Naka, Ryosuke (Hokkaido University) | Tatekawa, Takuto (Hokkaido University) | Kodama, Jun-ichi (Hokkaido University) | Sugawara, Takayuki (Hokkaido University) | Itakura, Ken-ichi (Muroran Institute of Technology) | Hamanaka, Akihiro (Kyushu University) | Deguchi, Gota (NPO Underground Resources Innovation Network)
Underground Coal Gasification is expected to be efficient technique for coal energy recovery from deep or complex coal seam since directional drilling technique is advancing in these days. Authors have been performing small-scale UCG model tests to clear gasification and combustion process in UCG. Then, we found that radial cracks were initiated from the cavity formed in the artificial coal seam. Understanding mechanism of the crack initiation is important for clarification of the detail process of combustion and gasification and assessment for environmental risks. In this study, thermal stress analysis was performed on the small-scale UCG model tests to consider the initiation mechanism of the cracks by assuming that combustion and gasification of coal were progressing through the following three processes which are often observed in coal carbonization: (A) thermal expansion, (B) softening and melting and (C) thermal contraction. It was found that tensile stress was induced in the vicinity of the cavity in the tangential direction in process C. Direction of principal stress in the coal was almost parallel to tangential or radial direction of the cavity and the magnitude of it exceeded coal tensile strength. It was also found that tensile stress zone was extended into deeper coal seam with increase in temperature and time and compressive stress zone was formed outside of the tensile stress zone. It can be considered that the radial cracks initiated at the surface of the cavity since tangential tensile stress exceeded tensile strength of coal. Then, radial cracks were arrested at the boundary of tensile stress zone and compressive stress zone after they were propagating in coal seam.
Underground Coal Gasification (UCG) is a technique to use coal energy more efficiently and cheaply. In UCG, oxidant is injected into underground through an injection well to gasify coal seam, and syngas is recovered from a production well (Fig. 1). It is expected that UCG increases available amount of coal energy because even low-grade, complex and deep coal can be used by UCG.
It is pointed out that UCG has risks of surface subsidence and groundwater pollution because cracks are likely to initiate in coal seam by combustion and gasification. Therefore, clarification of initiation and growth mechanisms of the cracks is significant for stability assessment of ground as well as assessing environmental risks.
We performed small-scale UCG model tests on massive coal and crushed coal samples to clear gasification and combustion process in UCG. It was found that radical cracks were initiated in an artificial coal seam made by massive coal as well as crushed coal (Fig. 2 (Kodama et al., 2016)). Similar radial cracks were also observed in large-scale UCG model test (NPO Underground Resources Innovation Network, 2016).
Hamanaka, Akihiro (Kyushu University) | Itakura, Ken-ichi (Muroran Institute of Technology) | Su, Fa-qiang (Henan Polytechnic University) | Deguchi, Gota (Underground Resources Innovation Network) | Kodama, Jun-ichi (Hokkaido University)
Underground coal gasification (UCG) is a process of producing combustible gases by the in-situ conversion of coal into gaseous products. Coal resources abandoned under the ground for either technical or economic reasons can be recovered with economically and less environmental impacts by UCG; therefore, this technology is regarded as a clean coal technology. UCG has several advantages of low investments, high efficiency, and high benefits compared to conventional coal gasification. However, some environmental risks such as gas leakage, surface subsidence, and underground water pollution are difficult to control because the process is invisible. The reactor in UCG is unstable and expands continuously due to fracturing activity caused by coal combustion. It is, therefore, considered that acoustic emission (AE) is an effective tool to monitor the fracturing activities and visualize the inner part of coal. For this study, UCG model experiments were conducted using coal blocks of 0.55 × 0.60 × 2.74 m to discuss the applicability of AE monitoring for the estimation of the crack generations during UCG process and the extent of the gasification area. Temperatures were also monitored because the crack generations were strongly related to thermal stress occurred by coal combustion and heat transfer. The monitoring results of AE agreed with the measured data of temperatures and the gasification area; the source location of AE was detected around the region temperature increased and the gasification area. Additionally, the gasified coal amount can be predicted by using the data of product gas. Therefore, AE monitoring combined with the prediction of reacted coal amount are expected to be a useful tool as monitoring system of the gasifier in the underground.
Underground coal gasification (UCG) is a technique to extract energy from coal in the form of heat energy and combustible gases through the chemical reactions in the underground gasifier. This technique enables to utilize coal resources that remain unrecoverable in underground due to either technological or economic reasons. Most coal mining in Japan was closed by 2001 because of complicated geological conditions for mining development and high prices of domestic coal. However, abundant unused coal resources remain underground, but they are not recoverable because of technical and economic reasons. Such coal resources are estimated to be 30 billion tons. For that reason, UCG has a great potential to recover vast amounts of energy from these coal resources.
Nakoshi, Takao (Hokkaido Road Management Engineering Center) | Kimura, Katsutoshi (Muroran Institute of Technology) | Kamikubo, Katsumi (Civil Engineering Research Institute for Cold Region Public Works Research Institute) | Ochi, Masashi (Nippon Data Service Co., Ltd.)
Wave overtopping often caused traffic hindrances on Route 231 on the coast of the Hamamasu District, Ishikari City. To ensure safe traffic, a wave splash barrier behind the wave-dissipating seawall was designed. The splash height of overtopping waves was measured and the necessary wave splash barrier height was designed. Wave force distribution on the wave splash barrier was also measured via hydraulic model tests. Storm waves equivalent to the design waves hit the site on October 2015, but the wave splash barrier was undamaged, indicating applicability of the design method to the site.
In Japan, necessary traffic routes have been established by constructing roads on limited land areas along the coastline. With the progress of shoreline recession in recent years, sections affected by high waves have increased. Although it is necessary to elevate seawall crests and establish wave-dissipating blocks in such sections, it is often difficult due to environmental or financial reasons. Wave splash barriers are effective in such cases and have already been constructed in different areas. Although Kimura, et al. (1998, 2001, 2003, 2006), Kamikubo, et al. (2009, 2011), and Yamamoto, et al. (2008) have conducted studies of actual sites, no commonly used design methods have been established.
Traffic hindrances due to overtopping waves occurred frequently on Route 231, which is a two-lane road along the shoreline of the Hamamasu coast of Ishikari City. Therefore, a wave splash barrier was constructed in November 2013. Kamikubo, et al. (2015) have presented an overview of measures against overtopping waves on the road.
This paper presents the background of the construction of a wave splash barrier on the road and analyzes the wave overtopping status when the site was hit by high waves after the construction.
FIELD OBSERVATION OF WAVE OVERTOPPING
The road is a 1.3-km-long straight road along the shoreline as shown in Fig. 1. The water depth at the front of the seawall h = +1.1 to −3.1 m, and the water is deeper on the left side of the figure. Although wavedissipating blocks are placed on the entire section, the crest width varies greatly by location.
Hamanaka, A. (Kyushu University, Fukuoka) | Itakura, K. (Muroran Institute of Technology) | Su, F. Q. (Muroran Institute of Technology and Henan Polytechnic University) | Deguchi, G. (Underground Resources Innovation Networks, NPO, Sapporo) | Kodama, J. (Hokkaido University, Sapporo)
Underground coal gasification (UCG) is a process of producing combustible gases by the in-situ conversion of coal into gaseous products. Coal resources abandoned under the ground for either technical or economic reasons can be recovered with economically and less environmental impacts by UCG; therefore, this technology is regarded as a clean coal technology. UCG has several advantages of low investments, high efficiency, and high benefits compared to conventional coal gasification. However, some environmental risks such as gas leakage, surface subsidence, and underground water pollution are difficult to control because the process is invisible. The reactor in UCG is unstable and expands continuously due to fracturing activity caused by coal combustion. It is, therefore, considered that acoustic emission (AE) is an effective tool to monitor the fracturing activities and visualize the inner part of coal. For this study, UCG model experiments were conducted using coal blocks of 0.55 × 0.60 × 2.74 m to discuss the applicability of AE monitoring for the estimation of the crack generations during UCG process and the extent of the gasification area. Temperatures were also monitored to understand the inner part of coal blocks because the crack generations were strongly related to thermal stress occurred by coal combustion and heat transfer. The monitoring results of AE agree with the measured data of temperatures; the source location of AE was detected around the region temperature increased. AE monitoring are expected to provide a useful data to visualize the gasifier in the underground.
Underground coal gasification (UCG) is a technique to extract energy from coal in the form of heat energy and combustible gases through the chemical reactions in the underground gasifier. This technique enables to utilize coal resources that remain unrecoverable in underground due to either technological or economic reasons. Gasification reaction in UCG process is promoted by enlargement of the oxidation surface around the gasification channel with crack initiation and development inside the coal seam. Fracturing activities inside the coal seam are accelerated with an increase of thermal stress caused by exothermic reactions and heat transfer, as a result, gasification reaction and cavity growth is promoted. Cavity growth influences gasification efficiency because it is directly proportional to the coal consumption. At the same time, some of environmental issues have to be cared such as gas leakage, groundwater pollution, and surface subsidence associated with the cavity growth (Bhutto et al., 2013; Imran et al., 2014; Kapusta and Stańczyk, 2011; Kapusta et al., 2013, Shu-qin et al., 2007). Therefore, techniques to evaluate the fracture activity around the gasification area have to be developed for precise control of coal gasification in-situ and minimizing environmental impacts.
The geometric parameters of fractures such as orientation, frequency, size, and shape are used in the modeling of a fractured rock mass. One investigation to acquire geological data for setting these parameters is the geological observation of a gallery wall performed based on geologists’ traditional techniques. However, it is necessary to reduce the work volume associated with geological observations of gallery walls because numerous geological observations must be conducted during construction of an underground facility for radioactive waste disposal. To reduce the differences in data quality attributable to geologists’ individual skills, it is also necessary to provide a method to acquire objective data that are unaffected by a geologist’s subjectivity. Three-dimensional laser scanning (3DLS), which can acquire point clouds indicating the gallery wall surface shape, is useful as a means for achieving these goals. To develop methods of geological observation applying 3DLS, acquisition of geometric data of fractures distributed on the gallery wall has been conducted using 3DLS data while clarifying the geological observation data necessary for modeling of a fractured rock mass. The acquired geometric data of fractures were compared with data acquired by a geologist. Consequently, the fractures were extracted by visible reading of images generated from 3DLS data, and a trace map was generated as geometric data. Strike/dip data of fractures were also acquired. The results confirmed that the geometric data of fractures derived from 3DLS data reached levels comparable to data acquired by a geologist.
An examination of rock-bolt hole drilling at a tunnel face showed that drilling efficiency is greatly influenced by the geological conditions of the surrounding rock mass. Therefore, by assessing the geological conditions around a tunnel, the drilling efficiency could be estimated quantitatively. Accordingly, we have developed a geological drill survey and 3-dimensional visualization system (GENESIS/GSV), which gathers data related to drilling works, processes it and visualizes the 3-dimensional geological structure. This paper outlines GENESIS/GVS and its practical use, and shows examples of identifying the geological structure based on daily drilling work using the system. The validity of using the system for tunneling is also discussed.. Keywords: ARMS8, Rock Mechanics, Sapporo, Slope Stability, Template 1. Introduction During tunnel construction, a drill log survey using a hydraulic rock drill is often conducted in order to investigate the geological conditions ahead of the tunnel face.
Su, F. (Muroran Institute of Technology) | Kitagawa, M. (Muroran Institute of Technology) | Itakura, K. (Muroran Institute of Technology) | Deguchi, G. (NPO Sapporo) | Ohga, K. (Hokkaido University) | Kodama, J. (Hokkaido University)
During UCG operations, evaluation of the coal combustion cavity growth and precise control of the reactor are important to ensure effective combustion and efficient gasification. Enlargement of the oxidation surface around the gasification channel with crack initiation and development inside the coal seam directly influence the gasification efficiency.
To investigate the distribution and extent of fracture activity, and to evaluate the propagation of the combustion area in the UCG model experiments and small-scale field tests, we used acoustic emissions (AE) monitoring and temperature measurements. In this work, the AE signals from sensors CH1–CH6/CH8 were monitored and stored: the arrival time, number of events, and peak amplitude. The AE source locations and moment tensor analysis were investigated to establish a crack distribution model. With the monitoring results of temperature changes and AE activity, results showed coal fracture generation and extension in the gasifier. Results also showed that many AE events occurred during coal combustion. The AE activity was related closely to the temperature change occurring inside the coal. This AE generation apparently results from crack initiation and extension around the coal combustion area, which occur because of thermal stress. Moreover, coal consumption was investigated by calculating the cavity volumes formed in the gasifiers, and by estimating the reaction process of coal gasification based on the stoichiometry using product gas compositions. Results from the estimated values of coal consumption support experimental observations for the rational error range (about 15%) and provide a better evaluation of UCG effects.
Results show that the local temperature change strongly affects AE activities when fracture occurs inside the coal. A crack distribution model constructed with AE data is useful to evaluate the gasification process. It provides necessary data and parameters for UCG simulation development. The AE techniques are reliable for application to the real-time process control in UCG trials.
Itakura, K. (Muroran Institute of Technology)
Recently, the development of digital measurement technology has enabled the instantaneous acquisition of numerous data. Image data of digital cameras and time series data recorded over a long period by high-speed A/D converters are some examples.
For this study, Primitive Pattern Decomposition (PPD) was developed to extract arbitrary data patterns easily and quickly from a large digital data sequence. The primitives use nine patterns. PPD with the primitives is applied for the detection of crack locations from torque logging data obtained during drilling. In a rotational drilling machine, the torque directly reflects the drilling energy. It is also sensitive to discontinuities such as layer boundaries, layer separations, and cracks in the rock mass. Torque logging data are obtained from a roof bolter with thrust, rotation, and stroke during drilling for the roof rock of the roadway in underground coal mines. Results show that the crack distribution along the drill hole is obtainable by application of PPD to the torque data. Moreover, by compiling the crack distributions of roof drillings in the same area, a 3-D roof rock geostructure can be reconstructed. Comparing core sample inspection and drill hole observation using an optical fiber scope to results estimated using this system, the accuracy of discontinuity detection is probably less than 45 mm, with a fitting ratio higher than 60%.
Results of these experiments demonstrated that the PPD method can easily and flexibly produce a pattern to be extracted. It is possible to extract it rapidly from original signals. Furthermore, PPD is useful to obtain a rough approximation of complex signals and for the rough estimation of signal characteristics as a type of spectrum analysis.
Kodama, J. (Hokkaido University) | Nakaya, M. (Hazama-ando Co. Ltd.) | Nara, Y. (Tottori University) | Goto, T. (Muroran Institute of Technology) | Fukuda, D. (Hokkaido University) | Fujii, Y. (Hokkaido University) | Kaneko, K. (Horonobe Institute for the Subsurface Environment Technology)
In this study, cyclic freeze-thaw tests were carried out on rock samples in order to characterize fracture propagation by freeze-thaw action. The fracture process was characterized by macroscopic observation by digital camera and changes in water absorption. Fracture initiation and propagation inside of the specimen were also observed using X-ray CT scanner. It was found that the fracture process depends on the rock type. For rocks which possess preferred orientation of preexisting fractures and remarkable anisotropy in P-wave velocity, only one fracture propagated dominantly and split the rock specimen in half. Gradual increase in aperture of the fracture was observed not only on surfaces but also inside of the specimen. On the other hand, for isotropic rock, propagation and branching of several fractures occurred and rock samples were split into several parts.
The effects of loading rate on the strength of frozen rock as well as the failure process at sub-zero temperatures were investigated. There was a marked difference observed in the fracture initiation stress and rock strength between frozen dry rock samples and frozen rock that contained water. There was no significant change observed in strength parameters of dry rock whereas in frozen rock containing water, rock strength increased with loading rate. Interestingly the magnitude of the change in strength was much greater for tensile strength than compressive strength of wet rock. It is postulated that these changes in mechanical properties may be explained in part by a reduction in the stress concentration within the interstitial spaces and cracks of the rock.