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**Industry**

**Oilfield Places**

**Technology**

A hybrid method for constraining all three principal in situ stresses and their directions around vertical boreholes at great depths is described. It involves hydraulic fracturing tests for estimating the minimum horizontal stress and its direction. The vertical stress is computed from the weight of the overlying strata. In order to estimate the maximum horizontal stress additional field and laboratory efforts are employed. Geophysical logging using such tools as the Borehole Televiewer or the Formation Micro Imager captures oriented images of borehole breakouts, from which breakout span as it varies with depth is obtained. Laboratory tests of core samples in a polyaxial cell render the true triaxial strength criterion of the rock. Using the condition of limit equilibrium between the local state of stress at the edges of breakout-borehole wall intersections and the strength criterion, a non linear equation emerges from which the maximum horizontal principal stress is derived, thus completing the estimation of the prevailing state of stress. Two field case histories are described in which the hybrid method was used: the KTB, Germany scientific ultra deep hole, and the Taiwan Chelungpu Fault Drilling Project (TCDP).

Hydraulic fracturing (HF) is the most common method of estimating the state of in situ stress around vertical holes at great depths. In such holes HF typically induces vertical fractures. Correct analysis of pressure vs. time records and of any of the available fracture delineation logging techniques leads to reliable estimations of the least horizontal stress

ISRM-ISRS-2010-007

International Symposium on In-Situ Rock Stress

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)

A method for measurement of stress change is developed to monitor rock stress using a borehole. Two dimensional state of stress change within rock mass in a plane perpendicular to a borehole axis can be measured by this method, which is named the Cross-sectional Borehole Deformation Method (CBDM). In this paper, the theory of the CBDM is described, as well as the prototype instrument with the laser displacement sensor. Analyzing influence factors on measurement result theoretically, it makes clear that stress change within rock mass can be estimated by the CBDM.

Knowledge of rock stress is one of fundamental importance for designing and constructing rock structure, such as underground openings, since the mechanical behavior of rock mass around it is affected by initial stress. Furthermore, the induced stress measurement is performed to estimate the stability of a rock structure under construction and confirm the design of it. In order to measure initial stress, many methods have been suggested. On the other hand, there are a few methods for stress change around an opening under construction. For example, the stress change of an underground power house has been measured by a vibrating wire strain gauge in Japan (Kudo et al. 1998). However, using this gauge, only stress in one direction in a plane perpendicular to a borehole axis can be measured. Furthermore, this gauge has a rigidity which affects on measured results. In this paper, a method for measurement of stress change is developed to monitor rock stress using a borehole. Two dimensional state of stress change within rock mass in a plane perpendicular to a borehole axis can be measured by this method. This method was named the Cross-sectional Borehole Deformation Method (CBDM) by Tanguchi et al. (2003) and Obara et al. (2004).

ISRM-ISRS-2010-017

International Symposium on In-Situ Rock Stress

SPE Disciplines:

Jia, L. (Institute of Crustal Dynamics, China Earthquake Administration, China University of Geosciences) | Wang, C. (Institute of Crustal Dynamics, China Earthquake Administration) | Chen, Q. (China University of Geosciences) | Jiang, Z. (China University of Geosciences)

In order to better cooperate with the YJ-95 piezomagnetic stress meter for stress measurement, design and develop the piezomagnetic stress meter intelligent data analysis system. In this paper accuracy assessment and reliability analysis of the intelligent data analysis system of the piezomagnetic stress meter are discussed in theory and practical application on the detail. This system first processes error and estimates precision about measured value by least-squares method, studentized residuals method and coordinate transformation. Precision estimation have two steps: assessing the accuracy of stress components and assessing the accuracy of the principal stress. Then the system analyses the reliability of results depending on existing databases, the in-situ stress condition and other criteria. The above process is carried out by the system intelligently, and the final results of the analysis can be presented.

In-situ stress measurement is an important method to study the crustal stress state and tectonic stress field. Accurate stress data is essential to the mining, water conservancy, civil construction, underground caverns, and other constructions. However, stress measurement errors are ineluctable. According to error theory, as long as there are the number of observations is more than that of variables (stress components), we can estimate the error, in other word the accuracy can be assessed. In addition the actual stress value always is unknown, so it is not possible to compare measured values with unknown values in order to validate the accuracy of measurement results. As a result, it is necessary to use appropriate criterions for reliability analysis of the results so as to the reasonable utilization. The piezomagnetic stress meter intelligent data analysis system aimed at the YJ-95 piezomagnetic stress meter. So the piezomagnetic stress meter intelligent data analysis system can get the accurate outcome, and the reliability of the results is scientific assessed.

ISRM-ISRS-2010-021

International Symposium on In-Situ Rock Stress

SPE Disciplines:

Lin, Weiren (Japan Agency for Marine-Earth Science and Technology) | Byrne, Timothy B. (University of Connecticut) | Tsutsumi, Akito (Kyoto University) | Yamamoto, Yuhji (Kochi University) | Sakaguchi, Arito (Japan Agency for Marine-Earth Science and Technology) | Yamamoto, Yuzuru (Japan Agency for Marine-Earth Science and Technology) | Chang, Chandong (Chungnam National University)

To determine three-dimensional stress orientation, we carried anelastic strain recovery (ASR) measurements out using drill core samples taken from a scientific ocean deep drilling project. The lithology of the core samples is mudstone or siltstone with larger porosities ranged from 35% to 45%. We glued strain gauges on their cylindrical surface, and successfully obtained high quality anelastic strain data in at least six directions. And then, we determined the three-dimensional stress orientations by the strain-time curves. The stress orientations obtained from the ASR core measurements were consistent with those from drilling induced borehole breakouts and tensile fractures observed in electrical image of borehole logging.

Following Ocean Drilling Program (ODP), the Integrated Ocean Drilling Program (IODP) begun from 2003. Deep drillings related with geodynamics such as seismogenic zone drillings are one of its important scientific targets. Therefore, determination of in situ stress state is an important and necessary research item in such ocean drilling projects. As an IODP scientific deep drilling project, NankaiTrough Seismogenic Zone Experiments (NanTroSEIZE) is undergoing in the southwest Japan subduction zone to understand the physics of an active fault (Kinoshita et al., 2006). Determination of current in-situ stress is one of the main scientific objectives of NanTroSEIZE. Unfortunately, there is no foolproof method by which magnitudes and orientations of threedimensional in-situ stress can be reliably measured at large/great depth, although various field and laboratory measurement techniques have been proposed. In the cases of ocean scientific deep drilling projects, we suggest that a combined application of borehole method (s) and core-based method (s) be employed. As one of them, a simple and inexpensive method to determine in-situ stress from anelastic strain recovery (ASR) measurement of oriented cores can be considered as having a relatively explicit theoretical basis in comparison to other core-based methods.

ISRM-ISRS-2010-029

International Symposium on In-Situ Rock Stress

SPE Disciplines:

The thermo-mechanical strength distributions of the lithosphere underneath Chinawere obtained by 3-D crustal velocity model of China, local isostasy equilibrium constrained geothermal inversion, and 4-layered rheological model of lithosphere. Map of the lateral strength variation at compression condition is presented for whole China continent, and the thermal thickness of the lithosphere is also calculated. The relative strength ratio of crust/mantle (SC/SM) in the vast region of Tibetan plateau is larger than 10, corresponding to a stronger crust but weak upper mantle. Off Tibetan plateau, the lithosphere of typical “jelly sandwich” rheology that SC/SM ratio is less than 1, occurs in Junggar, Tarim, Sichuan basin, and Dabieshan as well as the southeastern and northeastern part of Sino-Korean platform. However, the SC/SM ratios are larger than 3 in the northern part of Northeast China,Tian Shan and South China fold belt. The “crèmebrûlée” model is more suitable for describing the lithosphere rheology of these regions. For other portion of China, the SC/SM ratios are mainly in range of 1 to c.3, corresponding to a slight stronger crust and a weaker upper mantle. Accordingly, the most of China Mainland exhibit “crème-brûlée” layered lithosphere rather than “jelly sandwich” one, with exception of Junggar, Tarim and Sichuan basin as well as some areas in Sino-Korean platform. This result means that the lithosphere beneath most area of China continent is mechanically weak. It is the upper crust rather than the upper mantle portion bears the elastic stress. In earthquake-prone regions of China landmass, the mechanical behavior of crust and mantle is decoupled.

The continental area of China is made up of juxtaposed strongly deformed parts and relatively stable regions, and the crustal structure, lithosphere thickness and temperature distributions beneath China mainland exhibits significant lateral variations (Wang, 2001).

ISRM-ISRS-2010-122

International Symposium on In-Situ Rock Stress

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)

Wan, Y. G. (Institute of Disaster-Prevention Science and Technology) | Sheng, S. Z. (Institute of Geophysics, China Earthquake Administration) | Lin, T. L. (National Taiwan University) | Wu, Y. M. (National Taiwan University)

We estimate the absolute stress value at the earthquake source region of a small cubic area in Homestead valley fault segment which broke during the Landers earthquake, using seismic stress drop and rotations of stress axes during the event. We obtained the pre-seismic compressive, intermediate and extensional principal stress values as 323, 319 and 312MPa in the depth of 8∼12 km. The shear stresses on the fault plane before and after the event are 6MPa and 1MPa respectively. The results show that normal stress increased after the earthquake, which helps to terminate the rupture process. The pre-seismic shear-stress is greater than that after the quake and their differences roughly correspond to the seismic stress drop. The post-seismic shear-stress is positive, which means no overshooting in co-seismic slip. The differential stress value is less than the absolute stress value, and the shear stress is also relatively small.

ISRM-ISRS-2010-033

International Symposium on In-Situ Rock Stress

SPE Disciplines:

Kang, H. (Coal Mining and Designing Branch, China Coal Research Institute) | Si, L. (Coal Mining and Designing Branch, China Coal Research Institute) | Zhang, X. (Coal Mining and Designing Branch, China Coal Research Institute)

In-situ stress testing methods frequently used in underground coal mines in China were introduced, including stress relief, hydraulic fracturing, geological structure information, earthquake focal mechanism and underground stress mapping. The stress data records obtained by the small borehole hydraulic fracturing testing rig used in underground coal mines were given more emphasis. Based on the testing data, the relationship between in-situ stresses and depth, and the changes of the ratio of the maximum horizontal principal stress to vertical stress were analyzed. There exist three types of in-situ stress fields. Depth, geological structures and rock properties are the main factors affecting in-situ stresses. Complicated geological conditions result in obvious scatter in testing data. However, the magnitude of in-situ stresses basically increase with the depth in the general trend; the increasing rate of horizontal stresses is larger than that of vertical stress in shallow sites, and gradually decrease as the depth increases.

1 INTRODUCTION

Coal measures are extremely complicated geological bodies. When compared with other geological materials, they have two distinct characteristics: firstly, they are cut by various discontinuities, such as joints and fractures, which sharply change the strength and deformation characteristics of them, and cause the great difference of strength between rock mass and a small rock block; secondly, there are active stresses in the coal measures, and the orientation and magnitude of stresses strongly influence deformation and damage characteristics of surrounding rock mass. Most coal mines in China are operated in underground, and there are a variety of deposition conditions of coal seams. The stress fields in the coal measures are complex and irregular, because of the mixed influence of faults, folds, subsided columns and so on. Deep mining brings about the unfavorable effects of high in-situ stresses, high temperature, high hydraulic pressure and violent mining disturbance.

ISRM-ISRS-2010-020

International Symposium on In-Situ Rock Stress

Industry:

- Materials > Metals & Mining > Coal Mining (1.00)
- Energy > Oil & Gas > Upstream (1.00)

Oilfield Places:

- Asia > China > South China Sea > China Basin (0.97)
- Asia > China > Shanxi Province (0.97)

SPE Disciplines: Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)

With the advancement of technology and the possibility of making large underground excavations in difficult geological conditions, dynamic analysis of underground structures against earthquakewaves has been necessitated. In this research, factors affecting response of underground structures to earthquake loading has been reviewed using finite difference numerical method. In modeling, Tabas, Naghan and El-Centro earthquakes are used as typical earth shakes. The changes in amplitude of displacement, velocity and acceleration as a function of changing desired model parameters are recorded and facilitated a sensitivity analysis. Results indicate that design parameters such as diameter and depth of underground structures have greater impacts than geotechnical parameters. Among geotechnical parameters, density and elastic modulus show greater impacts. Friction angle and cohesion have great influence only in terrestrial environments during dynamic analysis and it can be stated that these two parameters have little impacts in rocky environments in the process of dynamic analysis.

There has been a lot of controversy around the issue of earthquake especially for the last century. Once it was believed that effects of earthquake on tunnels and underground spaces is not very important and have long been assumed they have the ability to sustain earthquakes with little damage. But due to significant damages in some underground structures for instance the 1995 Kobe Japan earthquake (Parra-Montesinos et al., 2006), the 1999 Chi-Chi Taiwan earthquake and the 1999 KocaeliTurkey earthquake (Iwatate et al. 1997); few rock mechanic engineers subscribe to this viewpoint anymore. Therefore strong motions or shallow tunnels can be considered as factors which can cause considerable damages to underground structures. The primary purpose of current research in this field is to present methods for analyzing stresses and defections developed in underground structures when they are subjected to arbitrary earthquake loading.

ISRM-ISRS-2010-094

International Symposium on In-Situ Rock Stress

SPE Disciplines:

This paper describes the three-dimensional numerical modeling of in situ stress distributions in a limited seismic region of the Earth’s crust. The model involves a vertical strike-slip planar fault that resides in the crust and reaches the Earth’s surface. Stress distribution in faulted areas can be calculated and then used to assess the potential of regional seismic hazard. The second goal of this study is application of a constitutive relation which represents the governing equation of the failure process and specifies the dependence between stress, fault slip, slip rate, and other relevant physical properties. There are several laboratory-derived friction constitutive laws among which the slip-weakening was adopted in this paper to simulate the failure process based on stick-slip behavior of faults. The finite element code (ABAQUS) is used to model the mechanical behavior of fault illustrating the distribution of stress and deformation in the crust.

The dynamic rupture along a fault during an earthquake is a highly complex process involving many factors such as fault geometry, the initial stress field and the constitutive law. Since most of earthquakes occur by sudden slippage along pre-existing faults, the frictional behavior of faults and the constitutive friction law is the main factor in earthquake mechanism (Scholz 1998). There are several frictional laws which express the stick-slip behavior such as Amontons-Coulomb friction law (Jeager, Cook et al. 2007; Voisin, Renard et al. 2007), slip-weakening lawinwhich coefficient of friction is dependent on slip (MariagiovannaGuatteri & PaulSpudich 2000; Senatorski 2002; Olsen-Kettle, Weatherley et al. 2008; Liu & Shi 2009) and rate and state friction law (Chen & Lapusta 2008). All of these constitutive laws are derived from laboratory experiments. In this study, linear slip weakening law is adopted to simulate the frictional behavior of fault.

ISRM-ISRS-2010-119

International Symposium on In-Situ Rock Stress

SPE Disciplines:

Excavation induced stress change is a significant and considerable factor to drive the brittle failure in the underground opening. Rockburst, as a type of brittle failure, has became a great threat to the construction of mining, traffic tunnels, hydropower station etc. With Hoek-Brown brittle parameters

The stability of underground openings can be drastically influenced by excavation-induced stress change (Kaiser et al. 2001). Martin et al. (1999) pointed that as in situ stress magnitudes increase, the fractures growing parallel to the excavation surface due to the induced stress will dominate the process of brittle failure, and the failure regions are localized near the opening perimeter at intermediate depths while at great depths the whole boundary of the excavation may be enveloped by the brittle fractures. Rockburst, as a kind of brittle failure, always results in the damage of equipment, delay of construction and even wounds and deaths of workers. Several failure phenomena observed in B-auxiliary tunnel are presented in Figure 1 (photographed by China Railway Shisi Group). On Nov. 28, a very strong rockburst happened in the drainage tunnel of Jinping II and caused 7 deaths and 1 wounded (Liu 2010).

ISRM-ISRS-2010-096

International Symposium on In-Situ Rock Stress

SPE Disciplines:

- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.47)
- Management and Information > Professionalism, Training, and Education > Communities of practice (0.41)
- Management and Information > Information Management and Systems > Knowledge management (0.41)