scholarly journals Numerical Analysis of the integrity of potential host rocks – modelling thermo-hydro-mechanical processes in the containment providing rock zone

2021 ◽  
Vol 1 ◽  
pp. 173-174
Author(s):  
Carlos Guevara Morel​​​​​​​ ◽  
Jobst Maßmann ◽  
Jan Thiedau
Keyword(s):  
Numerical Analysis ◽  
Nuclear Waste ◽  
Fluid Pressure ◽  
Fracture Network ◽  
Crystalline Rock ◽  
Salt Rock ◽  
Safety Requirements ◽  
Rock Types ◽  
Verification Period ◽  
The Individual

Abstract. The disposal of heat-generating nuclear waste in deep geological formations is an internationally accepted concept. Several repository systems are under discussion in Germany, whereby claystone, salt or crystalline rock could act as the host rock. In this contribution we focus on repository systems where the Containment Providing Rock Zone (CRZ) ensures safe enclosure of the waste and thus the geologic barrier is essential. Even though the various rock types considered differ substantially in their mechanical, hydraulic, thermal and chemical behavior, they must all meet the same safety requirements as defined by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) in 2020. As part of these safety requirements, it must be shown that the integrity of the CRZ is guaranteed for the verification period, i.e. the retention of the properties essential for the containment capacities must be demonstrated over 1 million years. Therefore, the formation of new pathways must be avoided and temperature development must not significantly impair the barrier effect. The anticipated stresses and fluid pressures should not exceed the dilatancy strength and the fluid pressure capacity, respectively. In order to assess the compliance of these requirements, numerical modelling is an essential and powerful tool. Even though great progress has been made regarding the efficiency of computational methods, multiphysical modelling on different length scales over long time periods is still a challenging task. Moreover, since readily available solutions do not exist, adapted methods have to be developed and evaluated, in order to verify concepts and numerical implementations. The BGR gained experience in the field of thermal, hydraulic, mechanical (THM) numerical analysis of the integrity of the CRZ in salt rock and clay stone joined research projects on German disposal options. For crystalline rocks, first concepts are currently being developed within the CHRISTA II project. Compared to clay stone and salt rock, special features have to be taken into account: First of all, crystalline rock is characterized by fractures and other discontinuities. Thus, it cannot be assumed that an undisturbed area of sufficient size can be found for the entire nuclear waste. Consequently, several smaller CRZs must be defined, each providing undisturbed rock. Numerical analysis must deal with smaller CRZs and mechanical and hydraulic boundary conditions that are influenced by fractures. In addition, the processes in the individual CRZs may influence each other (e.g. Temperature distribution). Preliminary modelling approaches and results of numerical THM analyses, considering an upscaled fracture network, are presented.

scholarly journals Influence of reservoir geology on seismic response during decameter-scale hydraulic stimulations in crystalline rock

Solid Earth ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 627-655 ◽  
Author(s):  
Linus Villiger ◽  
Valentin Samuel Gischig ◽  
Joseph Doetsch ◽  
Hannes Krietsch ◽  
Nathan Oliver Dutler ◽  
...  
Keyword(s):  
Seismic Response ◽  
Fluid Pressure ◽  
Shear Zones ◽  
Test Site ◽  
Fracture Network ◽  
Crystalline Rock ◽  
Hydraulic Fractures ◽  
Geothermal Systems ◽  
Hydraulic Stimulation ◽  
Shear Dilation

Abstract. We performed a series of 12 hydraulic stimulation experiments in a 20m×20m×20m foliated, crystalline rock volume intersected by two distinct fault sets at the Grimsel Test Site, Switzerland. The goal of these experiments was to improve our understanding of stimulation processes associated with high-pressure fluid injection used for reservoir creation in enhanced or engineered geothermal systems. In the first six experiments, pre-existing fractures were stimulated to induce shear dilation and enhance permeability. Two types of shear zones were targeted for these hydroshearing experiments: (i) ductile ones with intense foliation and (ii) brittle–ductile ones associated with a fractured zone. The second series of six stimulations were performed in borehole intervals without natural fractures to initiate and propagate hydraulic fractures that connect the wellbore to the existing fracture network. The same injection protocol was used for all experiments within each stimulation series so that the differences observed will give insights into the effect of geology on the seismo-hydromechanical response rather than differences due to the injection protocols. Deformations and fluid pressure were monitored using a dense sensor network in boreholes surrounding the injection locations. Seismicity was recorded with sensitive in situ acoustic emission sensors both in boreholes and at the tunnel walls. We observed high variability in the seismic response in terms of seismogenic indices, b values, and spatial and temporal evolution during both hydroshearing and hydrofracturing experiments, which we attribute to local geological heterogeneities. Seismicity was most pronounced for injections into the highly conductive brittle–ductile shear zones, while the injectivity increase on these structures was only marginal. No significant differences between the seismic response of hydroshearing and hydrofracturing was identified, possibly because the hydrofractures interact with the same pre-existing fracture network that is reactivated during the hydroshearing experiments. Fault slip during the hydroshearing experiments was predominantly aseismic. The results of our hydraulic stimulations indicate that stimulation of short borehole intervals with limited fluid volumes (i.e., the concept of zonal insulation) may be an effective approach to limit induced seismic hazard if highly seismogenic structures can be avoided.

Time-lapse monitoring of fractured rock response to hydraulic stimulation using pressure tomography

2020 ◽  
Author(s):  
Márk Somogyvári ◽  
Mohammadreza Jalali
Keyword(s):  
Dynamic Response ◽  
Fractured Rock ◽  
Fluid Pressure ◽  
Time Lapse ◽  
Fracture Network ◽  
Industrial Applications ◽  
Geothermal System ◽  
Fractured Reservoir ◽  
Hydraulic Stimulation ◽  
The Individual

<p>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.</p><p>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.</p><p>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.</p><p>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.</p>

scholarly journals Influence of reservoir geology on seismic response during decameter scale hydraulic stimulations in crystalline rock

2019 ◽  
Author(s):  
Linus Villiger ◽  
Valentin Samuel Gischig ◽  
Joseph Doetsch ◽  
Hannes Krietsch ◽  
Nathan Oliver Dutler ◽  
...  
Keyword(s):  
Seismic Response ◽  
Mechanical Response ◽  
Fluid Pressure ◽  
Shear Zones ◽  
Test Site ◽  
Fracture Network ◽  
Crystalline Rock ◽  
Hydraulic Fractures ◽  
Geothermal Systems ◽  
Hydraulic Stimulation

Abstract. We performed a series of 12 hydraulic stimulation experiments in a 20 × 20 × 20 m foliated, crystalline rock volume intersected by two distinct fault sets at the Grimsel Test Site, Switzerland. The goal of these experiments was to improve our understanding of stimulation processes associated with high-pressure fluid injection used for reservoir creation in enhanced or engineered geothermal systems. In the first six experiments, pre-existing fractures were stimulated to induce shear dilation and enhance permeability. Two types of shear zones were targeted for these hydroshearing experiments: i) ductile ones with intense foliation and ii) brittle-ductile ones associated with a fractured zone. The second series of six stimulations were performed in borehole intervals without natural fractures to initiate and propagate hydraulic fractures that connect the wellbore to the existing fracture network. The same injection protocol was used for all experiments within each stimulation series so that the differences observed will give insights into the effect of geology on the seismo-hydro-mechanical response rather than differences due to the injection protocols. Deformations and fluid pressure were monitored using a dense sensor network in boreholes surrounding the injection locations. Seismicity was recorded with sensitive in-situ acoustic emission sensors both in boreholes and at the tunnel walls. We observed high variability in the seismic response in terms of seismogenic indices, b-values, spatial and temporal evolution during both hydroshearing and hydrofracturing experiments, which we attribute to local geological heterogeneities. Seismicity was most pronounced for injections into the highly conductive brittle-ductile shear zones, while injectivity increase on these structures was only marginal. No significant differences between the seismic response of hydroshearing and hydrofracturing was identified, possibly because the hydrofractures interact with the same pre-existing fracture network that is reactivated during the hydroshearing experiments. Fault slip during the hydroshearing experiments was predominantly aseismic. The results of our hydraulic stimulations indicate that stimulation of short borehole intervals with limited fluid volumes (i.e., the concept of zonal insulation) may be an effective approach to limit induced seismic hazard if highly seismogenic structures can be avoided.

Nuclear Waste Repositories in Crystalline Rock- an Overview of Flow and Nuclide Transport Mechanisms

1994 ◽  
Vol 353 ◽  
Author(s):  
Ivars Neretnieks
Keyword(s):  
Nuclear Waste ◽  
Field Experiments ◽  
Fracture Network ◽  
Crystalline Rock ◽  
Transport Mechanisms ◽  
The Matrix ◽  
Nuclide Transport ◽  
Waste Repositories ◽  
And Diffusion ◽  
Diffusion Properties

AbstractNuclides eventually escaping from nuclear waste repositories in crystalline rock will move with the seeping water in the fracture network. Most important nuclides interact physically or chemically with the rock and are expected to be considerably retarded allowing them to decay to insignificant concentrations. Velocity variations may allow some portions of the nuclides to move faster. Matrix diffusion and sorption on the surfaces of the rock are by far the most powerful retardation mechanisms and depend, in addition to the sorption and diffusion properties, directly on the magnitude of the “flow wetted surface”which is the contact surface between the mobile water carrying the nuclides and the fracture surfaces over which the nuclides diffuse into the matrix.A number of field experiments have been performed over the last 15 years to help validate the concepts and models and to obtain data. A number of such experiments are described and discussed in relation to the above issues.

Changes of Selected Structural and Mechanical Properties of the Strzelin Granites As Induced By Thermal Loads / Wpływ Obciążeń Termicznych Na Zmiany Niektórych Strukturalnych I Mechanicznych Właściwości Granitów Strzelińskich

2012 ◽  
Vol 57 (4) ◽  
pp. 951-974 ◽  
Author(s):  
Andrzej Nowakowski ◽  
Mariusz Młynarczuk
Keyword(s):  
Mechanical Properties ◽  
Nuclear Waste ◽  
Structural Changes ◽  
Structural Parameters ◽  
Total Porosity ◽  
Fracture Network ◽  
Qualitative Description ◽  
Temperature Changes ◽  
Heating And Cooling ◽  
Waste Repositories

Abstract Temperature is one of the basic factors influencing physical and structural properties of rocks. A quantitative and qualitative description of this influence becomes essential in underground construction and, in particular, in the construction of various underground storage facilities, including nuclear waste repositories. The present paper discusses the effects of temperature changes on selected mechanical and structural parameters of the Strzelin granites. Its authors focused on analyzing the changes of granite properties that accompany rapid temperature changes, for temperatures lower than 573ºC, which is the value at which the β - α phase transition in quartz occurs. Some of the criteria for selecting the temperature range were the results of measurements carried out at nuclear waste repositories. It was demonstrated that, as a result of the adopted procedure of heating and cooling of samples, the examined rock starts to reveal measurable structural changes, which, in turn, induces vital changes of its selected mechanical properties. In particular, it was shown that one of the quantities describing the structure of the rock - namely, the fracture network - grew significantly. As a consequence, vital changes could be observed in the following physical quantities characterizing the rock: primary wave velocity (vp), permeability coefficient (k), total porosity (n) and fracture porosity (η), limit of compressive strength (Rσ1) and the accompanying deformation (Rε1), Young’s modulus (E), and Poisson’s ratio (ν).

Time Constraints on Seepage Through Fractured Regions on the Vestnesa Ridge off the W-Svalbard Coast

2021 ◽  
Author(s):  
Hariharan Ramachandran ◽  
Andreia Plaza-Faverola ◽  
Hugh Daigle ◽  
Stefan Buenz
Keyword(s):  
Fluid Flow ◽  
Effective Stress ◽  
Gas Hydrate ◽  
Fluid Pressure ◽  
Fracture Network ◽  
The Arctic ◽  
Stability Zone ◽  
Methane Seepage ◽  
Carbonate Formation ◽  
Hydrate Stability

<p>Evidences of subsurface fluid flow-driven fractures (from seismic interpretation) are quite common at Vestnesa Ridge (around 79ºN in the Arctic Ocean), W-Svalbard margin. Ultimately, the fractured systems have led to the formation of pockmarks on the seafloor. At present day, the eastern segment of the ridge has active pockmarks with continuous methane seep observations in sonar data. The pockmarks in the western segment are considered inactive or to seep at a rate that is harder to identify. The ridge is at ~1200m water depth with the base of the gas hydrate stability zone (GHSZ) at ~200m below the seafloor. Considerable free gas zone is present below the hydrates. Besides the obvious concern of amount and rates of historic methane seeping into the ocean biosphere and its associated effects, significant gaps exist in the ability to model the processes of flow of methane through this faulted and fractured region. Our aim is to highlight the interactions between physical flow, geomechanics and geological control processes that govern the rates and timing of methane seepage.</p><p>For this purpose, we performed numerical fluid flow simulations. We integrate fundamental mass and component conservation equations with a phase equilibrium approach accounting for hydrate phase boundary effects to simulate the transport of gas from the base of the GHSZ through rock matrix and interconnected fractures until the seafloor. The relation between effective stress and fluid pressure is considered and fractures are activated once the effective stress exceeds the tensile limit. We use field data (seismic, oedometer tests on calypso cores, pore fluid pressure and temperature) to constrain the range of validity of various flow and geomechanical parameters in the simulation (such as vertical stress, porosity, permeability, saturations).</p><p>Preliminary results indicate fluid overpressure greater than 1.5 MPa is required to initiate fractures at the base of the gas hydrate stability zone for the investigated system. Focused fluid flow occurs through the narrow fracture networks and the gas reaches the seafloor within 1 day. The surrounding regions near the fracture network exhibit slower seepage towards the seafloor, but over a wider area. Advective flux through the less fractured surrounding regions, reaches the seafloor within 15 years and a diffusive flux reaches within 1200 years. These times are controlled by the permeability of the sediments and are retarded further due to considerable hydrate/carbonate formation during vertical migration. Next course of action includes constraining the methane availability at the base of the GHSZ and estimating its impact on seepage behavior.</p>

The Experimental and Numerical Analysis of Stress as a Tool for Optimizing the Selected Structural Components of an Injection Device

2015 ◽  
Vol 816 ◽  
pp. 482-489
Author(s):  
Peter Sivák ◽  
Oskar Ostertag
Keyword(s):  
Numerical Analysis ◽  
Service Life ◽  
Structural Components ◽  
Injection Device ◽  
Paper Briefly ◽  
Technical Parameters ◽  
Structural Parts ◽  
The Individual ◽  
Important Basis

The improvement of the main monitored technical parameters of the production devices is connected with the optimization of the individual structural components and construction nodes of these devices. Mainly those critical structural nodes of the devices that have a significant impact on the quality of the products manufactured by this device and its service life are studied. This paper briefly presents the results of experimental and numerical analysis of stresses and strains of selected structural components of the injection device, such as the connecting and guiding columns and the adjustable push plate. The relevant analyses of stresses and strains have been connected with the performance of a series of experimental and computer simulations. The results of these simulations then became an important basis for further optimization of selected structural parts of the injection device.

scholarly journals Design and Experiments of a Piezoelectric Motor Using Three Rotating Mode Actuators

Sensors ◽  
2019 ◽  
Vol 19 (23) ◽  
pp. 5184 ◽  
Author(s):  
Roland Ryndzionek ◽  
Łukasz Sienkiewicz ◽  
Michał Michna ◽  
Filip Kutt
Keyword(s):  
Numerical Analysis ◽  
Traveling Wave ◽  
Laser Interferometry ◽  
Experimental Tests ◽  
Impedance Analysis ◽  
Piezoelectric Motor ◽  
Resonant Frequencies ◽  
Rotary Motors ◽  
The Individual ◽  
Geometrical Dimensions

This paper represents a numerical and experimental investigation of the multicell piezoelectric motor. The proposed design consists of three individual cells that are integrated into the stator, double rotor, and a preload system combined into a symmetrical structure of the motor. Each of the cells is characterized by a traveling wave and rotating mode motor. A finite element numerical analysis is carried out to obtain optimal geometrical dimensions of the individual cell in terms of generated vibrations and resonant frequencies of the structure. The results of the numerical analysis are compared with analytical calculations based on the equivalent circuit theory. Experimental tests are also presented, including laser interferometry measurements of vibrations generated at the surface of the stator, impedance analysis, as well as measurements of mechanical characteristics of the complete motor. The final stage of the study concludes that the presented motor can provide relatively high torque compared with other traveling wave rotary motors.

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