Primary Stress State Research Articles

In-situ Primary Stress Detection Based on Seismic Tomography Measurements and Numerical Back-analysis for an Underground Radwaste Repository

In order to understand how the in-situ primary stress state has evolved with subsidence and uplift in a granitic rock mass for anticipated of a radioactive waste repository in Hungary, the authors investigated the applicability of seismic tomography as an interpretive tool. Very high P wave velocity (Vp) values were obtained during the tomographic scanning of the study area of the repository, and these were compared with existing findings of in-situ and laboratory seismic measurements.Apart from seismic tomographic survey, dynamic FEM numerical modelling, empirical calculations of residual stresses, laboratory measurements of compression wave (ultrasonic) velocities on intact rock cores, in-situ primary stress measurements as well as site geological model were integrated to evaluate the use of seismic tomography for identifying possible in-situ stress increases around the excavation.A detailed calibration modelling was carried out based on the site seismic tomography measurements and during the large-scale modelling. It was observed that the increasing Vp is directly related to simulated increasing directional loadings on the rock mass. Using a measured wave raypath it was possible to check the different in-situ stress parametrizations which resulted in the best approximation to the measured Vp values.It was concluded that the rock mass under investigation to extend the repository must have higher in-situ stress values than the area of the constructed deposition chambers nearby. The results of this research indicated that seismic tomography is a useful tool for determining relative stress around and within the vicinity of underground excavation.

Numerical study of response behaviors of natural gas hydrate reservoir around wellbore induced by water jet slotting

The trial production of natural gas hydrate reservoirs remains poor. Reasonable reservoir reconstruction, which can improve formation permeability, is an important approach to increasing the efficiency and enhancing production. In this work, water jet slotting is proposed to reconstruct an natural gas hydrate reservoir near a wellbore. The spatial slots formed by water jet slotting not only directly constitute high-permeability channels, but also generate disturbances to the surrounding in-situ sediment. Water jet slotting disturbances to nearby sediment was investigated using a three dimensional flow-structure coupling model to evaluate the proposed reconstruction method. The reservoir at the SH2 site in the Shenhu area of the South China Sea was used as the reference. A horizontal slotting arrangement along the vertical well was adopted. The results demonstrate that water jet slotting can change the primary stress state of the sediment around the wellbore, and generate a dominant stress relaxation zone and small stress concentration zone. Within the stress relaxation zone, the in-situ compressive stress was remarkably reduced or even transformed into tensile stress, accompanied by sediment displacement and volumetric expansion strain. This is conducive to loosening the sediment around the wellbore and improving the permeability characteristics. In addition, the influence of the water jet slotting parameters including slot radius, spacing, and number on disturbances to the nearby sediment was studied. Reservoir responses to water jet slotting under balanced and unbalanced bottom-hole pressures were compared and analyzed. This study provides a reference for natural gas hydrate reservoir reconstruction using water jet slotting. Cited as: Huang, M., Su, D., Zhao, Z., Wu, L., Fang B., Ning, F. Numerical study of response behaviors of natural gas hydrate reservoir around wellbore induced by water jet slotting. Advances in Geo-Energy Research, 2023, 7(2): 75-89. https://doi.org/10.46690/ager.2023.02.02

The soviet Experience with International students: The Geopolitical Aspect

The relevance of this topic is due to the aggravated geopolitical confrontation in the contemporary world, as well as an increasingly noticeable emphasis on “soft power” and “smart power”. The purpose of this article is to briefly analyse the Soviet experience of using higher education as a tool of “soft power”. In this article, we divided this experience into two main stages. The first was the formation of Communist ideology as only one in the world and the beginning of its expansion (1920–1940s of the XX century.). The second one — was the stage of the bipolar world, when the USSR dealt with international students who came from the countries of “victorious socialism”. At the first stage, the education of international students could not be considered as a full-fledged tool of “soft power”, since it was more about the propaganda treatment of international students than a more subtle impact . Interaction with international students in Soviet universities at the second stage (during the cold war) had another serious flaw. Since it was about the representatives of the countries of “victorious socialism”, the ideological work was carried out on the residual principle, and the primary stress state placed on the training of technical personnel. Thus, it can be concluded that, taking into account the Soviet experience and the vagueness of spheres of influence in the contemporary world, it is advisable to rely on the involvement of international students in the Humanities, who after returning home are more likely to occupy senior positions in the state apparatus and/or become leaders of public opinion.

Time influence of tunnel support on the factor of safety

Before taking any measures to build a tunnel, the rock (soil) is in a primary stress state, which means that the stress state is a function of the thickness of the overburden. At the moment when the measures necessary to excavate a tunnel are taken, the rock state changes from primary to secondary, leading to stress concentration, especially in the tunnel abutments. If the rock is capable of accepting these stresses, a state of equilibrium is reached after certain deformations. Plastic deformations can occur if the stresses are larger than the strength of the rock mass. To avoid excessive deformations or collapse of the rock and the tunnel excavation, it is necessary to place a support. The achieved factor of safety is a function of both the support type and the time when the support is installed. This paper shall present a numerical example of different pressures considered in order to obtain the rock’s reaction curve.

Wechselwirkungen zwischen tiefliegendem Tunnelvortrieb und bestehendem Tunnel im Bereich von Störzonen – Modellierung versus Realität

AbstractThe influence of a tunnel drive on an existing tunnel is primarily the result of the tunnel geometries, the relative location of the two tunnels, the geological‐geomechanical conditions and the associated primary stress state. During construction of the Brenner Base Tunnel, there are constructionally relevant interactions in some places between the running tunnels and the exploratory tunnel (EKS) running centrally and 12 m deeper. The rock mass stress as a result of driving the running tunnels here leads to deformations of the previously driven exploratory tunnel. Due to the high loading on the outer linings of deep tunnels and the high stiffness of the already hardened shotcrete support, slight deformation can cause local cracking in the support layer. This article compares the observed deformations in a fault zone intersecting at a flat angle with numerical simulations, which enables investigation of the geomechanical problems. Comparison of the calculation results with the real deformation behaviour permits the validation of the numerical simulations and the reliability and limits of such analyses.

A novel tool for simulating brittle borehole breakouts

Breakouts are important both as a threat to borehole or tunnel stability and as a tool for primary stress state interpretation. This work suggests a semi-analytical method based on conformal mapping for the prediction of breakouts in brittle rock. After introducing the underlying theory, the developed algorithm is outlined. The single components are described and verified. Finally, the applicability and efficiency of the resulting tools is assessed. The aim is to provide a simple, fast and easy to use tool for the assessment of borehole and tunnel stability and for the estimation of the in situ stress in brittle rock.

Equilizing of the Primary Stress State in the Rock Mass, Simulated by a Model of Layer in an Elastic-Viscous Medium

Abstract This paper is devoted to the analysis of the stress development process in the homogeneous and non-homogeneous rock mass. The rock-mass model consists of an elastic-viscous medium containing a layer (Fig. 1) that displays distinct geomechanical strain properties. When examining the process of stress equilizing in time, the Norton-Bailey power creep law was applied in the numerical analysis. The relationship between effective stresses and time, the modulus of elasticity, Poisson’s coefficient, and creep compliance were obtained. It was demonstrated that the relationship between effective stress and time or creep compliance, for the assumed conditions in a homogeneous rock-mass, was approximated by hyperbolic functions (10 and 16). The process parameter included a certain value of creep compliance or of time at which there occurred a half-way equilizing of primary stresses. An analogous function binds effective stresses with creep compliance. Our model studies indicated a number of relationships between bulk and shear strain with time and creep compliance in the homogeneous and non-homogeneous rock mass, presented in Figs. 2-14, expressed by the functions of those specific parameters. The relationships obtained in this work resulted from our model assumptions. However, they demonstrated the influence of the geomechanical strain properties of rocks on the process of shaping the primary stress state in the rock mass and the tendency to reduce the principal stress differences in time. Our research results suggested the necessity to simulate the primary stress state as an initial condition of the geomechanical numerical analysis concerning the rock-mass behaviour showing rheological properties.

Coupled thermo–hydro–mechanical simulation of CO2 enhanced gas recovery with an extended equation of state module for TOUGH2MP-FLAC3D

As one of the most important ways to reduce the greenhouse gas emission, carbon dioxide (CO2) enhanced gas recovery (CO2-EGR) is attractive since the gas recovery can be enhanced simultaneously with CO2 sequestration. Based on the existing equation of state (EOS) module of TOUGH2MP, extEOS7C is developed to calculate the phase partition of H2O–CO2–CH4–NaCl mixtures accurately with consideration of dissolved NaCl and brine properties at high pressure and temperature conditions. Verifications show that it can be applied up to the pressure of 100 MPa and temperature of 150 °C. The module was implemented in the linked simulator TOUGH2MP-FLAC3D for the coupled hydro–mechanical simulations. A simplified three-dimensional (3D) 1/4 model (2.2 km × 1 km × 1 km) which consists of the whole reservoir, caprock and baserock was generated based on the geological conditions of a gas field in the North German Basin. The simulation results show that, under an injection rate of 200,000 t/yr and production rate of 200,000 sm3/d, CO2 breakthrough occurred in the case with the initial reservoir pressure of 5 MPa but did not occur in the case of 42 MPa. Under low pressure conditions, the pressure driven horizontal transport is the dominant process; while under high pressure conditions, the density driven vertical flow is dominant. Under the considered conditions, the CO2-EGR caused only small pressure changes. The largest pore pressure increase (2 MPa) and uplift (7 mm) occurred at the caprock bottom induced by only CO2 injection. The caprock had still the primary stress state and its integrity was not affected. The formation water salinity and temperature variations of ±20 °C had small influences on the CO2-EGR process. In order to slow down the breakthrough, it is suggested that CO2-EGR should be carried out before the reservoir pressure drops below the critical pressure of CO2.

Design and Evaluation of a Subterranean Work Lining from Layers of Shotcrete and Steel Arch

A steel concrete lining is today the common ground massive retaining structure implemented shortly after the excavation of an opening. The constituent elements of the steel concrete lining are layers of shotcrete and steel arch frames. This simple structure has many unique features which set it apart from the conventional reinforced concrete constructions. Without simplification these features doesn’t allow to carry out design and evaluation of steel concrete lining by relevant design and evaluation methods convenient to the conventional reinforced concrete constructions. The main differences are construction process, yielding curve of structure and determination of external load. In the case of a steel concrete lining its external load is product of mutual work of both the ground massive and the lining. The value of load depends on the yielding curves of the lining and the ground massive. The yielding curve of a ground massive is objective and conditioned on strength strain properties of ground massive and primary stress state. The yielding curve of a steel concrete lining relates lining design specification. The process of construction affects the value of load too. The article analyzes the steel concrete lining focusing the influence of lining design parameters on its yielding curve. It looks for a way to control the ground massive behavior and as well as to engage it more in an effort with lining to stabilize subterranean work. The themes of the analysis are: static and deformation parameters of the cross-section of the steel concrete lining with regard to the construction stage and dependency of hardening shotcrete; interaction between of steel concrete lining and ground massive based on yielding curves; assessment of a stress state across the steel concrete lining section in the steel concrete lining constituent elements. The analysis is carried out on mathematical models that combine analytical and numerical methods.

A Study Of The Membrane State Of FlorenceCathedral Dome

Cracks appeared in Brunelleschi's dome of Florence Cathedral as soon as it was finished. Since the primary stress state for a shell structure is the membrane state, it can be supposed that those cracks were produced by membrane forces. An analysis by membrane theory of this dome shows that cracks correspond to the points with greatest tensions. This analysis also shows the need of two rings and a lantern load, as believed by Brunelleschi himself.

Viscoplastic creep of rocks around horizontal tunnels

The aim of this note is to present a mathematical model to describe dilatancy and/or compressibility of the rocks surrounding a horizontal tunnel, during their deformation by creep. The constitutive equation proposed for the purpose can describe both dilatancy and compressibility depending on the stress state existing at the particular point of the rock under consideration. Due to excavation, the primary stress state in the rock changes to a secondary one. Generally around the tunnel there are areas where the rock is in a dilatant state, while in others the rock is compressible, and finally in others the rock is in an elastic state. Mathematical criteria are given to outline the boundaries of such areas. The boundaries between the domains may also vary in time, since generally the stress state slowly varies with time. Since dilatancy can be related to the damage and failure of rocks, the constitutive equation can be used to predict damage and failure, localization of the damage, etc. (TRRL)

Determination of the primary stress state in tectonic disturbed soft shales : Dolezalova, M; Hudek, J Proc International Symposium on the Geotechnics of Structurally Complex Formations, Capri, 1977, Vol 1, P199–210

Determination of the primary stress state in tectonic disturbed soft shales : Dolezalova, M; Hudek, J Proc International Symposium on the Geotechnics of Structurally Complex Formations, Capri, 1977, Vol 1, P199–210

In-situ stress measurements of the primary stress state in the Sonnenberg tunnel in Lucerne, Switzerland

The results of some primary stress measurements performed in 1970 in the Sonnenberg area near the city of Lucerne, have shown excessive horizontal stress components. In the paper an attempt is made to find correlations between this situation and some models which are suitable to describe the tectonics of the investigated region.