Andreiko, S. (Mining Institute of the Ural Branch of the Russian Academy of Science, Perm National Research Polytechnic University) | Baryakh, A. (Mining Institute of the Ural Branch of the Russian Academy of Science, Perm National Research Polytechnic University) | Lobanov, S. (Mining Institute of the Ural Branch of the Russian Academy of Science) | Fedoseev, A. (Mining Institute of the Ural Branch of the Russian Academy of Science)
On the base of mathematical modeling of stress-strain state in underworked layered salt massif the influence of clay bands on the creation of conditions for gas-dynamic phenomena in the process of potash deposits development has been analyzed. During numerical implementation clay bands were simulated by modified Goodman joint elements. The results of modeling of mine technical situations that precede gas outburst are presented for room-and-pillar method and longwall mining.
More than 30 potash deposits are developed or are in stage of prospecting in the world. Total world reserves are estimated in 100 billion tons of K2O. First underground potash mining was started at a small mine near Stassfurt, Germany, in 1861. From that time rapid development of potash industry began, first in Germany and then in the former USSR, France, the USA, Canada, Italy and England.
Underground mining of potash beds is considerably difficult due to gas-dynamic phenomena (GDP) practically at all potash deposits of the world. Sudden outbursts of gas and salt, roof rock failure, combined type phenomena, rock bump near working face-these are GDP which present real danger to miners’ life, damage expensive excavating and treating equipment, disturb regular work process of potash mines. During the sylvinite beds mining at Verchnekamskoe (Russia) and Starobinskoe (Belarus) potash deposits more than 500 GDPs occured which in some cases led to fatal outcomes and made considerable damage to property of the enterprises. Thus, solution of GDP problem which is faced with during the underground mining of potash layers is one of the most urgent tasks of mining engineering.
2. Study of the gas-dynamic phenomenon mechanism
Improving mine safety demands the research of the mechanism of different gas-dynamic phenomena.
The presence of gas accumulation near the surface of mined areas and meeting major failure criteria are the conditions for GDP realization. In potash mines gas accumulation occurs, as a rule, in clay bands and layers. Geomechanical preconditions for GDP occurrence are the opening of clay joints, leading to formation of channels for free gases migration and anthropogenic gas saturated zones appearance due to the influence of mining. It means that mathematical models having a claim on description of gas-dynamic phenomena during the development of such deposits should consider deformation of clay joints.
Overwhelming majority of GDP at Verchnekamskoe deposit is realized as sudden roof’s rock failure (floor failure) which is accompanied by release of gas (Fig. 1). These gas-dynamic phenomena occur due to near-contact gases pressure in the roof or the floor of mine working.
Park, Sehyeok (Seoul National University) | Xie, Linmao (Seoul National University) | Kim, Kwang-Il (Seoul National University) | Kwon, Saeha (Seoul National University) | Min, Ki-Bok (Seoul National University) | Choi, Jaiwon (NexGeo Inc.) | Yoon, Woon-Sang (NexGeo Inc.) | Song, Yoonho (Korea Institute of Geoscience and Mineral Resources)
The first hydraulic stimulation for enhanced geothermal system (EGS) development in Korea had been conducted in the PX-2 well of 4,348 m depth in Pohang EGS site from January 29th to February 20th, 2016. Treatment histories of injection rate, wellhead pressure and corresponding induced microseismicity data were obtained from the stimulation test upon 140 m long open hole section at the well bottom. Wellhead pressure was up to 89 MPa and considerable level of flow rate was attempted up to 47 L/sec. Microseismicity observation showed a trend of lager and more frequent seismicity occurrence in shut-in phase than in injection phase. The injectivity index during the stimulation periods had increased as 2.7 times in January 30th at the wellhead pressure of 73 MPa. Postulating the existence of a major fracture zone intersecting the open hole section, the transmissivity and the corresponding equivalent aperture of the fracture were evaluated. Required breakdown pressures by hydrofracturing and hydroshearing mechanisms were estimated based on the various scenarios on the in-situ stress condition, major fracture zone orientation and shear failure criteria.
1.1. Pohang EGS development site
The first enhanced geothermal system (EGS) development project in Korea was launched at the end of 2010 in Pohang. Five boreholes are located within 5 km from the site (Fig. 1): BH-1 of 1.1 km depth, BH-2 of 1.5 km depth, BH-3 of 0.9 km depth, BH-4 of 2.4 km depth, and EXP-1 of 1 km depth. The Pohang EGS site is owned and operated by NexGeo Inc., and it is located at 129°22’46.08’’E, 36°06’23.34’’N. Drilling of PX-1 and PX-2 wells were finished with final depths of 4,127 m and 4,348 m, respectively, and it is planned to be expanded to a triplet system, i.e., a fluid circulation system with three wells in the target reservoir, after stimulations in PX-2 and PX-1.
Zhai, Hao (The University of New South Wales) | Canbulat, Ismet (The University of New South Wales) | Hebblewhite, Bruce (The University of New South Wales) | Zhang, Chengguo (The University of New South Wales)
Weak rock mass strength estimation is a long-lasting challenge associated with geotechnical engineering due to its complex nature and limited definition. Weak rock masses normally refer to low strength, highly fractured decomposed and tectonically disturbed rocks which have properties intermediate from brittle rocks to ductile soils. Since the behavior of weak rock mass has not been fully understood, it is a common practice to apply existing empirical approaches, which are developed for competent rock masses influenced by joints, to determine their mechanical properties. This paper reviewed the current empirical approaches, and detailed weak rock mass strength calculations based on rock matrix, joint layout, joint condition and external factors. The limitations associated with these methods are discussed, and suggestions are provided for the selection of suitable methods.
Determination of weak rock mass properties is a significant challenge in geotechnical engineering. In general, weak rocks are considered to be the transitional material between competent rocks and soil, therefore, their behavior converges to competent rock at its upper bound and soil at the lower bound. Despite significant amount of research, methods to estimate in-situ behavior and strength of weak rock masses remain to be relatively fragmented and incomplete. The difficulty of determining their behaviour is mostly caused by the complex nature and inadequate definitions. Different origin and alteration process of weak rocks result in variant properties that inevitably influence their overall behaviour. Hence, it is important to understand the differences in their property that inherited from both previous phase and alteration process and to adopt suitable approaches to estimate their strength according to these features.
In practice, the term weak rock commonly refers to both young sedimentary rocks with low compressive strength and heavily altered hard rock with intense structures [1-3]. Based on the origin and geological alterations, weak rock can be classified as young sedimentary rock, weathered competent rock and tectonically disturbed competent rock as shown in Fig. 1. Young sedimentary rocks such as mudstone and claystone contain poor lithification and weak particle cementation. The strength of them can be described by the ISRM definition of weak rocks with uniaxial compressive strength (UCS) being 0.5 MPa to 25 MPa [2, 3]. Weathered competent rocks such as sandstone can also be considered as weak rock. During prolonged exposure, some rock mass components start to break down and crack along pre-existing micro fractures. As a result of weathering, the well-developed, interconnected defect fabric deteriorates the integrity of the rock mass, thus lead to a reduction of the overall mechanical strength. This type of rock is well represented in Rock Mass Rating (RMR) and Geological Strength Index (GSI) classification systems as poor quality rocks with ratings lower than 25 and 20 respectively or less than 0.1 in Q system. In practice, there is a tendency to consider tectonically disturbed competent rocks, which preserves limited original structures formed in lithification, as weak rock mass . Due to destruction of original structure during folding and shearing, it’s common to observe widely existing intensive fractures. Thus, this type of rock has very low mechanical properties similar to other types of weak rock masses. Marinos and Heok’s study of flysch in 1998 provides a good example of such weak rock [4-6].
Buocz, Ildikó (Budapest University of Technology and Economics) | Rozgonyi-Boissinot, Nikoletta (Budapest University of Technology and Economics) | Török, Ákos (Budapest University of Technology and Economics)
Shear strength along rock discontinuities by means of direct shear strength tests were performed on granitic and clayey rocks. Both rock types represent potential host rocks of radioactive waste. The paper focuses on the effect of one parameter in particular influencing the shear strength: the angle enclosed by the plane of the sample surface and the shear plane, in the direction of shear. 3D surface roughness measurements were carried out on 18 Opalinus Claystone sample (Switzerland) and 6 on granitic rock sample (Hungary) surfaces. An interval of angle was analyzed in the direction of shear and against the direction of shear. Direct shear strength tests were carried out under constant normal loading (CNL) conditions, the influence of the angle on the shear strength values were determined. Additionally, a percentage value was calculated for how much the range of both peak and residual shear strength values change, with taking into consideration the effect of upslope and downslope shear.
Designing and constructing in rock masses require a detailed and sound knowledge of the mechanical properties of the rock materials involved. Failure of rocks and determination of appropriate failure envelope has been studied for a long time [1, 2], however one of the most important parameters controlling the strength of rock masses is the shear strength of the discontinuities . The discontinuities are usually a weak point of the rock mass; in particular, the features of their surfaces, such as roughness, joint strength, brittleness, humidity, mineralogy, (etc.) determine its strength. Mechanical, test related factors i.e.: loading conditions, type of testing machines, (etc.) influence the shear strength as well. The mechanical parameters defining the shear strength, i.e. friction angle and apparent cohesion, are investigated by means of laboratory direct shear strength tests. The basic principles of such analyses and test methods have been already published [4, 5, 6].
In this paper one parameter influencing shear strength was investigated in particular: the angle between the sample surface and the shear surface in the direction of the shear, resulting in “upslope” or “downslope” shearing. The paper compares the results of upslope and downslope shearing for granites and claystones. Both rock types represent target formations for hosting radioactive waste in Hungary [7, 8] and in Switzerland [9, 10, 11], respectively.
The current study presents a numerical modeling approach for three-dimensional simulation of hydro-shearing in jointed rocks for the generation of a man-made, multi-fracture heat exchanger in Hot Dry Rock (HDR) Geothermal reservoir. Various literatures have suggested the presence of in-situ fractured rock mass even in massive granite formations. Thereby, 3D numerical modelling is essential, since the prediction of fracture growth in 3D is key to investigation of different fracture designs and furthermore various operational parameters in order to optimize the heat exchanger design and the resulting energy production. The numerical approach incorporates the Dynardo’s approach of homogenized continuum method to simulate the hydro-shearing process in jointed rocks unlike vast majority of commercial and scientific approaches which use the discrete modelling technique. The main motivation of the continuum approach is the numerical efficiency of the 3D coupled hydraulic- mechanical simulations of hydraulic fracturing process in comparison to other alternatives. The input parameters of the numerical model from the best available well log and reservoir data are calibrated from diagnostics measurements such as Micro-seismic events, Bottom Hole Pressure, etc to assign the correct level of forecast quality to the important mechanisms of hydraulic fracturing. The numerical procedure is applied to a prospective Granite reservoir in Thuringen, Germany within the purview of a German joint research project - optiRiss. The integrated approach involves ANSYS as a pre-processor and solver, Dynardo’s fracturing simulator on top of ANSYS, Tamino - a post-processing tool and optiSLang - an environment for optimization and uncertainty quantification.
Energy consumption in the world has seen an ever increasing upward trend since the early part of the 20th century. Still a major chunk of the total energy supply comes from non-renewable energy resources with approximately 66 % supplied only by crude oil, natural gas and coal reserves. The large share of these resources are not just restricted to developing economies such as China, Brazil, etc. but also economic powerhouses such as USA and the European Union . Geothermal Energy provides tremendous benefits in terms of reducing dependence on fossil fuels, reducing greenhouse emissions and generating new economic and employment opportunities. Geothermal Power systems aim to extract the inexhaustible heat available beneath the earth's surface. Natural Geothermal springs are rare to find and hard to locate and in order to reduce dependence on naturally occurring hydrothermal reservoirs, Hot Dry Rock/Enhanced Geothermal Systems (EGS) provide a suitable alternative. EGS reservoirs are set-up by drilling wells beneath the earth's surface and creating a man-made permeable fracture network between the wells. In the process, Hydraulic Fracturing is a well-stimulation technique used for creating artificial heat exchanger by creating a network of permeable fractures between injection and production wells.
Kajzar, Vlastimil (Institute of Geonics of the CAS) | Pavelek, Zdenek (HBZS, a.s., Lihovarská, VSB-Technical University of Ostrava) | Konícek, Petr (Institute of Geonics of the CAS) | Kukutsch, Radovan (Institute of Geonics of the CAS)
Relatively accurate data regarding the temperature distribution in a rock mass can be obtained from measurements carried out in short boreholes drilled directly into mine workings. Such thermal field measurements were undertaken in selected mine workings in all active mines of the Ostrava-Karviná District (OKR), situated in the Czech part of the Upper Silesian Coal Basin. A temperature field survey of the rock mass at high depths was performed in the years 2011-2013, with a total of 204 valid in situ measurements of initial temperature in the carboniferous rock mass recorded, ranging from 27.0°C to 48.9°C. All measurements were registered in a newly created database.
During the project, many archival sources of individual temperature measurements made in the OKR were found, complementing and extending the existing knowledge base regarding the distribution of the temperature field.
A new spatial distribution model of primary temperature values for the carboniferous massif in the Czech part of the Upper Silesian Coal Basin was the final output of the presented project. The main purpose of this spatial modelling and data visualisation was to create an image of the area’s complex topological arrangement based on the available data. Following the generated thermal model, a 3D map of OKR temperature fields at around 1000 m depth was generated. In order to determine the temperature at a particular position in the thermal model, an executable plugin was created. It is also possible to import the final thermal model of the carboniferous massif in the Czech part of the Upper Silesian Coal Basin into commercial computational software for the determination of appropriate mine air conditions.
Mining activities involved in the extraction of energy resources in the Czech Republic are reaching ever more significant depths, a phenomenon that has led to increased consideration of mine worker safety, as well as the indirect safety of the general public. The project “Mine workings design and direction at depths of 800 metres and greater” focuses on the safe direction and operation of mine workings at depths exceeding 800 metres, due to the fact that at such depths, increased rock stresses and temperatures may lead to potentially dangerous anomalous stress phenomena or disturbances in ventilation and air quality. The latter in particular require specific procedures to be undertaken by the national Mines Rescue Service when dealing with such emergency situations.
Many design codes recommend using the limit states design in different geotechnical design applications including rock slope stability. The Generalized Hoek-Brown (GHB) failure criterion is widely adopted for rock characterization. However, a partial factor is defined for the uniaxial compressive strength only in international codes like the Eurocode. The limit states slope stability using the reduced uniaxial compressive strength does not provide the same factor of safety when the reduced equivalent Mohr-Coulomb (MC) parameters are utilized in the analysis. This study aims at developing a partial factor for the Geological Strength Index such that the limit states design of weak rock slopes using the reduced GHB parameters becomes equivalent to the design using the reduced MC parameters. A parametric study has been carried out using the Limit Equilibrium Analysis approach for weak rock slopes considering several parameters such as the uniaxial compressive strength, the geological strength index, slope inclination and height, and water depth. It is concluded that the proposed partial factor should range from 1.10 to 1.15 according to the values of the uniaxial compressive strength and geological strength index. These two parameters are the most influential on the proposed partial factor.
Hoek-Brown (HB) criterion has been utilized for over three decades to characterize the failure envelope of rock mass . It has gained a lot of attention from rock mechanics researchers and practitioners due to its ability to quantify the effect of discontinuities on the rock mass strength, especially weak rocks . Hoek et al. [3, 4, 5] defined the rock mass strength by two main parameters, the Uniaxial Compressive Strength (σ ci) and the Geological Strength Index (GSI).
Limit equilibrium analyses (LEA) of rock slopes have been carried out for a long time using equivalent Mohr- Coulomb (MC) parameters (cohesion “c” and friction angle “Φ”). The equivalent MC parameters are determined by linear interpolation of the inherently-nonlinear GHB failure envelope in the same expected range of confining stresses. With the recent advances in the geotechnical software applications, it became now possible to carry out the LEA utilizing GHB parameters.
It is critical to obtain the rock strength along the wellbore to control drilling problems such as pipe sticking, tight hole, collapse and sand production. The purpose of this research is to predict the uniaxial compressive strength based on data of sonic travel time, formation porosity, density and penetration rate. For prediction of UCS, artificial neural networks were developed between UCS and input data resulting a practical correlation. In this research, a long well segment possessing complete and continuous data coverage has been analysed, and collected data of the wellbore are used to correlate data of the four mentioned input parameters of artificial neural networks with uniaxial compressive strength data as network targets. Selection of input parameters is based on a vast literature review in this area. Due to the fact that standard experimental test methods based on established standards require costly equipment and that the methods for sample preparation is difficult and time-consuming, indirect methods are more favourable. Using these methods, the UCS values are predicted in a simpler, faster and more economical way. In this study, it is concluded that artificial neural networks are a good predictor of rock strength, and can reduce drilling costs significantly. It is observed in this paper that UCS predicted values by neural networks are very close with lab and field data, which is concluded by analysis of network performance results including mean squared error and correlation coefficient. It is also concluded in this study that input parameters which are chosen in this study, have deep effects in UCS prediction studies, and should be considered in other scientific studies. Conclusions show that using artificial neural networks to predict UCS of formation rocks in petroleum fields around the world, would ease UCS estimation, optimize drilling plans and decrease costs.
A geomechanical model requires a great deal of input information including measurements of magnitude of vertical and minimum stresses, pore pressure, rock mechanics properties and drilling experiences, all oriented to determine the magnitude of maximum horizontal stress. To conduct a geomechanical reservoir characterization, it is essential to have the knowledge of the in-situ stress magnitudes and rock mechanical properties .
Coal seams in the Upper Silesian Coal Basin are currently extracted under more and more disadvantageous geological and mining conditions. Mining depth, geological dislocations and mining remnants are factors which affect the rockburst hazard during underground mining to the greatest extent. This hazard can be minimized by employment of active rockburst prevention, where long-hole destress blasts in roof rocks (torpedo blasts) have an important role. The main goal of these blastings is to either destress local stress concentrations in rock mass and to fracture the thick layers of strong roof rocks to prevent or minimize the impact of high energy tremors on the excavations. Sometimes, these blastings are performed to make the roof rocks caving behind the longwall face easier. The efficiency of blasting is typically evaluated from the seismic effect, which is calculated based on seismic monitoring data (seismic energy) and the weight of the charged explosive. This method, as used previously in the Czech Republic, was adopted in a selected Polish hard coal mine in the Upper Silesian Coal Basin. This method enables rapid and easy estimation of destress blasting effectiveness, adjusted to conditions occurring in the designed colliery. Destress blasts effectiveness may be evaluated via the seismic source parameters analysis as well, as was carried out in the selected colliery in the Polish part of the Upper Silesian Coal Basin. These parameters provide information, for example, on its size, state of stress and occurrence of slip mechanism in the source of provoked tremors. Long-hole destress blasting effectiveness in selected colliery has been evaluated using the seismic effect method and seismic source parameters analysis. The results were compared with each other and conditions were observed in situ.
Rockburst is a dangerous phenomenon occurring during deep underground hard coal mining in the Upper Silesian Coal Basin (USCB). To minimize this hazard, special prevention techniques are applied. One of them are destress blasts in the rocks surrounding the coal seam, especially in roof rocks. The main purpose of such blasts is to reduce stress concentrations in these rocks, and to reach a new advantageous energy equilibrium state by the rock mass due to stress drop. Fracturing the thick layers of strong roof rocks to prevent or minimize the impact of high energy tremors on the excavations is important too. Sometimes blasts are performed to facilitate roof rocks caving and goaf formation. Hanging-up of strong roof rocks behind longwall face may be responsible for high-energy tremors occurrence in close distance from the longwall face, which is dangerous for the working crew.
The paper describes the investigation results on specific strain energy in hard rocks (urtite, ijolite and apatite-nepheline ore) under uniaxial compression, tension and triaxial compression. The ways of strain energy accumulation have been shown in the investigated rock samples under different loading modes on the pre-pick and pick stage. The connection between energy accumulation capability of the rocks and their strength and deformation characteristics has been established. The investigation results have been examined in respect to forecasting rockburst hazard in the hard rock mass.
The development of highly stressed hard rock mass is often accompanied by dynamic pressure manifestation with different destruction degree of mining workings: from slabbing and bursting to rockbursts and mining-induced earthquakes. The last two are the most dangerous ones. The consequences of such rock pressure manifestation during mining can be catastrophic and lead to great property losses and even to human fatalities. To prevent these events special arrangements for rock pressure forecasting have been developed.
In recent time when forecasting rockburst hazards of the rock mass, much more attention is given to the questions of strain energy accumulation and transfer in the rocks of different hierarchical level [1-6]. The physical modeling is the main method to investigate energy processes in the rock mass. It is carried out on the adequate rock mass physical models – samples because of similar occurring processes. Thus, we can suppose that the processes of strain energy accumulation and transfer in the specimens and in the rock mass are the same. Therefore, it is necessary to establish the critical energy during the specimens’ failure and a character of energy accumulation during the specimens loading in order to forecast rockburst hazards in the rock mass.
By now vast experience has been accumulated in conducting such experimental research [4, 5, 7–9]. However, the issues of establishing the interconnection between energy parameters and physical-mechanical properties under different stress-strain conditions have not been solved yet.