Wang, Yiteng (Chinese Academy of Sciences / University of Chinese Academy of Sciences) | Wang, Chuanying (Chinese Academy of Sciences) | Han, Zengqiang (Chinese Academy of Sciences) | Zou, Xianjian (Chinese Academy of Sciences)
In the process of engineering investigation of rock mass, it is of great significance to obtain precise rock structural surface properties for rational design of the project and prevention of geological disasters during construction. A new three-dimensional topological feature description method for the rock face was proposed. This method is based on the circular structural profile line observed in the borehole and digital borehole camera technology application in three-dimensional topographical feature of the rock structure plane. This paper is based on the borehole wall plane development graph as the basic data, the basic information such as the occurrence of rock structural planes on the borehole wall is analyzed, and the structural surface section lines extracted from the borehole plane development diagram of the digital image processing technology are used. According to the three-dimensional information feature of the profile line on the structural wall of the hole wall, the profile feature parameters of the profile lines in each direction are calculated, and the structural surface is formed by referring to the correspondence between the profile feature parameters and the structural surface roughness coefficient (JRC). Then the roughness coefficient rose diagram is made to describe the three-dimensional roughness of the rock structure surface.
In recent years, China has gradually attached importance to the use of underground space in cities and the exploration and development of deep resources. Many major projects have gradually been put on the agenda, and engineering safety issues have also increased. The use of test technology by engineering designers to obtain precise and accurate engineering properties of deep rock masses is of great significance to the rational design of the project and the prevention of geological disasters during construction.
The study of rock structural planes is the basic work of analyzing the engineering properties of rock masses. Numerous studies and experiments have shown that the mechanical properties of the rock face are not only related to the characteristics of the wall rock and the combined state of the face, but also affected by the surface morphology of the face.
For a hard structural surface with a small degree of filling, the surface morphology of the structural surface is the main influencing factor controlling the mechanical properties of the structural surface (Gao et al.,2010). However, obtaining information on deep rock structural planes by drilling and coring has many limitations. First, during the core drilling process, due to the rotational displacement of the core, the exact information of the rock structural plane is destroyed. Secondly, the disturbances such as high-speed rotation of the drill bit and the circulation of the drilling fluid in the coring tube generate a structural plane on the core affect the determination of structural surface closure (openness) and structural surface filling. It can be seen that using core data as a source of rock structural plane information is not accurate enough. Therefore, it is necessary to propose an in-situ measurement technique to directly measure the structural plane of the hole wall of the drilled hole and obtain the surface morphology information of structural plane on the hole wall.
Su, Fangsheng (Chinese Academy of Sciences / University of Chinese Academy of Sciences) | Gao, Yaohui (Chinese Academy of Sciences / University of Chinese Academy of Sciences) | Pan, Pengzhi (Chinese Academy of Sciences) | Qiu, Shili (Chinese Academy of Sciences)
Evolution characteristics of the irreversible strains and mechanical parameters of Jinping marble were investigated using the combination of uniaxial and triaxial compression tests as well as triaxial cyclic loading and unloading tests. Compared with the results of stress-strain curves, strength and failure modes under different conditions, it was found that the testing results were valid under cyclic loading and unloading tests. The experimental results also demonstrated that with the increase of the rock damage, the lateral irreversible deformation was always greater than the radial one but less than the volume one. The irreversible strains could be quantified by two damage stages, namely, the linear and the nonlinear rising stage. The linear increase of irreversible deformation was mainly caused by micro crack initiation and propagation, and the nonlinear increasing trend could be attributed to micro cracks through each other. The relationship between these irreversible strains and equivalent plastic shear strains was established. Moreover, the physical meanings of these fitting parameters were analyzed.
At present, with the increasing depth of mining, civil engineering and underground laboratory, excavation of deep tunnels always causes stress concentration, thus resulting in brittle fracturing of surrounding rock (Liu et al., 2016). This failure behavior is strongly influenced by the confined pressure (Qiu et al., 2010). Therefore, in order to study the engineering stability, it is meaningful to investigate the mechanical properties of hard rock.
In the last decades, a large number of researchers have studied the mechanical response of hard rock under uniaxial and triaxial loading tests. On one hand, some researchers focus on the rock damage evolution and irreversible strains characteristics using the damage-control tests. For instance, Martin and Chandler (1994) investigated damage stress of Lac du Bonnet granite and distinguished stress-strain curves based on the characteristics of crack development. Eberhardt et al. (1999) studied the failure process of granite using the normalized method. Qiu et al. (2014) paid attention to the irreversible strains in the pre-peak stage using cyclic loading and unloading tests of confined pressure. On the other hand, the variation of mechanical parameters from damage variables has also been studied by many researchers. For instance, Su and Yang (2006) summarized the deformation and strength characteristics of marble with different types of crystal particles. Zhang (2010) established the elastoplastic coupling model of marble. Zhao et al. (2010, 2014) discussed the dilatancy of rock under triaxial tests.
Han, Zengqiang (Chinese Academy of Sciences) | Wang, Chuanying (Chinese Academy of Sciences) | Wang, Yiteng (University of Chinese Academy of Sciences / University of Chinese Academy of Sciences)
In situ stress measurement is usually done based on borehole methods such as hydraulic fracturing and borehole breakout. Borehole shape changes under the action of in situ stress and similarly we can calculate in situ stress based on borehole shape analysis. In this study, borehole shape is analysed under far field stress state based on theories of elastic mechanics. We theoretically prove a circular hole under far field stress state will be elliptical in shape, and derive the equations between the elliptical parameters and the stress. Formulas for calculating ellipse shape and azimuth by using deformation data of 3 points on borehole are deduced. A new method for in situ stress measurement based on borehole shape analysis is proposed and a new contact type borehole shape measuring device is developed. Through laboratory test, this method is proved to be feasible in measuring the magnitude and direction of in-situ stress.
The in-situ stress is the natural stress which is not disturbed by engineering in the formation, also known as the initial stress of rock mass or the stress of original rock. It is the fundamental force that causes the deformation and destruction of underground or open-pit excavation projects, such as mining, hydropower, civil construction, railway, highway and military. It is a necessary prerequisite to determine the mechanical properties of the engineering rock mass and to analyze the stability of the surrounding rock. In recent 30 years, with the continuous development of in-situ stress measurement, various measuring methods and measuring instruments are developing constantly. Amadei &; Stephansson (1997) divide the commonly used in-situ stress measurement methods into borehole based methods and core based methods. Among them, the most applications in rock engineering are based on drilling method, and the typical methods are stress relief method and hydraulic fracturing method. The hydraulic fracturing method is a direct in-situ measurement method which is directly obtained by hydraulic fracturing. It is especially suitable for measuring deep in-situ stress, but there are some limitations in determining the direction of the principal stress and the requirements of the testing environment. The stress relief method is an indirect measurement method which can get the in-situ stress by measuring the displacement of the borehole wall caused by in-situ stress. The method can accurately get the magnitude and direction of the in-situ stress. But the testing process is more tedious, each step needs to be completed by the drilling rig, especially the line problem when the sensor is laid, which seriously limits the application of this method in the deep borehole. Therefore, it is an important direction for the development of the in-situ stress measuring technology to break through the deterministic problem of the principle stress direction, the limitation of the measurement depth and the complexity of the testing process, realizing the rapid and accurate in-situ stress measurement in the deep borehole with the complex environment.
During circular process of gas injection and withdrawal, the salt cavern for gas storage experience rapid temperature changes. The thermal effect coupling with the boundary conditions generates thermal stress, which induce the micro-fractures rock salt at the wall of underground cavity. Based on DEM, the Particle Flow Code is used to simulate the rock salt with interlayers. A novel hybrid DEM model is proposed, incorporating the rheological behavior of the pure rock salt, and the brittle character of interbedded mudstone. This model is capable of representing the macro-mechanical rock properties of laboratory observations. The high temperature decreases the compressive strength, makes the behavior of rock salt become more ductile instead of being brittle. The presence of interlayer induces more complex micro-cracking path, due to the heterogeneous heat transfer. Results illustrates the significant influence of temperature on the rock salt, resulting in the attenuation of the strength, induced thermal tensile cracking, and form weak zone around the interface of interlayers. The investigation of micro-mechanical response to the temperature influence can help us to predict the evolution of the damage zone around the interbedded salt cavern gas storage.
Rock salt is commonly accepted as host media for natural gas storage, as well as disposal of nuclear wastes, due to its characters, such as high solubility in pure water, very low permeability (Berest and Brouard 2003), creep behavior (Guessous et al., 1987), great potentiality of self-healing after damage (Chen et al., 2013), and relatively mechanical stability (Li et al., 2014; Zhu et al., 2016). The properties of non-halite evaporates varies different from one to another. (Jackson and Hudec, 2017), the behavior also changes when response to different temperature and confining pressure. High temperature changes the crystal structure of rock salt, which results in the variation of physical properties (Soppe et al., 1994; Cuevas 1997). Underground Gas Storage (UGS) is usually exposed to different temperature environment as the depth of its location varies. Therefore, an adequately capture and characterization of salt rock under different temperatures is essential for the design, construction, and operation of UGS.
As an alternative and promising simulation method, Discrete Element Method (DEM) can be applied to investigate the complexity of rock according to its discontinuum basis (i.e. the discrete element is independent to move from rock mass). DEM treats the rock material as an assembly comprising individual particles bonded at certain contacts modes, for simulating microscopic rock behavior including crack and deformation. Different from the conventional simulation methods, such as Finite Element Method (FEM), or Boundary Element Method (BEM), the cracks in PFC Modeling is the spontaneous consequence of breakage of bonds at which the bond strength is being exceeded by the motion of particles. The simulated fracture can be regarded as extensive microcracks to investigate its complex constitutive behavior. PFC modeling has advantage in study of micro-mechanical behavior of unconsolidated as well as complex non-elastic characters, according to direct application of Newton’s second law (Cundall and Strack 1979; Jing and Stephansson, 2007; Martinez, 2012).
Zou, Xianjian (Chinese Academy of Sciences) | Wang, Chuanying (Chinese Academy of Sciences) | Han, Zengqiang (Chinese Academy of Sciences) | Song, Huan (Wuhan Institute of Technology / University of Colorado Boulder)
Digital panoramic borehole camera technology has been widely employed in actual projects, and a large number of high-accuracy borehole camera images have been obtained. The borehole camera images accurately record the geological information, especially the feature parameters of discontinuities. However, since the acquisition of these features is usually done by hand, the workload is large and the results can be affected by human factors. To solve this problem, this paper presents an automatic interpretation method of discontinuities in borehole camera image. In this method, image gray, gradient values and projection method are employed to distinguish the occurrence region of structural planes. Then, standard sine function matching method is employed to search the discontinuities in the region. Lastly, the optimal sine curve is screened out and adopted as the feature curve of discontinuities. And the related parameters of feature curve are analyzed and converted into the parameters of discontinuities, such as the central position, orientation, dip angle and fracture width of discontinuity, which are required in engineering projects. This method can automatically identify the discontinuity in the borehole camera image continuously and quickly, and obtain the corresponding structural parameters. The method is stable and reliable, and greatly improves the working efficiency. It can realize the automatic interpretation of the discontinuities and the extraction of geometric parameters, and provide an effective and reliable solution for drilling information acquisition and borehole camera image signal processing.
Knowledge of a rock body’s structural stability is often necessary in geotechnical, geological, oil extraction, and also geological disaster control projects. The reliability of a project can heavily depend on the discontinuities found in rock bodies, such as joints, faults, weak planes, and bedding planes (Assous et al. 2013, Bae et al. 2011, Cunningham et al. 2004). A digital panoramic borehole camera system can obtain the high-definition images of boreholes that offer an accurate representation of their discontinuities characteristics. Therefore, the accurate recognition of discontinuities and rapid extraction of their parameters are highly valuable for actual drilling engineering (Deltombe &;Schepers 2004).
Present identification methods of discontinuities in borehole camera images mostly rely heavily on human reading. Generally, the structures identification and parameter extraction process may involve computer-aided pre-processing or characteristic curve fitting. Through using human-assigned control points on the discontinuities or other necessary parameters, the methods can obtain the sine curve’s parameters of discontinuities in borehole camera images (Lofi et al. 2012, Schepers et al. 2001), which is a laborious and slow process that requires human intervention. And the obtained discontinuities are often varying depending on individual readers (Han et al. 2013, Hurich &;Deemer 2013, Malone et al. 2013). This kind of manual identification process can become a drain on time and human effort in engineering practice, particularly when the boreholes are deep or numerous. Thus, the full-automatic recognition of discontinuities in borehole camera images for actual drilling engineering becomes urgent and very valuable.
Zhang, Qinghe (Chinese Academy of Sciences) | Peng, Aiwu (Chinese Academy of Sciences) | Liu, Yanjiao (Chinese Academy of Sciences / University of Chinese Academy of Sciences)
In a wave energy generation system, MPPT (maximum power point tracking) control can maximize the output power of a WEG (wave energy generator) according to the change of the waves. Due to the irregular movement of the waves and the characteristics of the LMMHD (liquid metal magneto-hydrodynamic) generator, the output of the LMMHD WEG is characterized by low voltage, high current, and irregular variation. Therefore, to withstand the large current the PCS (power conversion system) needs to be designed in parallel with multiple modules, which needs to solve the problem of current sharing. On the other hand, a control system with fast response speed is needed for the MPPT to track wave changes quickly. In this paper, a system model of the LMMHD WEG is analyzed firstly, and an MPPT control strategy that can automatically identify the sea state and can automatically realize current sharing is proposed based on the output characteristics of LMMHD WEG. Then, a MPPT control system is designed for a given LMMHD WEG, and a simulation experiment with changing sea state is carried out in MATLAB/Simulink. The comparison between the experimental results and theoretical values verifies the validity of the proposed MPPT control method.
With the increase of human activities at sea, the human demand for marine energy is also increasing. Wave energy has the characteristics of wide distribution, high energy density and large reserves, and it is one of the most promising marine energy. Wave energy generation is the main method of wave energy utilization. The WEGS (wave energy generation system) can provide continuous power supply for ocean power users such as UUV (Unmanned Underwater Vehicle) underwater charging platform, subsea scientific observation network, and ocean ranch, etc., which can solve the power problem of marine users. However, because of the characteristics of random variation of the waves, the wave energy captured by the WEG is also randomly changing. MPPT control can maximize the output power of a WEG according to the change of the waves. (Hugo and Martinez 2016; Chen, Yang and Yang 2017; Zheng, Yang, Lin, Huang and Duan 2107).
Liu, Yike (Chinese Academy of Sciences)
Imaging of multiples using reverse time migration generates inherent crosstalk artifacts due to the interference among different order of multiples. Traditionally, least-square reverse time migration of multiples (LSRTMM) has been used to address this issue by seeking the best match between the predicted and observed data. However, LSRTMM may need large number of iterations to attenuate strong coherent cross talk. Here, I present an alternative method to deal with the crosstalk noises. This approach separates a data into primaries and multiples similar to that applied in surface-related multiples elimination (SRME), and then isolates the multiples into different orders. We can take any specified, say the n-th, order of multiple data as the incident wave and the one order higher multiples, i.e. the (n+1)th order, as the corresponding reflected wavefield as input for imaging. Nevertheless, crosstalks of the same order of multiples for different reflectors still present in the image, which can be further attenuated by least-squares inversion. Here, the objective function is built to minimize the difference between a specific order of Born modeled multiples and the same order reflections in the observed data. This method is called least-squares reverse time migration of controlled order of multiples (LSRTM-CM). Numerical examples demonstrated that the LSRTM-CM can significantly improve the image quality compared to the conventional least-square reverse time migration of multiples.
Bond, Alexander (Quintessa Ltd.) | Chittenden, Neil (Quintessa Ltd.) | Fedors, Randall (United States Nuclear Regulatory Commission) | Lang, Philipp (Imperial College London) | McDermott, Christopher (University of Edinburgh) | Neretnieks, Ivars (KTH) | Pan, Peng-Zhi (Chinese Academy of Sciences) | Sembera, Jan (Technical University of Liberec) | Brusky, I. (Technical University of Liberec) | Watanabe, Norihiro (Helmholtz Centre for Environmental Research - UFZ) | Lu, R. (Helmholtz Centre for Environmental Research - UFZ) | Yasuhara, H. (Ehime University)
The evolution of fracture permeability can have important impacts for the resaturation of the facility and long-term transport of any radionuclides that escape the immediate area of disposal. Whereas such systems have been looked at both within the DEvelopment of COupled models and their VALidation against EXperiments (DECOVALEX) project and elsewhere, attempts to model a fully-coupled THMC system on a single fracture have been limited. Examples where THMC analysis in fracture rock has been addressed include Yasuhara and Elsworth (2006), Taron et al. (2009) and Zhang (2012), but with the exception of Yasuhara and Elsworth (2006), the emphasis has been largely on theoretical studies, with no direct comparison against well-constrained small-scale experimental data. There is however a large body of knowledge concerning THM behavior with non-reactive transport in fractures (e.g. Berkowitz, 2002; Neuman, 2005) and a wide range of work examining chemical interactions in fractured systems (e.g. Watson et al., 2016) but modelling efforts incorporating THM and C processes for single fractures are rare. The objective of this Task (Task C1: one of 5 Tasks in the previous phase of DECOVALEX; please see www.decovalex.org for more information on DECOVALEX including numerous examples of this type of collaborative research) is to use the experimental data of Yasuhara et al., 2006 and 2011 to model evolving single fractures incorporating coupled THMC effects for novaculite (quartzite) and granite fractures. This work is not focused on blind prediction, rather it is concerned with building experience and understanding of the physical processes in operation in single fractures on the basis of experimental data and to understand how to represent such processes through numerical and/or semi-analytical models. The Task has had significant technical contributions from six teams (abbreviations, where used, are shown in bold), as well as input from Neretnieks, 2014 and Sandia National Laboratory: - BGR/UFZ - Germany-Federal Institute for Geosciences and Natural Resources and the Helmholtz Centre for Environmental Research.
Zhang, Yuhao (Chinese Academy of Sciences) | Shi, Xilin (Chinese Academy of Sciences) | Ma, Hongling (Chinese Academy of Sciences) | Yang, Chunhe (Chinese Academy of Sciences) | Ye, Liangliang (Chinese Academy of Sciences) | Han, Yue (Chongqing University) | Zhang, Nan (Chongqing University)
ABSTRACT: Reasonable operating pressure is the key to ensure the safe operation of salt cavern gas storage. The determination of upper and lower limit operating pressure is particularly important. Take into account the characteristics of a bedded salt mine, a set of methods is proposed for determining the upper and lower limit operating pressure. Upper limit operating pressure is determined by formation pressure gradient, cavern roof depth, and cavern wall compact principles. The lower limit operating pressure is determined by the mechanical calculation results (volume shrinkage, key point displacement and plastic zone distribution) and other people's research results. The specific research object is a bedded salt gas storage cavern in China's Jintan. The upper limit operating pressure is determined as 18 MPa by the above proposed method. The simulation model is established by FLAC3D software, based on the physical mechanical experimental parameters of the cavern in the mining area and combined with the data of sonar measurement. The model is used to simulate the mechanical behavior of the cavern in low pressure operating conditions. By analyzing volume shrinkage, plastic zone volume, key point displacement and plastic zone distribution, the lower limit operating pressure is designed as 8 MPa. The research results can provide a reference for operating pressure of bedded salt cavern gas storage.
The stable supply of natural gas is important for the stable development of China's society. Increasing the proportion of natural gas in the energy structure is an important measure to improve the environment and manage urban haze from coal power plants and automobiles. Because of the geological conditions and energy reserves of China, it is urgent to build underground natural gas storage. In 1999, salt caverns were first considered for natural gas storage in China. In order to ensure that the West-East Gas Transmission Project and the Sichuan-to-East Gas Transmission Project can better address the Yangtze River Delta gas supply demands. China plans to build salt cavern gas storage in Jintan, and Yingcheng. In 2005, the first salt cavern for natural gas storage was completed and put into operation in Jintan. Compared with other countries’ deposits, the salt rock layer presents the characteristics that “the number of salt rock layers is large, the thickness of single layers is thin and the content of insoluble impurities is high“ (Yang et al., 2009). The natural gas storage design, construction, and operations in these bedded salt formations cannot simply copy the experience of storage in salt domes and thick bedded salt strata. This study on bedded salt rocks is mainly focused on the mechanical properties of layered salt rocks in China. A large number of on-site sampling tests and numerical simulations show that the interlayer is usually the first part to be impaired (fractured or sloughed) (Yin et al., 2006, Li et al., 2006). It can be seen that interlayers are important for the stability of salt cavern gas storage.
Based on remolded clay sediment obtained from a certain area in the South China Sea, a series of tri-axial unconsolidated untrained compression tests on clay sediment containing tetrahydrofuran hydrate were performed under the conditions of different hydrate saturation and confining pressure. The stress-strain curve of clay sediment containing hydrate consists of three stages: elastic (when strain is less than 1.5%), plastic (when strain is between 2% and 6%) and strain hardening (when strain is greater than 6%) stage, and very different from that of clay sediment which behaves only plastic failure. Moreover, the undrained shear strength of clay sediment containing hydrate increases about 1~7 times than that of clay sediment and decreases by 50% after hydrate dissociation.
Gas hydrates, as a potential energy resource, are usually occurring as ice-like solids and widespread in sand and clay sediments in deep marine continental margins and permafrost regions. The mechanical properties of hydrate-bearing sediments are one of the most important information and parameters for analyzing the stability of seabed and foundation during the exploration of gas hydrates. Because of the low temperature and high pressure conditions which hydrates form in natural sediments, the synthesis and tri-axial compression tests in laboratory on hydrate-bearing sediments have been more difficult to simulate in situ environment and perfectly be accomplished than clay and sand sediments. The study on mechanical property of sediments containing gas hydrate has always been a hot research topic since 2000.
Until now, most of studies on mechanical properties of hydrate-bearing sediments are focus on sand sediment containing methane hydrate and based on triaxial compression test results in laboratory. Previous researches (Winters 2007, Waite 2004, Hyodo 2007/2013, Masui 2005/2007, Miyazaki 2011, Masui 2005/2007) show that the strength of hydrate-bearing sediments depends on hydrate saturation, confining pressure, grain size distribution, density, temperature, strain rate, synthesis method and other factors. However, there are seldom researches (Yun 2007, Song 2014) on mechanical properties of clay sediment containing hydrate, and the part of reason is because gas hydrate is more difficult to be synthesized in clay sediment in laboratory. Yun compared triaxial test results of sand, silt and kaolinite containing tetrahydroguran hydrate and concluded that the stress-strain behavior of clay sediment containing hydrate is very different from sand sediment containing hydrate and strongly relative to the grain size and cementation. Therefore, the further study on shear strength and stress-strain behavior of clay sediment containing hydrate is necessary and should to be taken more attention.