コアディスキングの表面形状に基づく三次元地圧方向の評価法

Inversion of borehole breakout orientation data : J Geophys Res V96, NB12, Nov 1991, P20093–20107

A review of observations on crevasse orientation relative to principal stress or strain rate shows that in many instances crevasses are not aligned in the optimum direction perpendicular to the direction of principal tensile stress. Moreover, a number of studies report the occurrence of strike‐slip motion, with the crevasse walls moving parallel to each other. These observations cannot be explained by the conventional model for crevasse formation, introduced over a century ago by William Hopkins. Instead, it is proposed that crevasses are the manifestation of mixed‐mode fracturing, combining the opening mode usually associated with crevassing, with the shearing mode, which is equivalent to strike‐slip faulting. In a biaxial stress field, the crevasse will tend to orient itself in the optimum direction, thus minimizing the shearing motion, but this direction is achieved asymptotically. This means that on glaciers subject to tension and lateral shear, the orientation of crevasses may differ by up to 20° from the optimum direction. Further, by incorporating the shearing mode, the depth to which fractures can penetrate may be significantly larger than for the opening mode only. The weight‐induced lithostatic stress increases with depth below the surface, so that at some depth, the walls of a crevasse cannot physically separate. However, it is possible that the shearing motion acts over greater depths, thus allowing the fracture to penetrate deeper without separation of the fracture surfaces. This might explain why, on some glaciers, crevasse traces persist well into the ablation area, where surface melting should have obliterated all visible traces of crevasses formed upglacier, if these crevasses penetrated only a few tens of meters, as predicted for the opening mode.

The development of the experimental geothermal site at Soultz-sous-forets, has provided awealth of data on the regional stress field in the local granite basement, down to depths larger than 4 km. Borehole imaging logs have outlined the pervasiveness of drilling induced fractures which yield very accurate indication on the maximum horizontal principal stress (N175o ± 6o E). They outline the existence of both time dependent compression breakouts in the direction of the minimum principal stress and thermal borehole elongations in the direction of the maximum horizontal principal stress. The magnitude of the minimum principal stress has been derived from large scale injection tests during which induced seismicity was monitored. The growth of the seismic cloud indicates that the vertical stress profile is not controlled by the equilibrium of optimally oriented pre-existing faults or fractures but that the development of large scale failure processes in this rock mass satisfies a standard Coulomb criterion, with zero tensile strength.

Influence of the principal tensile stresses on delamination fracture mechanisms and their associated morphology for different loading modes in carbon/epoxy composites

We have performed a laboratory study of cryogenic fracturing for improving oil/gas recovery from low-permeability shale and tight reservoirs. Our objective is to develop well stimulation techniques using cryogenic fluids, e.g. liquid nitrogen (LN) to increase permeability in a large reservoir volume surrounding wells. The new technology has the potential to reduce formation damage created by current stimulation methods as well as minimize or eliminate water usage and groundwater contamination. The concept of cryogenic fracturing is that sharp thermal gradient (thermal shock) created at the rock surface by applying cryogenic fluid can cause strong local tensile stress and initiate fractures. We developed a laboratory system for cryogenic fracturing under true triaxial loading, with a liquid nitrogen delivery/control and measurement system. The loading system simulates confining stresses by independently loading each axis up to about 5000 psi on 8"×8"×8" cubes. Both temperature in boreholes and block surfaces and fluid pressure in boreholes were continuously monitored. Acoustic and pressure-decay measurements are obtained before and at various stages of stimulations. Cubic blocks (8"×8"×8") of Niobrara shale, concrete, and sandstones have been tested, and stress levels and anisotropies are varied. Three schemes are considered: gas fracturing without cryo-stimulation, gas fracturing after low-pressure cryogen flow-through, gas fracturing after high-pressure flow-through. Pressure decay results show that liquid nitrogen stimulation clearly increases permeability, and repeated stimulations further increase the permeability. Acoustic velocities and amplitudes decreased significantly following cryo-stimulation indicating fracture creation. In the gas fracturing without the stimulation, breakdown (complete fracturing) occurs suddenly without any initial leaking, and major fracture planes form along the plane containing principal stress and intermediate stress directions as expected theoretically. However, in the gas fracturing after cryogenic stimulations, breakdown occurred gradually and with massive leaking due to thermal fractures created during stimulation. In addition, the major fracture direction does not necessarily follow the plane containing principal stress direction, esp. at low confining stress levels. In tests, we have observed that cryogenic stimulation seems to disrupt the internal stress field. The increase of borehole temperature after stimulation affects the permeability of the specimen. While a stimulated specimen is still cold, it keeps high permeability because fractures remain open and local thermal tension is maintained near the borehole. When the rock becomes warm again, fractures close and permeability decreases. In these tests, we have not used proppants. Overall, fractures are clearly generated by low and high-pressure thermal shocks. The added pressure of the high-pressure thermal shocks helps to further propagate cryogenic fractures generated by thermal shock. Breakdown pressure is significantly lowered by LN stimulation with breakdown pressure reductions up to about 40% observed.

In order to investigate the effect of welding residual stress field on the fatigue crack propagation direction, the plate specimen which had crossed beads and asymmetric residual stress field, and another specimen which had one bead parallel to the load axis and located in the center of the specimen width were prepared.The material was high tensile strength steel and the welding was done by electron beam welding. Fatigue crack propagation tests were conducted under constant amplitude loading and stress ratio was nearly equal to zero.As the results, it was clarified that the direction of maximum tensile principal stress on a certain point beyond the crack tip was rotated to the direction of load axis, the crack tip coming near and loading being applied. And also it was recognized that the fatigue crack even though on the crossed beads specimen was propagated straightly perpendicular to the load axis, and that the fatigue crack propagation rate was plotted on same scatter band obtained in base metal against to ΔKeff.

Introduction Single-phase epoxy polymers are usually relatively brittle materials and are frequently toughened by the incorporation of a rubbery phase [1]. Two important toughening mechanisms have been identified for such two-phase materials, which consist of a rubbery phase dispersed in a matrix of a cross-linked polymer. The first is localized shear yielding, or shear banding, which occurs between rubbery particles at an angle of approximately + 45 ° to the direction of the maximum principal tensile stress [1-3]. Due to the large number of particles involved, the volume of thermoset matrix material which can undergo plastic yielding is effectively increased compared to the single-phase polymer. Consequently, far more irreversible energy dissipation is involved and the toughness of the material is improved. The second mechanism is the internal cavitation, or interfacial debonding, of the rubber particles, which then enables the subsequent growth of these voids by plastic deformation of the epoxy matrix [1-4, 5]. This irreversible hole-growth process of the epoxy matrix also dissipates energy, and so contributes to the enhanced fracture toughness. Recently, attempts [4, 5] have been made to model the toughening mechanisms and this work has clearly demonstrated the importance of establishing a sound finite element analysis representation of the effects of the rubbery phase on the properties of the rubber-toughened polymer. Indeed, this is the aim of the present work, and some most interesting initial results are the subject of the present letter. Predictive modelling of particulate-filled composite materials has frequently been based on the assumption that the material consists of a collection of cylinders, each containing a sphere at its centre [4, 6, 7]. The size of the cylinders as a function of the overall volume fraction can be deduced from assuming a uniform array [4, 6], or assuming a random array [7]. Such analysis is relatively straightforward since the constraints required to model the interactions of surrounding particles may be simply applied by forcing the sides of the cylinder to remain straight. However, this material model is inherently inaccurate since it does not reflect the overall isotropy of the material. The more accurate material model used in the present work is a collection of spheres, each consist-

Past RC panel tests performed at the University of Houston show that reinforced concrete membrane elements under reversed cyclic loading have much greater ductility when steel bars are provided in the direction of principal tensile stress. In order to improve the ductility of low-rise shear walls under earthquake loading, high seismic performance shear walls have been proposed to have steel bars in the same direction as the tensile principal direction of applied stresses in the critical region of shear walls. This paper presents the results of reversed cyclic tests on three low-rise shear walls with SMA bars. The height, width, and thickness of the designed shear walls are 1.0 m, 2.0 m, and 0.12 m, respectively. SMA bars are provided in the directions of 27 degrees to the horizontal that are in the diagonal direction. The reinforcing bars of the shear walls are in vertical and horizontal directions. The ratios of both SMA and reinforcing bars are 0.24%. The main parameter used in the study is the type of SMA bar, namely Superelastic and Martensite SMA bars. The test results from the walls with SMA bars are also compared to a conventional wall without SMA bars. Test results also show that the maximum shear strengths of the tested walls are affected by the SMA bars. It was found that the shear wall with Martensite SMA bars has greater residual displacement. In contrast, the shear wall with superelastic SMA bars has less residual displacement. At the ultimate state, one of the four superelastic SMA bars buckled, resulting in less energy dissipation capacity than the expected value. Preventing buckling of SMA bars is the research focus in the near future.

The Suki–Kinari hydropower station is situated in a landform with high mountains and deep valleys in northwest Pakistan. Affected by some geological conditions such as topography and faults, the in situ stress distribution of the project area is highly complex. To determine the in situ stress associated with this project, hydraulic fracturing in situ stress tests are conducted in four representative boreholes. Generally, the relation of the three principal stresses is σ H > σ z > σ h . However, the measured stress directions associated with the headrace tunnel and the powerhouse area are clearly different. The direction of the maximum horizontal principal stress of the headrace tunnel is NNE–NEE, whereas that of the underground powerhouse area is NNW–NWW. To identify the mechanism responsible for this difference, the present study simulates the stress field in the project area based on the three-dimensional finite element regression analysis method. Two theories are proposed on the basis of the numerical simulation results. On the one hand, the direction of the maximum horizontal principal stress in the rock mass above the valley elevation is controlled mainly by topography. On the other hand, the fault will disturb the stress field, causing noticeable changes in the stress direction and value of the fault-affected zone. By comparing the characteristics of the stress direction distribution at three elevations, the direction of maximum horizontal principal stress of the shallow strata is determined to be affected mainly by the topography and faults, and topography may be the most important factor. Moreover, with an increase in strata depth, the influence of the topography and faults on the stress direction decreases gradually. After reaching a particular strata depth, the direction of maximum horizontal principal stress tends to be the same as that of the regional tectonic stress, i.e., NNE.

The key components of railway rolling stock are almost constantly in the multiaxial stress state in service. To evaluate the fatigue strength of the structure, stress parameters should be obtained beforehand. The traditional projection method assumes that the maximum principal stress is the maximum tensile stress, and then the projection process is performed without taking the tension/compression states of principal stresses into account. To address these inadequacies, a method for fatigue parameter determination was proposed based on principal stress projection toward a spherical direction cosine group. This method can determine the direction of maximum tensile stress and preserve the tension/compression attribute of stress during calculating the maximum stress and the minimum stress. Therefore, the physical meaning of present improved method is more apparent than the traditional projection method, and its algorithm is simple and effective. The fatigue strength assessments of a welded bogie frame were performed utilizing the traditional projection method and present method, respectively. Results obtained show that maximum stresses calculated by the improved method can be 19.4% higher than those obtained by the traditional one at some nodes, while the minimum stresses and the stress ratios derived by these two methods can even be of opposite sign. The present method, which takes the tension/compression properties of principal stresses into consideration, is more reasonable, as compared with the traditional projection method, and thus, its fatigue strength assessment results are more credible.

The inversion of earthquake focal mechanisms is one of the few tools available for determining principal stress directions at seismogenic depths. Various methods have been proposed for performing such inversions. For three of the most commonly used methods, including one that has been proposed by Jacques Angelier, we discuss the physical assumptions and the error determination and then we propose an extension for one of the methods. All four methods are then applied for evaluating the stress field in the Upper Rhine graben. They are applied to seismic data recorded with a temporary monitoring network that was deployed 12 hours after the magnitude Mw = 4.4 Sierentz earthquake, which occurred on July 15, 1980. While differences in principal stress directions can be as much as 28° depending on the method used for the principal stress direction determination (orientation of the minimum principal stress has been found to range from N051°E with a 27° plunge to N090° E with a 20° plunge), the 90% confidence level associated with each method varies from 11° to 27°. Moreover, these various methods yield fairly diverse values for the R factor that characterizes relative differences between principal stress magnitudes (from R = 0.7 with a 0.2 90% confidence level to R = 0.3 with a 0.2 90% confidence level). Furthermore all three methods leave some focal mechanisms unexplained. These are then declared to be the result of heterogeneity and are not considered for the inversion. It is concluded that earthquake focal mechanisms inversions lack resolution for stress field evaluation at depth if no proper attention is given to the event independence hypothesis. When proper attention is given to this hypothesis, a resolution of the order of 15° may be achieved. The minimum principal stress orientation derived with these various focal mechanisms inversions differs by 4 to 36° from the orientation determined from borehole breakouts observed in Basel, in a 5 km deep well (N054°E ± 14°), located some 20 km from Sierentz. The solution that fits best borehole breakout observations is that which satisfies the minimum number (three) of prerequisite physical assumptions.

To study the initiation, propagation, and mechanical behaviour characteristics of fractures on the wall of a single hole during hydraulic fracturing for in-situ stress measurement, and to improve the accuracy of stress direction discrimination in hydraulic fracturing in-situ stress measurement. This paper adopts methods such as theoretical analysis, numerical simulation, and experimental research. On the basis of obtaining basic mechanical parameters through rock mechanics tests, and using the Weibull distribution to describe the inhomogeneity of rocks, a model based on COMSOL software is constructed. It studies the laws and influencing factors of fracture propagation on the borehole wall under different stress environments. The simulation results show that under different stress conditions, the initiation direction of fractures on the borehole wall is mainly parallel to the direction of the maximum principal stress and perpendicular to the direction of the minimum principal stress. At the same time, the propagation and closure of fractures are jointly controlled by the mechanical properties of rocks and the in-situ stress field. In addition, natural cracks have little impact on the propagation of fractures on the borehole wall, and the propagation of hydraulic fractures is not affected by natural cracks. This research achievement provides a reliable theoretical basis for accurately identifying the direction of underground stress, and has important value in guiding tunnel support design, preventing and controlling rockburst and other engineering practices.

Based on the comprehensive analysis of wellbore characteristics in a deep shale gas field, the in-situ stress state of the shale reservoir was assessed in this study for the Longmaxi formation in the Dingshan area, Southwestern China. The data obtained from hydraulic fracturing, drilling-induced fractures, and in-situ core testing were used to determine the magnitude and direction of the maximum principal horizontal stress in five wells. Besides, hydraulic fracturing and cross-multipole array acoustic logging (XMAC) were employed to determine the vertical variation of the in-situ stress. Based on the logging interpretation and mechanical test results, the spatial distribution of rock mechanical parameters in the Dingshan area was assessed by the amplitude variation versus offset (AVO) seismic inversion. A 3D heterogeneous mechanical inversion model was realized via the ANSYS and CATIA3D finite element software packages, providing the area in-situ stress field simulation. The depth, fault strike, and position change effects on the main stress, horizontal stress difference, and horizontal stress difference coefficient were numerically simulated. The results show that the maximum principal stress azimuth was mainly concentrated in the NE20°-NE80° sector. Moreover, the development zone of natural fractures was related to the area with the highest principal stress differences. The maximum principal stress variation in the study area was mainly in the compressive range from −135 to −45 MPa, gradually increasing from east to west and south to north. The stress type mainly depended on the depth, fault strike, and rock mechanical parameters, while the stress difference and stress difference coefficient near the fault structure were relatively small. This study’s findings are considered instrumental in improving the borehole stability, determining the casing setting point, and optimizing the well location in deep shale reservoirs with similar geological conditions.

In fully mechanized top‐coal caving (FMTC), the effects of coal caving are closely related to the degree of damage to the top‐coal which is determined by the stress state therein. Therefore, research into the evolution of mining‐induced stress field is key to understanding failure mechanisms of top‐coal. By taking the 12 309 FMTC face in Wangjialing Coal Mine (China) as the engineering background and using field measurement and numerical simulation, we studied the evolution of the principal stress field in FMTC under high horizontal stress. The research results demonstrated that in situ stress in Wangjialing Coal Mine was a high horizontal stress and lateral pressure coefficients in the north‐south and east‐west directions were 1.52 and 0.45, respectively. Under the influence of mining, principal stresses were concentrated in front of the coal wall, with concentration factors of 2.26, 2.24, and 2.85, respectively. The minimum principal stress in unconstrained sides of the top‐coal in the roof‐control zone was tensile. The directions of intermediate and minimum principal stresses in the top‐coal were more sensitive to the influence of mining compared to the maximum principal stress. During rotation of the principal stress direction in top‐coal, the location (19 m in front of the coal wall) where the maximum and intermediate principal stresses were equal and the location of the coal wall are key positions.

Effect of confining pressure on damage accumulation of rock under repeated blast loading