary fault characterization in the Bedretto Underground Laboratory, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20127, https://doi.org/10.5194/egusphere-egu2020-20127, 2020

D786 |
EGU2020-2950
Ajay Kumar Sahu and Ankur Roy

It well known that fracture networks display self-similarity in many cases and the connectivity and flow behavior of such networks are influenced by their respective fractal dimensions. One of the authors have previously implemented the concept of lacunarity, a parameter that quantifies spatial clustering, to demonstrate that a set of 7 nested natural fracture maps belonging to a single fractal system, but different visual appearances have different clustering attributes. Any scale-dependency in the clustering of fractures will also likely have significant implications for flow processes that depend upon fracture connectivity. It is therefore important to address the question as to whether the fractal dimension serves as a reasonable proxy  for the connectivity of a fractal-fracture network or is it the lacunarity parameter that may be used instead. The present study attempts to address this issue by studying the clustering behavior (lacunarity) and connectivity of fractal-fracture patterns. We compare the set of 7 nested fracture maps mentioned earlier which belong to a single fractal system, in terms of their lacunarity and connectivity values. The results indicate that while the maps that have the same fractal dimension have almost similar connectivity values, there exist subtle differences such that both the connectivity and clustering change systematically with the scale at which the networks are mapped. It is further noted that there appears to be an exact correlation between clustering and connectivity values. Therefore, it may be concluded that rather than fractal dimension, it is the lacunarity or scale-dependent clustering attribute that control connectivity in fracture networks.

How to cite: Sahu, A. K. and Roy, A.: Clustering and Connectivity of Fractal-Fracture Networks: Are they related?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2950, https://doi.org/10.5194/egusphere-egu2020-2950, 2020

D787 |
EGU2020-5157
Lisa Maria Ringel, Márk Somogyvári, Mohammadreza Jalali, and Peter Bayer

This study is aimed at the characterization of discrete fracture networks (DFN) by a transdimensional inversion methodology. It has been demonstrated that the reversible-jump Markov chain Monte Carlo (rjMCMC) is suitable for the inversion of two-dimensional (2D) DFNs. Based on given statistical information and measured data, the algorithm identifies the main characteristics of a DFN correctly.

For this reason, the method will be extended to the inversion of three-dimensional (3D) DFNs which allows more realistic examples. Two main difficulties arise here. First, further constraints have to be defined to limit the number of unknowns due to the high dimensionality of the inversion problem. Second, the forward modelling is a restricting factor concerning the computational costs and the robustness of the iteration. The assumptions made to simplify the governing fluid equations are to be evaluated and the resulting limitations are presented, e.g. small Reynolds number, smooth fracture walls, impermeable rock matrix. Moreover, the errors caused by the numerical solution of the partial differential equation are estimated to verify the correctness of the implementation.

How to cite: Ringel, L. M., Somogyvári, M., Jalali, M., and Bayer, P.: A fast and robust approach for simulating the pressure diffusion in three-dimensional discrete fracture networks applied to inversion problems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5157, https://doi.org/10.5194/egusphere-egu2020-5157, 2020

D788 |
EGU2020-6136
Liz Elphick, Christoph Schrank, Adelina Lv, and Klaus Regenauer-Lieb

Deformation bands are sub-seismic brittle structures found in granular materials. These structures exhibit two spatial distributions: [1] non-linear decay of spacing associated with the damage zone of a fault, and [2] periodic, constant spacing not associated with faults. Periodically spaced deformation bands are of interest as they can be pervasive through porous (>5% φ) formations and are known to impact fluid flow. Bands can act as conduits or barriers to fluid flow and are commonly identified in petroleum reservoirs. An understanding of the factors controlling their distribution is therefore of great importance.

Here, we test a novel mathematical theory postulating that material instabilities in solids with internal mass transfer associated with volumetric deformation are due to elastoviscoplastic p-waves termed cnoidal waves. The stationary cnoidal wave model for periodic compaction bands predicts that their spacing is controlled by important material properties: the permeability of the weak phase in the pores, the viscosity of the weak phase, and the inelastic volumetric viscosity (strength) of the solid grains. A semi-analytical parametric study of the dimensional non-linear governing equations yields a surprisingly simple scaling relationship, which requires testing in the field. Stronger units with higher permeability are predicted to exhibit a wider spacing between deformation bands.

We test the cnoidal-wave model on natural deformation bands from Castlepoint, North Island, New Zealand. These bands are hosted by Miocene turbidites of the Whakataki formation, which formed in tectonically controlled trench-slope basins associated with the onset of subduction of the Pacific plate beneath the Zealandian plate along the Hikurangi subduction margin. Adjacent sand- and siltstone beds exhibit significant differences in deformation band spacing. Spacing statistics derived from field mapping and laboratory measurements of host-rock permeability and strength are employed to test the scaling relation predicted by the cnoidal wave model. Inconsistencies between theoretical and observed spacing are discussed critically.

How to cite: Elphick, L., Schrank, C., Lv, A., and Regenauer-Lieb, K.: Can stationary cnoidal waves explain periodic deformation bands in porous sandstone?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6136, https://doi.org/10.5194/egusphere-egu2020-6136, 2020

D789 |
EGU2020-6751
Márk Somogyvári and Mohammadreza Jalali

Hydraulic stimulation using high-pressure fluid injection has become the common technique for rock mass treatment in various industrial applications such oil & gas, mining and enhanced geothermal system (EGS) development. Hydraulic stimulation is associated with creation of new fractures or dilation of existing fractures that could alter the flow regime in the stimulated reservoir. In this context, it would be beneficiary to understand the dynamic response of the discrete fracture network (DFN) to the stimulation activities rather than comparison between the changes in injectivity and/or transmissivity.

In this work, a 2-D fully coupled hydro-mechanical model is developed to simulate the dynamic response of a fractured reservoir to hydraulic stimulation. The model calculates stresses, fracture fluid pressure and flow inside the fractures, and modifies the physical properties of the individual fractures given these values. All these alterations will be calculated and applied after each simulation timestep. The results of this synthetic modelling will be used to test the time-lapse pressure tomography approach.

Pressure tomography will be simulated at multiple timesteps, to capture the hydraulically active fractures within the system. The used tomographic interpretation will be based on the transdimensional DFN inversion, where model parametrization could change over time. With this methodology we can model the newly opened fractures by the stimulation.

The time-lapse inversion will use the result of the previous timestep as the initial solution for improved efficiency. We test the proposed methodology on outcrop based synthetic 2-D DFN models. The results could capture the changes of permeability (i.e. aperture) as a direct response to hydraulic stimulation.

How to cite: Somogyvári, M. and Jalali, M.: Time-lapse monitoring of fractured rock response to hydraulic stimulation using pressure tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6751, https://doi.org/10.5194/egusphere-egu2020-6751, 2020

D790 |
EGU2020-10913
Peter Achtziger-Zupancic and Simon Loew

Reliable predictions of the distribution of permeabilities on site scale are economically relevant in a wide range of geoscientific disciplines. Not only are predictions important for modeling hydrogeological conditions at site scale but also for using the underground safely and sustainably.

Scale dependent, different geological processes are influencing the distribution of hydrogeological properties. A dataset of about 5000 inflows from individual transmissive fractures draining to about 660 km of drifts and 57 km of boreholes has been compiled into depths of 2000 mbgs of the Variscan age German Ore Mountains (Erzgebirge/Krušné hory). Fracture closure with increasing depth is a main process controlling the distribution of transmissivities. Additionally, orientation, age and mode of fault zones exert a major control on the local distribution of inflows. These factors are locally overprinted by with the presence of contact metamorphic aureoles around Variscan granitic intrusions as seen from transmissivity reversals with depth. However, as seen from a decreasing trend of mean log hydraulic conductivity and permeability, the contact metamorphism exerts minor control on the rock mass hydrology with depth than the decreasing secondary porosity provided by fractures.

These findings are in accordance with results deduced from a worldwide permeability compilation of about 30000 single in-situ permeability measurements to depths of 2000 mbgs. Geological influences on the distribution have been analyzed on permeability-depth relationships using log-log regressions. Depth is generally the most important geological factor, resulting in a permeability decrease of three to four orders of magnitude in the investigated depth range. Beside depth, most influential factors are the long-term tectono-geological history described by geological province which locally is overprinted by current seismotectonic activity as determined by peak ground acceleration. Although petrography might be of local importance, only a low impact has been observed for the global dataset, besides lithologies allowing for karstification.

In summary, the multi-variate analysis of the datasets has improved our generic understanding of the distribution of hydrogeological properties and provides a basis to model hydrogeological processes in fractured crystalline rocks.

How to cite: Achtziger-Zupancic, P. and Loew, S.: Geological Processes to Consider for Modeling the Distribution of Hydrogeological Properties in Fractured Crystalline Rocks on a Site Scale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10913, https://doi.org/10.5194/egusphere-egu2020-10913, 2020

D791 |
EGU2020-7089
Michael Kettermann, Volker Schuller, Andras Zamolyi, and Mira Persaud

Normal faults are common in sedimentary basins and often associated with reservoirs in interbedded sands and clays. Fault rocks therefore also consist of some mixture of sand and clay. Outcrop studies have shown, that these fault rocks can occur as homogeneous mixtures, (multiple) parallel layers of sand and clay without intense grain-scale mixing, or complex structures with brittle clasts of one material embedded in a ductile sheared matrix of the other. Both, the composition and the structure of the fault rock affect its the overall frictional strength at any given position.

The strength of faults in sedimentary basins is crucial information when producing fluids from faulted reservoirs in critically stressed conditions. Increasing pore pressure during injection phases bears the risk of fault reactivation. To minimize the risk of reactivation while maximizing the recovery, our goal is to improve the prediction of fault friction. The predicted friction coefficient can then be used in dynamic reservoir models to calculate the maximum allowed pore pressure increase. 

From literature we compile the friction coefficients for various homogeneous sand-clay mixtures at different effective normal stresses, measured in laboratory tests. The resulting function shows a linear increase of the friction coefficient with increasing sand content, while normal stress only shows an effect for stresses larger than expected at reservoir conditions. We can now use this function to predict the friction coefficient for any given homogeneous sand-clay mixture.

However, fault rocks are often not homogeneous mixtures. To gain insights into natural fault rock compositions, we investigate field and sample data in 2D and 3D from outcrops in northwest Borneo/Malaysia. These show the complex structure of fault rocks on various scales for faults with displacements from cm to decameter range.

In exploration and production workflows, commonly algorithms such as the shale gouge ratio are applied to predict the average volume of clay (Vclay) in the fault rock, based on the amount of clay in the unfaulted rock and the displacement. The average Vclay is then loosely correlated to a friction coefficient, often proprietary to the used software packages. We propose that the structure of the fault rock, i.e. the distribution of clay and sand, affects the frictional properties estimated for the average Vclay.

We use discrete element numerical simulations to study the effect of complex fault rock structures on the fault friction coefficient. We reproduce natural structures from outcrop and sample data and calibrate the mechanical properties of the individual components in the model to fit the natural prototype. In direct-shear tests we then measure the friction coefficient of the entire modelled fault rock. Preliminary results show a discrepancy between the friction coefficient of a homogeneous sand-clay mixture and a more complex geometry with the same clay volume. This suggests errors in currently used approaches that are solely based on Vclay.

How to cite: Kettermann, M., Schuller, V., Zamolyi, A., and Persaud, M.: The strength of layered siliciclastic fault rocks as a function of composition and structure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7089, https://doi.org/10.5194/egusphere-egu2020-7089, 2020

D792 |
EGU2020-12857
Qiang Xie, Yuxin Ban, Xiang Fu, and Chunbo He

Quantitative evaluation of the fracture morphology of shale is an essential prerequisite for assessing the complexity of hydraulic fracturing fracture networks during shale gas exploitation. Brazilian tests coupled with digital image correlation and acoustic emission technique were conducted on black shale in Sichuan Basin in China, the corresponding relationships between the characteristics of the frequency band of acoustic emission power spectra and the micro-damage mechanism of rock specimens were established, and the fracture morphology was quantitatively evaluated. The bedding layer leads to the differences in power spectra characteristics, micro-damage mechanism and fracture morphology of shale. The tension and shear failure of shale matrix induce high-frequency acoustic emission signals, and the tension and shear failure of shale bedding induce low-frequency acoustic emission signals. With the increase of the angle between the bedding layer and loading direction, the dominant frequency and secondary dominant frequency gradually diffuse from low-frequency band to high-frequency band, and the quantitative ratio of high frequency to low frequency  H:L gradually increases. The H:L  of 0° shale specimen is 4.28%: 95.72%, and the fracture is a straight line in shape. The H:L of 30° and 60° shale specimens are 15.89%: 84.11% and 36.93%: 63.07% respectively, and their fractures are arched in shape. The H:L of 90° specimen is 93.85%: 6.15%, and the fracture is composited arc-straight line in shape. The results can provide references for analyzing micro-seismic data in situ, and provide a theoretical basis for controlling fracture trajectory in hydraulic fracturing in shale reservoirs.

How to cite: Xie, Q., Ban, Y., Fu, X., and He, C.: Evaluating the fracture morphology of shale specimen by the means of AE power spectrum characteristics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12857, https://doi.org/10.5194/egusphere-egu2020-12857, 2020

D793 |
EGU2020-16033
Mohammadreza Jalali, Zhen Fang, and Pooya Hamdi

The presence of fractures and discontinuities in the intact rock affects the hydraulic, thermal, chemical and mechanical behavior of the underground structures. Various techniques have been developed to provide information on the spatial distribution of these complex features. LIDAR, for instance, could provide a 2D fracture network model of the outcrop, Geophysical borehole logs such as OPTV and ATV can be used to investigate 1D geometrical data (i.e. dip and dip direction, aperture) of the intersected fractures, and seismic survey can mainly offer a large structure distribution of the deep structures. The ability to combine all the existing data collected from various resources and different scales to construct a 3D discrete fracture network (DFN) model of the rock mass allows to adequately represent the physical behavior of the interested subsurface structure.

In this study, an effort on the construction of such a 3D DFN model is carried out via combination of various structural and hydrogeological data collected in fractured crystalline rock. During the pre-characterization phase of the In-situ Stimulation and Circulation (ISC) experiment [Amann et al., 2018] at the Grimsel Test Site (GTS) in central Switzerland, a comprehensive characterization campaign was carried out to better understand the hydromechanical characteristics of the existing structures. The collected multiscale and multidisciplinary data such as OPTV, ATV, hydraulic packer testing and solute tracer tests [Jalali et al., 2018; Krietsch et al., 2018] are combined, analyzed and interpreted to form a combined stochastic and deterministic DFN model using the FracMan software [Golder Associates, 2017]. For further validation of the model, the results from in-situ hydraulic tests are used to compare the simulated and measured hydraulic responses, allowing to evaluate whether the simulated model could reasonably represent the characteristics of the fracture network in the ISC experiment.

 

References

  • Amann, F., Gischig, V., Evans, K., Doetsch, J., Jalali, M., Valley, B., Krietsch, H., Dutler, N., Villiger, L., Brixel, B., Klepikova, M., Kittilä, A., Madonna, C., Wiemer, S., Saar, M.O., Loew, S., Driesner, T., Maurer, H., Giardini, D., 2018. The seismo-hydromechanical behavior during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in situ stimulation experiment. Solid Earth 9, 115–137.
  • Golder Associates, 2017. FracMan User Documentation.  Golder Associates Inc, Redmond WA.
  • Krietsch, H., Doetsch, J., Dutler, N., Jalali, M., Gischig, V., Loew, S., Amann, F., 2018. Comprehensive geological dataset describing a crystalline rock mass for hydraulic stimulation experiments. Scientific Data 5, 180269.
  • Jalali, M., Klepikova, M., Doetsch, J., Krietsch, H., Brixel, B., Dutler, N., Gischig, V., Amann, F., 2018. A Multi-Scale Approach to Identify and Characterize the Preferential Flow Paths of a Fractured Crystalline Rock. Presented at the 2nd International Discrete Fracture Network Engineering Conference, American Rock Mechanics Association.

How to cite: Jalali, M., Fang, Z., and Hamdi, P.: Construction of 3D Discrete Fracture Network Model using Structural and Hydrogeological Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16033, https://doi.org/10.5194/egusphere-egu2020-16033, 2020

D794 |
EGU2020-18745
Quan Liu, Rui Hu, Pengxiang Qiu, Ran Tao, Huichen Yang, Yixuan Xing, and Thomas Ptak

Compared to porous media, fractured aquifers are generally characterized by a more pronounced hydraulic heterogeneity. To describe hydraulic properties of fractured subsurface, investigation methods such as hydraulic tests, tracer tests and hydrogeophysical tests have been widely used. In recent years, thermal tracer tests are obtaining more attention because thermal response signals can be easily and economically obtained at a high resolution, e.g. using distributed temperature sensing (DTS) systems. Some studies have even employed the travel-time-based thermal tracer tomography (TTT) to reconstruct the aquifer heterogeneity (Somogyvári M. et al., 2016; Somogyvári M. and Bayer P., 2017). In this study, we further develop and apply the TTT method for a field scale investigation of the hydraulic properties at a geothermal test site in Göttingen, Germany, equipped with five instrumented experimental wells.

Presently, using travel-time-based thermal tracer tomography to describe the hydraulic connectivity or conductivity is limited to the condition that the heat transfer must be convection dominated. Thus, the field experiments have to be divided into two steps. A full length well warm water injection test is firstly conducted to obtain information about the basic hydrogeological conditions, such as the fracture insertion depth and the connectivity between the wells. Subsequently, four multilevel thermal tracer tests are performed. The temperature changes in all five wells are recorded using a DTS system. Finally, based on the travel-time-based inversion method, the hydraulic conductivity distribution of the fractured aquifer can be obtained.

Preliminary test results showed that the orientation of transmissive fractures is mainly along the E-W direction at our test site. Given the good hydraulic connectivity, the first thermal tracer tomographical tests in a fractured aquifer were performed between two wells positioned along this direction. As next, we will work on the reconstruction of the fracture distribution between those two wells.

How to cite: Liu, Q., Hu, R., Qiu, P., Tao, R., Yang, H., Xing, Y., and Ptak, T.: Tomographical field investigation of hydraulic properties of a fractured aquifer using active thermal tracer testing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18745, https://doi.org/10.5194/egusphere-egu2020-18745, 2020

D795 |
EGU2020-21671
Andreas Englert, Wolfgang Gossel, and Peter Bayer

Understanding of subsurface flow and transport is of major interest supporting optimal design for several societal relevant technologies, such as waste disposals, geothermal or groundwater production facilities. To advance measurement and modeling techniques and refine them for practical applications, we develop the fractured aquifer test site Rock Garden at the Martin-Luther University Halle.

 

The Rock Garden test site is situated beneath the courtyard of the Faculty of Natural Sciences III and is 60 m x 60 m in size. Fractured Rotliegend series of konglomerates, sand- and siltstones are investigated at the site by 6 drillings. A central borehole (B3) is 40 m in depth and developed as an open borehole between 15 m – 40 m below surface. Five boreholes are developed as groundwater observation wells of about 20 m depth and are equipped with filterscreens between 10 m - 20 m below surface. Natural groundwater levels are on average about 3 m below surface and vary about 0,5 m around this value.

 

A first pumping test in B3 unraveled hydraulic connection to all of the five surrounding boreholes. The effective transmissivities are of the order of 10-5 m2/s and storativities are of the order 10-3. To understand hydraulically active fractures or fracture zones and their connection to the rock matrix at the Rock Garden site, we plan to performed a first flowmeter experiment in well B3 under natural and pumping conditions. Finally we plan to characterize these fractured zones in detail performing hydraulic and tracer tomography at the Rock Garden test site in the near future.

How to cite: Englert, A., Gossel, W., and Bayer, P.: Groundwater flow at the Rock Garden test site, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21671, https://doi.org/10.5194/egusphere-egu2020-21671, 2020