ABSTRACTS KEYNOTE LECTURE · ABSTRACTS KEYNOTE LECTURE KEYNOTE LECTURE #4 Wednesday, 21 Aug, 10:00...

THE 6TH INTERNATIONAL SYMPOSIUM ON IN-SITU ROCK STRESS 1

ABSTRACTS KEYNOTE LECTURE

KEYNOTE LECTURE #1

Tuesday, 20 Aug, 10:00 am – 10:50 am

Room: Hagi (萩)

RS2013-Keynote 01

Japan Trench Fast Drilling Project (JFAST): Investigation of the Large Shallow Slip of the 2011 Tohoku-oki Earthquake

James Mori

a*, Frederick Chester

b, Emily Brodsky

c, Shuichi Kodaira

d, Nobu Eguchi

d, Sean Toczko

d, Expedition

343 Scientists

aDisaster Prevention Research Institute,Kyoto University, Kyoto, Japan

bDept. Of Geology and Geophysics, Texas A&M University, College Station TX USA

aEarth and Planetary Sciences Dept., Univ. California Santa Cruz, Santa Cruz, CA USA

dJapan Agency for Marine-Earth Science and Technology, Yokohoma, Japan

*[email protected]

Abstract

The Japan Trench Fast Drilling Project (JFAST) was a project designed to investigate the region of very large slip on the shallow portion of the subduction megathrust during the 2011 Tohoku-oki earthquake. The huge fault slips in the region were the main cause of the large tsunami that devastated much of the northeast coast of Honshu, Japan. Using boreholes drilled from the seafloor to the plate boundary fault, studies included geophysical measurements of physical rock properties, core sampling of the plate-boundary fault zone and temperature measurements of the residual frictional heat. The thermal observations required a rapid mobilization for the drilling and instrument installations, so JFAST was planned and implemented in about a year following the earthquake.

The drill site was located in very deep water of about 6900 meters and the drilling, coring and instrument installation proved to be difficult. However, the project was successful in obtaining good core samples and installing and retrieving the sensors for the temperature measurements. All these data are currently being analyzed to understand the physical and frictional properties of the fault that produced the extrodinary amount of slip during the recent earthquake.

Keywords: JFAST, Tohoku-oki earthquake, fault friction, stress measurements, tsunami, megathrust

2 THE 6TH INTERNATIONAL SYMPOSIUM ON IN-SITU ROCK STRESS


KEYNOTE LECTURE #2

Tuesday, 20 Aug, 11:00 am – 11:50 am

Room: Hagi (萩)

RS2013-Keynote 02

The Relationship Between Stress Magnitudes and Rock Properties in Sedimentary Rock

Mark D. Zoback

a* and Hiroaki Sone

b

a Stanford University, Stanford, CA USA

b GeoForschungPotsdam, Potsdam, Germany * [email protected]

Abstract

It has been clear for many years that there is great practical utility to be able to directly estimate in situ stress magnitudes from measurements of the physical properties of rock. In this paper, we present a method for estimating stress magnitudes at depth. The methodology we propose is likely to be particularly useful in the oil and gas industry. In this case, interpolation and extrapolation of limited stress measurements at depth would have great utility in a variety of problems encountered in reservoir geomechanics, in particular wellbore stability and the prediction of variations of the “frac gradient” (the least principal stress) with depth.

A number of techniques for estimating stress magnitudes from physical properties have been proposed over the past 44 years based on the “instantaneous gravitational loading of an elastic earth with no lateral strain” model first proposed by Eaton (1969). Despite the many modifications of Eaton’s equation over the years that have been required to match field data (such as using “effective” elastic constants, adding arbitrary tectonic forces that vary with depth and utilization of anisotropic rock properties), these methods do not work. The fundamental problems with this approach are the assumption of instantaneous gravitational or tectonic loading with no lateral strain and that an elastic earth existed in absence of applied loads.

In this paper, we propose a new method for estimating stress magnitudes in sedimentary rock based on the viscoplasticity of the formations at depth. When the rock deformation is elastic, stress magnitudes are expected to reflect current tectonics. Utilizing stress measurements from sedimentary basins around the world found in different tectonic environments, it can be shown that the difference in the maximum and minimum principal stress are often consistent with predictions of frictional faulting theory (see review in Zoback, 2012). However when there is finite viscoplasticity of the sedimentary formations at depth, such as in clay-rich shales and diagenetically-immature (i.e., poorly-cemented) formations, the degree to which differential stress magnitudes relax over time controls stress magnitudes.

We utilize field data, laboratory measurements on core samples and viscoplatic theory to predict stress variations with depth in the Barnett shale, a gas-bearing formation in north central Texas. We demonstrate that that the stress magnitudes predicted by our viscoplastic relaxation theory match those determined independently and do not depend on assumptions about the geologic history of the Barnett shale over time, nor the use of effective rock properties.



KEYNOTE LECTURE #3

Wednesday, 21 Aug, 9:00 am – 9:50 am

Room: Hagi (萩)

RS2013-Keynote 03

Numerical Modeling of Mining-Induced Seismicity — Review and Case Study Example from the Malmberget Mine

Fredrik Perman

a, Jonny Sjöberg

a*, and Thomas Wettainen

b

a Itasca Consultants AB, Sweden

b LKAB, Sweden * [email protected] (corresponding author’s E-mail)

Abstract

Mining-induced seismicity is an important issue for safe and reliable future mining. This paper presents a brief review of different approaches for numerical modeling of mining-induced seismicity, with some examples from mines in Sweden. There are numerous examples of predictions of strain burst seismic events, but calibration and prediction of fault slip events has proven to be more difficult. Two different approaches for modeling fault slip can be identified: (i) using ESS (Excess Shear Stress) and continuum modeling, and (ii) using discontinuum modeling and simulating shear slip on geological structures explicitly. Some examples of the use of each of these two approaches are referenced in the paper. A general discussion on the use of modeling and an outlook for the future in terms of our ability to numerically model and predict seismic events in mining concludes the paper.

A detailed case study of modeling and using the ESS-concept in a slightly novel way is also presented. A detailed three-dimensional numerical model, using the FLAC3D finite difference code, was constructed for the LKAB Malmberget mine. The mining sequence in the model simulated mining in each orebody, in 21 mining steps, from year 2008 to year 2030. A differential stress criterion, based on Diederichs (1999) was used as an indicator of potential seismic events. Moreover, the nineteen largest recorded seismic events from the mine to date were analyzed. The maximum possible seismic moment and the major probable shear plane orientation were calculated for each location. ESS was calculated for disks (planes) with varying orientations and radius with 90% area with positive ESS. The resulting maximum radius of the disk and the mean ESS value were used to calculate the seismic moment. The results were compared with actual recorded seismic magnitudes, and predicted magnitudes for seismic events for future mining were evaluated.

Keywords: Mining-induced seismicity, Numerical model, Differential stress, ESS, Seismic moment, Fault slip



KEYNOTE LECTURE #4

Wednesday, 21 Aug, 10:00 am – 10:50 am

Room: Hagi (萩)

RS2013-Keynote 04

Role of stress evaluation on development of marine methane hydrate resources

Koji Yamamoto a*, Yu Nagano

a

a Japan Oil, Gas and Metals National Corporation, Chiba, Japan

* [email protected]

Abstract

Under the Japanese national research program for methane hydrate resource development, field survey operations including intensive data acquisition have been conducted in the Eastern Nankai Trough for a planned offshore production tests in the area. For a safe and stable operation, there are several geomechanics-related issues such as construction of boreholes in weak sediments, stability of geological formations, alternation of reservoir characters due to deformation and failure, and risk assessment for seafloor instability. To know stress states in the earth crust is an important subject of such geomechanical studies. On the other hand, mechanical nature of unconsolidated shallow submarine sediments are different from typical reservoir rocks petroleum engineers usually work on, and applicability of methods and techniques employed for such deep rocks should be carefully examined. Some log- and core-based techniques have been tried to apply to the sediments for the purposes and reveal geomechanical natures of the region.

Keywords: Methane hydrates, Deepwater shallow sediments, Unconsolidated, ASR, Breakout



KEYNOTE LECTURE #5

Thursday, 22 Aug, 9:00 am – 9:50 am

Room: Hagi (萩)

RS2013-Keynote 05

Integrated Evaluation of Stress Fields at Different Scales

Francois H. Corneta*, Bruno Figueiredo

a,b and Julie Maury

a

a Institut de Physique du Globe de Strasbourg, Strasbourg, France

bLaboratorio Nacional de Engenharia Civil, Lisboa, Portugal * [email protected] (corresponding author’s E-mail)

Abstract

Case studies are presented that help illustrate how integration of complementary in situ stress measurements and continuum mechanics modeling provides some understanding of the spatial variations of the regional stress field at various scales. This in turn is taken to advantage for providing some insight on the long term rheological behavior of the equivalent geomaterial or on the loading mechanisms at the origin of the measured stresses. The first example concerns a granitic rock mass at the km

3 scale in which creep along preexisting fractures is found to

dominate the long term behaviour of the rock mass. The second example refers to a sedimentary formation at the 100 km

3 scale in which fluid-solid interaction is found to be likely controlling the present day stress field. The

third example presents results at the lithospheric (107 km3) scale for a region of west-central Europe where the

crustal regional stress field is found to depend on the Lithospere-Asthenosphere Boundary.

Keywords:Integrated Stress Determination, Rock Masses, Long Term Rheology, Integration Volume.


ABSTRACTS TUESDAY PM

SESSION #1: EARTHQUAKE-1

Tuesday, 20 Aug, 2:45 pm – 3:15 pm

Room: Hagi (萩)

Chairs: Futoshi Yamashita, Yuta Mitsui

RS2013-Invited 01

The Tectonic Stress Tensor of Ms 8.0 Wenchuan Earthquake from Fault Slip Data and Coseismic Displacement

Furen XIE, Hongyan ZHANG* and Yi Du

Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics,

China Earthquake Administration, Beijing, China *[email protected] (corresponding author’s E-mail)

Abstract

The seismogenic structure of the Ms 8.0 Wenchuan earthquake is the Longmenshan fault zone. Its surface rupture occurs mainly on the Yingxiu-Beichuan fault and the Guanxian- Jiangyou fault, especially the former with the lengths of 240 km. By field investigation and observation, the geometrical characteristics and the spatial distribution of the tectonic stress tensor show that the surface rupture along the Longmenshan fault zone has segmented characteristics. The surface ruptures on the southwest subsegment of Yingxiu-Beichuan fault and on the Guanxian-Jiangyou fault are dominated by reverse-slip faulting, but on the northeast subsegment of Yingxiu-Beichuan fault is characterized with comparable horizontal and vertical motion. In addition, the results inverted from fault slip data measured by field work and from coseismic displacement both indicate that the recent tectonic stress field along Longmenshan fault is characterized by two different types of tectonic stress tensors. One is reverse-slip and the direction of the maximum compressional stress is nearly in EW direction. The other is strike-slip but with comparable vertical component, and the direction of maximum principal stress is also in EW direction. Though the two types of tectonic stress tensors have different stress regime, they both indicate that the characteristics of the recent tectonic stress field along the Longmenshan fault is dominated by near E-W maximum compressional dynamic environment.

Keywords: Longmenshan fault zone, Tectonic stress tensor, Fault slip data, Coseismic displacement



SESSION #2: INDUCED SEISMICITY & GEOMECHANICS-1


Room: Shirakashi-1 (白樫１)

Chairs: Kazuhiko Tezuka, Younane N. Abousleiman

RS2013-Invited 02

Zatim podany TOPIC: Blackpool (UK), 2011, and objective tool to distinguish between induced and natural

seismicity

Ivo Oprsala* and Leo Eisner

b

a Seismik s.r.o., Prague, Czech Republic

bInstitute of Rock Structure and Mechanics, Academy of Sciences, Czech Republic

*[email protected]

Abstract

The seismic activity induced by the hydraulic fracturing in relatively inactive Blackpool area (United Kingdom) in 2011 has stopped the exploration of the Bowland shale for at least 18 months. The two minor events of local magnitude (ML) 2.3 and ML1.5 had appeared approximately 10 hours after corresponding stimulation shut-in on April 1

st and May 27

th, respectively. Detailed analysis revealed that both felt events were preceded by series of

smaller events initiating during the hydraulic stimulations. Furthermore, relocation of aftershocks reveals spatial correlation between these events and stimulation intervals. Such evidence shows that felt seismicity can be induced even by moderate to small hydraulic stimulations. The information about seismicity appearing during stimulations can be used to prevent operators from inducing felt or damaging events. However, these small events may not always be connected to the injection and should not directly lead to stopping it, especially in places with higher natural seismic activity (e.g. Arkansas, USA).

We present an objective criterion to discriminate between induced and natural (background) seismicity as an addition to other (possibly more subjective) criteria. It is based on a quantitative evaluation of normalized cross correlation (NCC) between filtered histograms of earthquake rates and injection data (e.g. injected volume). We show that it is a reliable criterion for detection of induced seismicity on data examples for several other known case studies in Colorado, United Kingdom and Switzerland. We also present a geomechanical model that can explain the applied criterion-based positive NCC between injection activity and induced seismicity.

Keywords: Hydraulic, Fracturing, Induced, Seismicity, Criterion, Blackpool



SESSION #2: INDUCED SEISMICITY & GEOMECHANICS-1



Chairs: Kazuhiko Tezuka, Younane N. Abousleiman

RS2013-Invited 03

Microseismic Monitoring at the CCS and CO2-EOR Fields -Lessons from the Nagaoka Pilot CO2 Injection Site-

Ziqiu Xue

a * and Takahiro Nakajima

a

a Japan Petroleum Exploration Co., Ltd., Tokyo, Japan

* [email protected]

Abstract

Injection of CO2 will increase the formation pressure in the target reservoir. This will decrease the effective pressure and may also lead to geomechanical deformation of the rocks. To guarantee safety of storage, the CO2 injection must be controlled not to damage the caprock. In hydrocarbon and geothermal reservoirs, it is well known that the injection or production of fluids can induce microseismic events. Because of their small size, induced microseismicities usually are monitored using borehole instruments. The technology has been applied successfully to the CO2 injection sites. As part of the monitoring component of the Weyburn CO2 injection project, geophones were installed in a disused borehole to record the microseismic events in relation to changes in injection and production in nearby wells. The event magnitudes detected by the geophones ranged between -3 to -1 and the largest events recorded have magnitudes less than -1.0, and many are smaller than -2.0. This presentation will give a simple review on microseismic monitoring results observed at the onshore CO2 injection sites such as Weyburn (Canada) and Lacq (France), and will introduce the case study results from Nagaoka pilot site, including the field survey results after the earthquake hit the area in 2004 and 2007.



SESSION #3: CCS & NUCLEAR WASTE DISPOSAL


Room: Shirakashi-2 (白樫２)

Chairs: Toshinori Sato, Kimikazu Tsusaka

RS2013-Invited 04

Coupled Reservoir-geomechanical Analysis Associated with Geologic CO2 Storage in Deep Sedimentary Formations

Jonny Rutqvist

a*, Frederic Cappa

a, b, Alberto Mazzoldi

a, Antonio Rinaldi

a

aLawrence Berkeley National Laboratory, Berkeley, USA

bGeoAzur, University of Nice Sophia-Antipolis, Cˆote d’Azur Observatory, Sophia-Antipolis, France

* [email protected] (corresponding author’s E-mail)

Abstract

The importance of rock mechanics associated with geological storage of CO2 (GCS) is now widely recognized among GCS stakeholders, especially with respect to the potential for triggering notable (felt) seismic events and how such events could impact the long-term integrity of a CO2 repository (as well as how it could impact the public perception of GCS). This paper summarizes recent modeling studies related geomechanical aspects to GCS, including modeling potential fault reactivation and seismicity. We conclude that coupled reservoir-geomechanical analysis can be used to guide the site selection and evaluate the potential for injection-induced fault reactivation and seismic events, although it is difficult to make an absolute prediction of such an event and its magnitude. Whether a seismic event could actually occur very much depends on the local stress field as well as the reservoir properties, whether ductile rock (and faults) that can accommodate aseismically stress and strain induced by the injection or more brittle rock (and faults) that if critically stressed for shear might be more prone to induce seismicity. Thus, at any CO2 sequestration site, substantial efforts should be made to characterize the three-dimensional stress field and its spatial distribution in and around the injection zone.

Keywords: CO2 sequestration, modeling, geomechanics, faults, fluid pressure



SESSION #3: CCS & NUCLEAR WASTE DISPOSAL



Chairs: Toshinori Sato, Kimikazu Tsusaka

RS2013-Invited 05

Quantitative Damage Evaluation of Rock Mass from Acoustic Wave Monitoring

Jin-Seop Kim

a*, Won-Jin Cho

a, Heui-Joo Choi

a, and Gye-Chun Cho

b

a Radioactive Waste Disposal Research Division, KAERI, Taejeon 305-353, Korea

b Department of Civil & Environmental Engineering, KAIST, Taejeon 305-701, Korea

* [email protected]

Abstract

Although the acoustic emission (AE) technique has been recognized as a promising method for the real-time monitoring of a waste repository, very little effort has been made to use AE monitoring data to quantify rock mass damage. Thus, the purpose of this study is to propose a methodology for quantifying rock mass damage, that is, the degree of damage, corresponding strength of the rock mass, and their predictions, based on real-time AE monitoring. An in-situ damage evolution profile (normalized degree of damage vs. normalized stress) of KURT (KAERI Underground Research Tunnel) rock mass is first estimated from an incremental cyclic loading using a Goodman Jack test and a simultaneous AE monitoring. Subsequently, an in-situ damage evolution function of the KURT rock mass is successfully verified by taking into account 1) the Wigner-Ville Distribution with the theoretical wave dispersion model for a more reliable AE source location in a jointed rock mass, 2) the statistical Weibul model for the crack size determination in the rock mass, 3) the in-situ wave attenuation for AE energy correction not at the AE sensor location but at the AE source location where micro-crack are produced, and 4) the fractal theory for an estimation of scale-independent maximum AE energy. Consequently, it was found that the degree of damage derived from the proposed verification procedure is very close to that from the in-situ damage evolution curve. Therefore, it is proved that the methodology suggested in this study is reasonable and appropriate for quantifying the rock mass damage, and can also contribute to opening the way to a quantification of rock mass damage from microseismic observations in a waste disposal repository.

Keywords: Acoustic Emission (AE), Goodman Jack test, quantitative damage, rock mass


ABSTRACTS WEDNESDAY PM

SESSION #5: GEOMECHANICS-2

Wednesday, 21 Aug, 2:30 pm – 3:00 pm


Chairs: Ziqiu Xue, Ivo Oprsal

RS2013-Invited 06

Electrokinetic and Creep Effects on In-Situ Stresses and Pore Pressure Generation in Anisotropic Shale Formations

Younane N. Abousleiman

a, b, c, d *

aThe integrated PoroMechanics Institute, University of Oklahoma, U.S.A.

b ConocoPhillips School of Geology and Geophysics, University of Oklahoma, U.S.A.

cMewbourne School of Petroleum and Geological Engineering, University of Oklahoma, U.S.A.

dSchool of Civil and Environmental Engineering, University of Oklahoma, U.S.A.

* [email protected]

Abstract

Parameters related to the electrokinetic nature and creep behavior of shale that can affect the modeling and the estimation of in-situ stresses in shale formations are here in examined. Results show that for example the shale Cation Exchange Capacity (CEC) can affect the initial formation pore pressure values and consequently the in-situ effective stresses. In addition, the creep behavior (or time-dependent properties) of shale was shown to promote higher formation pore pressure generation due to additional compaction. Similarly, the time-dependent Young’s moduli and Biot’s pore pressure coefficients are shown to have a significant influence on the induced horizontal stress. The outcomes of this study suggest that the electrokinetic and creep behavior of shale should be properly accounted for in modeling the in-situ stress and pore pressure in shale formation. Ignoring one of these shale intrinsic characteristics may result in larger uncertainties on the quantitative values of the in-situ pore pressure and stress.



SESSION #5: GEOMECHANICS-2



Chairs: Ziqiu Xue, Ivo Oprsal

RS2013-Invited 07

Geomechanical Studies on Fractured Basement Oil/Gas Reservoir in Yufutsu, Japan

Kazuhiko Tezuka

a*, Tetsuya Tamagawa

a and Yusuke Kumano

a

a Japan Petroleum Exploration Co., Ltd., Tokyo, Japan

* [email protected]

Abstract

Geomechanical information is of crucial importance for optimizing development of fractured oil/gas reservoirs. Forming and re-activating fractures in a reservoir are strongly affected by stress environment; consequently the most of fractures contributing fluid flow are showing preferential orientations. This paper summarizes studies relating to the characterization and modeling for the fractured basement reservoir in Yufutsu oil/gas field. The studies include multi-scale fracture surveys using cores, borehole images, microseismic monitoring, and 3D seismic survey. Those data-sets are analyzed independently and interpreted in the context of stress environment and integrated to characterize the field scale fracture distribution. The flowing fractures confirmed in the production wells show orientations preferential to shear failure. The major fracture systems interpreted from microseismic distribution also have similar orientations and additionally provide us with a characteristic spacing. The ant-truck outputs from 3D seismic survey provide potential sub-seismic faults and can extrapolate the knowledge extracted from the microseimic data to all over the field. We conclude that an integrated interpretation combining stress and fractures is of most effective approach to delineate the major flow pathways in the reservoir and optimize the development.

Keywords: Geomechanics, Fracture, Stress, Borehole image, Microseismic, Ant-track, Fault



SESSION #6: DEEP MINING



Chairs: Hiroshi Ogasawara, Erling Nordlund

RS2013-Invited 08

Stress-related problems in deep mining

E. Nordlund

Division of Mining an dGeotechnical Engineering, Luleå University of Technology, Luleå, Sweden

[email protected]

Abstract

Mining at depth is associated with high stress magnitudes and often serious ground control problems. Hard rock and mining-induced seismicity and altered weak ore contact zones resulting in large deformations are typical for deep mining in Sweden. Since 2009 a research program focused on rock support design for deep mining conditions is conducted at Luleå University of Technology, Division of Mining and Geotechnical Engineering. The objective of the research is to improve the understanding of (i) the rock mass response and the rock support system performance and (ii) the interaction between the rock mass and the rock support system for mining at increasing depths. The program comprises field monitoring in three mines, large-scale dynamic loading of rock support systems and numerical analyses of different phenomena, failure mechanisms and details in the rock mass – rock support system interaction. The paper gives examples of activities and results from the program. The program has so far resulted in new knowledge as well as new questions. In order to improve the understanding of the rock mass – rock support interaction the details are important. Since the program will run for a number of more years only a few of the sub-projects are finished and therefore the main conclusions have yet to be drawn.

Keywords: Rock stress, Mining-induced seismicity, Deformations, Rock support


ABSTRACTS THURSDAY AM

SESSION #7: IN-SITU TEST METHOD

Thursday, 22 Aug, 11: 45 am – 12:15 pm

Room: Hagi (萩)

Chairs: Weiren Lin, Bezalel C. Haimson

RS2013-Invited 09

Difficulties and Possible Solutions for Deep Stress Measurements

Takatoshi Itoa*

a Institute of Fluid Science, Tohoku University, Sendai, Japan

* [email protected]

Abstract

Motion of rocks is so slow in nature that it takes a considerable time for us to realize the motion. Contrary to this, the stress state in rocks allows us to immediately infer crustal dynamics involving the rocks, which is typically represented as Anderson’s classification of faulting, i.e., normal, thrust, and strike-slip faulting. In recent years, geomechanical approaches applied to oil and gas or geothermal exploitation are becoming common place. For the mining industry, deeper and higher stress mining has become more important. Consequently rock stress and its effect on the mechanical behavior of rock masses at a few kilometers depths have drawn much attention. In situ test of hydraulic fracturing (HF) provides the only way to observe in situ stress magnitudes directly. The maximum and minimum horizontal stresses, SHmax and Shmin, are determined from critical borehole pressures, i.e., the reopening pressure Pr and the shut-in pressure Ps, etc, observed during the test. Keywords: RS2013, Rock stress, Fault sliding, Template



SESSION #8: CORE-BASED METHOD

Thursday, 22 Aug, 10: 00 am – 10:30 am

Room: Shirakashi-1 (白橿１)

Chairs: Kiyotoshi Sakaguchi, Douglas R. Schmitt

RS2013-Invited 10

Predicting the Shape of Drilling Induced Core Fractures in Three Dimensions

Douglas R. Schmitt

a*, Lei Zhang

a, and Claire Currie

a

a University of Alberta

* [email protected]

Abstract

Drilling induced core fractures are created by the pure tensions induced by concentrations of the stress field by the complex shape of the bit kerf and core stub at the bottom of the core hole. The earlier work of many authors from modelling and fractography has demonstrated that the rock fails under pure tension. The shapes that these fracture take range from petal-centreline through to saddles; and the orientations of the fractures have long indicated stress directions. The range of shapes and their spacing, however, indicates that the fractures themselves contain more information about stress states. However, even today attempting to predict the propagation of a fracture remains difficult despite greatly improved computational capacity and numerical modelling procedures. Instead, we apply a simple ‘Rankine’ tensile failure approach to ‘propagate’ a tensile fracture through a finite-element modelled state of stress concentration. Failure begins at the point with the greatest tension with the plane of the incipient fracture deciding the direction, in 3D, that the incipient fracture will propagate. This plane is extended through through the adjacent elements to find its intersection with the element boundaries. At each intersection the stress state is determined by interpolation and the fracture is re-oriented. This allows for growth of the fracture along a trajectory of maximum tension. While this may not necessarily be true to dynamic variations in the stresses due to the fracture propagation, a given fracture shape can rapidly be calculated and displayed. This is being incorporated into a methodology that allows for iterative modelling of observed fractures as a tool in stress state prediction.



SESSION #9: REGIONAL STRESS & CIVIL ENGINEERING

Thursday, 22 Aug, 10: 30 am – 11:00 am

Room: Shirakashi-2 (白橿２)

Chairs: Akira Sato, Tomoyuki Aoki

RS2013-Invited 11

Quantitative World Stress Map With Three Case Studies: Geologic Repository, Gold Mining and Deep Geothermal Energy

Arno Zang*, Ove Stephansson, Oliver Heidbach, Jeoung Seok Yoon and Moritz Ziegler

German Research Center for Geosciences, 2.6 Seismic Hazard and Stress Field, Potsdam, Germany

* [email protected]

Abstract

Mechanical stress remains an enigmatic physical rock property due to the facts that stress cannot be measured directly, it is difficult to visualize and it is affected by stress state-scale relations. Despite this, in situ rock stress has become a key parameter in rock engineering and solid Earth sciences, in general. In this contribution, we focus on the state-of-the-art techniques determining the in situ stress at a given site by using our recently proposed rock stress terminology and applying the best-estimated stress model approach. In this context, we inform the reader on the existing World Stress Map database (WSM) of stress orientations, and our new database on lithologic in situ stress magnitudes, rock strength and fracture mechanical properties, the so-called Quantitative WSM (Q-WSM). Three case studies highlight the road to the final rock stress model, which sometimes differs significantly from the recipes discussed. We address problems in obtaining reliable stress magnitudes in near-surface (0.5 km) hard rock of a Swedish radioactive waste disposal site, we report on stress perturbation due to mine excavation close to a vertical dyke structure in a deep (3.5 km) South African gold mine, and we investigate Kaiser stress observations in a crystalline geothermal reservoir at 5 km depth applying hydro-mechanical coupled discrete element modeling.

Keywords: Final rock stress model, Induced stresses (geothermal, mining, radioactive waste disposal), In situ rock stress, World Stress Map (WSM), Quantitative World Stress Map (Q-WSM)

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