Boeut, Sophea (Hokkaido University) | Oshima, Teppei (Hokkaido University) | Fujii, Yoshiaki (Hokkaido University) | Kodama, Jun-ichi (Hokkaido University) | Fukuda, Daisuke (Hokkaido University) | Matsumoto, Hiroyuki (Kushiro Coal Mine) | Uchida, Kazumi (Kushiro Coal Mine)
There are several case studies in which vibration produced by earthquakes or underground explosion affected the level and temperature of ground water, petroleum production, etc. These might be due to the change in permeability by transient stress disturbances creating new pathways, clearing particles clogging the pore spaces of the existing pathways. This paper investigated on the permeability change of intact and triaxially fractured Kushiro Cretaceous sandstone by transient axial and pore pressure disturbances. In the room temperature, the specimens dimensioned 30 mm in diameter and 60 mm in length were triaxailly compressed under 1 MPa of pore water pressure, and 3-15 MPa of confining pressures. The permeability was measured before (kI: intact rock) and after (kDI: disturbed intact rock) transient stress disturbances for pre-failure rock and before (kP: post-failure rock) and after (kDP: disturbed post-failure rock) transient stress disturbances for post-failure rock.
Under 0-11 MPa of the axial stress disturbance amplitudes, the permeability of the specimens decreased from kI to kDI and from kP to kDP due to the disturbance, yet it increased from kDI to kP resulted in rock failure. The permeability of pre-failure rock decreased larger with confining pressure and time; however, the decrease amount was almost constant by the disturbance amplitudes. For post-failure, the decrease amount of permeability became higher with the axial stress disturbances. This clarifies that the permeability of fractured Kushiro Cretaceous sandstone decreased by axial stress disturbance.
Under pore pressure disturbance amplitudes of 0.2 to 1.8 MPa, in the pre-failure regime, the permeability decreased at the lower disturbance amplitudes, but increased at higher disturbance amplitude. The permeability continued to increase by rock failure, though, in the post-failures, the permeability decreased by the pore pressure disturbances. The increase might be utilized for the enhanced methane gas recovery.
Transient stress disturbances from either earthquakes or the underground explosion may induce change in the underground properties. The seismic waves, for instance, resulted in the change in level, (Beresnev &; Johnson, 1994; Elkhoury et al., 2006; Wang &; Manga, 2009; Manga et al., 2012) and the temperature (Mogi et al., 1989) of the ground water or the petroleum production (Beresnev &; Johnson, 1994; Pride et al., 2008; Roegiers, 2016). The Union of Soviet Socialist Republic, the US, etc., did the underground nuclear explosion tests between the 1950s and 1960s; and these underground explosion tests may have reduced the number of earthquakes more than M8.0 (Fujii et al., 2017). These occurrences might be due to the change in permeability of rock mass by the transient stress disturbances creating microcracks, clearing the barrier particles clogged the pore spaces of the pathways, etc. (Manga et al., 2012). This paper focused on the change of permeability of intact and triaxially fractured Kushiro Cretaceous sandstone. Increase in permeability, if it is confirmed, may encourage its utilization to enhance gas recovery, to prevent future large earthquakes by inducing small earthquakes, and to de-route underground water flow for various purposes.
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).
Fukuda, D. (Hokkaido University / University of Tasmania) | Liu, H. (University of Tasmania) | Mohammadnejad, M. (University of Tasmania) | Chan, A. (University of Tasmania) | Cho, S. H. (Chonbuk National University) | Min, G. J. (Chonbuk National University) | Kodama, J. (Hokkaido University) | Yoshiaki, F. (Hokkaido University)
This paper introduces the Y-HFDEM code based on two-dimensional combined finite-discrete-element method (FEM/DEM) for numerical simulation of fracturing process in brittle and semi-brittle materials including rocks. The code has been successfully employed to simulate rock breakage under both quasi-static (e.g. uniaxial compression) and dynamic (e.g. rock blasting) loading conditions. However, the most challenging part in the application of the original Y-HFDEM code was its simulation time required to solve large-scale problems with massive number of nodes, elements and contact interactions. To overcome this limitation, this paper demonstrates the application of GPGPU (General Purpose Graphic Processing Unit) and CUDA (Compute Unified Device Architecture) C/C++ to parallelize the original sequential 2-D Y-HFDEM code along with related numerical algorithms using the GPGPU. Obtained results from verification examples demonstrate the capability of the proposed Y-HFDEM code in modelling larger scale problems in which massive computational effort is required.
Understanding the mechanism of the fracture process in rocks is important in the field of civil and mining engineering. Numerical methods have been increasingly applied recently to analyze the fracture process of rocks. For a realistic simulation of the fracture process of rock, numerical techniques must be capable of capturing crack onset and arbitrary crack growth, correct crack length within a given time interval as well as the propagating directions. In recent years, increasing attention has been paid on the techniques which bring together the advantages of the continuum-based and discontinuum-based computational methods. The combined finite-discrete element method (FEM/DEM) proposed by Munjiza (2004) has been employed successfully to model problems dealing with transition process from continuum to discontinuum such as rock fracturing and fragmentation (Mohammadnejad et al., 2018). ELFEN(2D/3D) (Rockfield, 2005) and Y(2D/3D) code (Munjiza, 2004) are two main implementations of the combined FEM/DEM. Several attempts have been made to extend the Y code such as Y-GEO(Mahabadi et al., 2012), IRAZU(Mahabadi et al., 2016), SOLIDITY (Xiang et al., 2016), HOSS with MUNROU (Rougier et al., 2014) and authors’ Y-hFdEM (Liu et al., 2015). The principles of the combined FEM/DEM are based on both continuum mechanics, cohesive zone modelling and contact mechanics which make it computationally expensive. Therefore, developing a capable parallel computation schemes is important in order to deal with larger scale problems with massive number of nodes, elements and contact interactions.
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.
Kim, Hyunjong (Pukyong National University) | Parthasarathy, Nanjundan (Pukyong National University) | Li, Kui Ming (Korea Marine Equipment Research Institute) | Choi, Yoon Hwan (Pukyong National University) | Oshima, Nobuyuki (Hokkaido University) | Lee, Yeon Won (Pukyong National University)
In order to design a wave energy generating system of a floating type, a 6– Degrees Of Freedom (DOF) motion technique was applied to Computational Fluid Dynamics (CFD) analysis on this floating body and the behavior is interpreted according to the nature of the incoming waves. In the numerical study, the tuning factor concept is employed, as it explains the relation between the maximum pitch angle and the length of the floating body and wavelength. The relation between tuning factor & pitch period for the generated waves is compared to analyze the effects of wave energy absorption. Linear waves are generated in a tank model using a single floater. In this paper, we focus on wavelength of a floater and 14 cases have been modeled for the same. It was found that the wave energy absorption varies in different parameters of wavelengths and also the different locations of the floating body in a wave affect the wave energy absorption, thus necessitating the importance of the location of a floater in a wave and its effect on wave energy absorption.
Ocean waves are powerful. It has an enormous amount of untapped energy left in it. This energy is practically exploited by means of a Wave Energy Converter (WEC). A WEC is a system; where in the physical movement of the device is converted into electrical energy (Nanjundan Parathasarathy et al. 2012, A Falcao 2010, B. Drew et al. 2009). WECs can be classified into four main types as they are designed according to the nature of the incoming waves. They are the Oscillating Water Column (OWC) WECs, point absorber WECs, overtopping WECs and attenuator type WECs. Among them, the attenuator type, whose motion is parallel to the wave direction, is a notable one. The Pelamis WEC is one of such devices. It is a long elongated body of cylindrical sections joined by hinges, which, when in motion with the waves drives a hydraulic motor. This motor in turn drives the electrical generators, thus producing electricity (A. Al-Habaibeh et al. 2010). Different from other WEC converters, the Pelamis is designed to collect the wave kinetic energy horizontally (sidewise) and vertically (up and down). To design a large-scale system like the Pelamis type in ocean, we need to study the motion of a floater, which is effected by the wave characteristics.
Changes at oil/brine and sandstone/brine interfaces can improve oil recovery in sandstone reservoirs by low salinity waterflooding (LSWF). There have been several attempts to understand the interfaces through both experiments and modelling, but the results are inconsistent and the existing models did not consider the important mechanisms on prediction. In this study, an integrated model incorporating surface complexation reaction, dissolution/precipitation of minerals, and speciation was developed to simulate LSWF in sandstone reservoirs. A separate surface complexation model was developed for oil-brine and kaolinite-brine interfaces. The effect of pH and divalent ions of calcium and magnesium concentration on surface species of oil and kaolinite is analyzed. These surface complexation models were combined and coupled with phase-equilibrium model to predict LSWF in sandstone reservoir. Deprotonation of Al and Si sites (>Al: SiO−) of kaolinite and both deprotonated carboxylic groups (−COO−) and calcium-adsorbed (−COOCa+) dominates the surfaces at various brines and its diluted composition. The pH rise in the formation water increases the dissociation of both oil and kaolinite and hence high repulsion between the surface. The desorption of oil was calculated from the surface species concentration of both oil and kaolinite. The simulation results on desorption of oil increases with dilution times (up to 20 times) and amount of equilibrated brine of seawater, formation water, 20 mmol/l CaCl2, and 20 mmol/l MgCl2. Consideration of phase-equilibrium significantly impact on the simulation results; the dissolution and precipitation of sandstone minerals increase the desorption of oil by few percentages, 6-10% in seawater flooding. Increased oil recovery in LSWF is strongly depend on the aqueous chemistry of injected solution and surface electrical properties, such as surface site density and equilibrium constants for ionization and ionic adsorption of oil and kaolinite. The simulation results of this study emphasize the importance of surface chemistry of oil and kaolinite as well as phase equilibrium reactions on improved oil recovery in LSWF. The key factors affecting electrostatics of oil and kaolinite and then oil recovery was identified and analyzed. The potential improvement of the model for better prediction is discussed. The proposed model could serve as a useful tool to predict oil recovery during LSWF in sandstone reservoirs.
Yamauchi, Masaki (Osaka University) | Hata, Yoshiya (Osaka University) | Murata, Akira (Kanazawa University) | Kuwata, Yasuko (Kobe University) | Koyama, Maki (Gifu University) | Nakashima, Tadayoshi (Hokkaido University) | Miyajima, Masakatsu (Kanazawa University)
During a scenario earthquake along the coast of Sea of Japan, generation of not only strong motion but also huge tsunami is predicted in Hegura Island, Wajima City, Ishikawa Prefecture, Japan. In this study, we estimated the difficult time for tsunami evacuation in Hegura Island during a future earthquake along the coast of Sea of Japan in order to evaluate the difficult area for tsunami evacuation. Then, we predicted the strong ground motion at 105 sites based on the characterized source model considering the empirical local site effects.
Serious human damages due to the 1993 Southwest Hokkaido Earthquake and the 2011 off the Pacific coast of Tohoku Earthquake motivated us to evaluate the effect of strong ground motion on tsunami evacuation at the same time. Regarding this motivation, Minato et al. (2015) performed the strong ground motion in the predicted tsunami attack area in Kushimoto Town centre, Wakayama Prefecture, Japan. Hata et al. (2015a) also carried out the hybrid evaluation between the strong motion prediction and the tsunami evacuation during a large-scale earthquake along the Nankai Trough. However, the hybrid evaluation focused on a scenario earthquake along the coast of Sea of Japan has not been carried out. In the coast of Sea of Japan, the serious human damages were occurred by the historical large scale earthquakes such as the 1993 main shock and the 1983 Mid Sea of Japan Earthquake. Thus, it is very important to evaluate difficult area for tsunami evacuation during a future large scale earthquake along the Sea of Japan. Actually, a serious damage due to the 1983 main shock was also reported in Hegura Island, Wajima City, Ishikawa Prefecture, Japan. In addition, the serious damage with an overall of Hegura Island during a future scenario Northern Noto Peninsula Earthquake was also assumed by Ishikawa Prefecture (2012).
ABSTRACT: One of the major challenges facing coal mines in Malawi is occurrence of hanging roof falls during the rainy season. Following this, the effect of humidity on the tensile strength of coal bearing rocks, arkose sandstone and fine-grained sandstone from Mchenga underground coal mine, was studied aiming to clarify the mechanism of roof fall in the underground mines and proposing countermeasures against the roof falls. Rock samples from the mine were treated in varying humidity levels. Some specimens were oven dried at 80°C and a desiccant was used to attain low humidity. Moderate humidity was obtained using magnesium-nitrate-hexahydrate, high humidity was achieved by pure water and some specimens were vacuum saturated. Subsequently, Brazilian test was carried out to obtain indirect tensile strength along the sedimentary plane. For comparison, Neogene tuffaceous Kimachi sandstone from Japan was also tested, following the same conditions. It was confirmed that indirect tensile strength of the rocks decreased with increase in humidity. The sensitivity of indirect tensile strength to humidity for arkose and fine grained sandstones was 12% or 29% larger than that for Kimachi sandstone. The stronger sensitivity for Mchenga roof rocks would be because the rocks contain illite, whose strength is very sensitive to water, while Kimachi sandstone contains only less sensitive zeolite for clay minerals.
Historically, mining has been imperative to human and social development, and the industry will continue to make investments that meet the increasing needs of society (ICMM, 2012). Moving from being an agrobased economy, Malawi has ventured into mining as its alternative economic driver towards the future. Coal offers an alternative for electrical energy in the country due to insufficient power generation from hydroelectric power stations along the Shire River in Malawi. This has put coal as the strategic commodity for the mining sector, focusing on the establishment of coal-fired power plants in the country. There are seven coalfields spread across the country, with four coalfields of bituminous coal in the Northern Region. Currently, four underground coal mining operations are running with additional two open pit mines in these coalfields.
ABSTRACT: This study introduces a method for determining Mode II dynamic fracture toughness using short-core-in-compression(SCC) rock specimens. The SCC specimens which have two notches are loaded dynamically by split Hopkinson pressure bar apparatus. A pulse shaping technique with copper disc pulse shaper was used to achieve the stress equilibrium state through the specimen before failure initiating. A High speed digital camera was used to observe the occurrence of shear cracks between the notch tips. The dynamic loading tests of SCC specimens were performed in the range of 202GPa/s ~ 343GPa/s and obtained Mode II dynamic fracture toughness was between 6.26MPa m1/2 ~ 7.51MPa m1/2. The Mode II dynamic fracture toughness showed 2.7~ 3.1 times of Mode II static fracture toughness in this study.
Theory of rock fracture mechanics has been applied to solve many rock engineering problems such as rock drilling, rock excavation, rock blasting and hydraulic fracturing. It is well known that rock materials contains many pre-existing microcracks which may cause complex fracturing processes depending on environmental conditions. When a pre-existing crack is subjected to an externally applied loading, stresses concentrate around the crack tip and accelerates the crack growth., to understand the stresses level around crack tip, the evaluation of stress intensity factor is of significant importance. Fracture toughness is a critical value of stress intensity factor which indicates the level of stress required for the pre-existing crack to propagate under a given crack arrangement. Three crack tip deformation modes are generally possible in fracture process, i.e. Mode I (crack opening), Mode II (crack sliding) and Mode III (crack tearing). In rock engineering problems, Mode I and Mode II or mixed mode I-II is very important and thus various researches have been conducted in the past [1-5] to evaluate the fracture toughness for these fracturing modes. However, according to the ISRM suggested methods, there is only one experimental method to determine Mode II fracture toughness while four methods have been already suggested for Mode I . In addition, experimental studies about Mode II fracture toughness of rocks are mainly for the case of quasi-static loading condition, while many rock engineering applications including rock drilling and rock blasting are carried out under dynamic loading condition. Because rocks show strong loading rate/strain rate dependency [e.g., 4,7], understanding the Mode II dynamic fracture toughness is also of significant importance.
Alam, A. K. M. Badrul (Horonobe Research Institute for the Subsurface Environment, NOASTEC) | Aramaki, Noritaka (Hokkaido University) | Tamamura, Shuji (Horonobe Research Institute for the Subsurface Environment, NOASTEC) | Ueno, Akio (Horonobe Research Institute for the Subsurface Environment, NOASTEC) | Murakami, Takuma (Horonobe Research Institute for the Subsurface Environment, NOASTEC) | Fujii, Yoshiaki (Hokkaido University) | Kaneko, Katsuhiko (Horonobe Research Institute for the Subsurface Environment, NOASTEC)
Oxidation of lignite with H2O2 solution to produce dissolved organic carbon (DOC) for generating biomethane by methanogen cultivation is an important stage of subsurface cultivation and gasification methods. To obtain more insights into this process, changes of lignite mechanical properties were investigated after its oxidation to produce DOC. Core specimens 30 mm in diameter and 60 mm in height were immersed into 1 wt.% H2O2 to achieve a liquid-to-solid ratio of 5:1. Values of pH and Eh were measured at arbitrary time intervals together with concentrations of H2O2 and DOC. P-wave velocity and density were measured before and after immersion. A series of uniaxial compression tests was carried out for both chemically reacted (H2O2-immersed) and non-reacted (H2O-immersed) specimens. The concentration of DOC, which is the substrate of methanogen cultivation, increased due to the oxidation of lignite at decreased pH by H2O2. P-wave velocity showed positive correlations with strength, static tangent modulus, and dynamic Young’s modulus. The average P-wave velocity (Vp) decreased by about 1.5% from its initial value due to the average density decrease of 0.6% resulting from the above chemical reaction. This decrease was associated with microcracking caused by swelling and grain boundary change due to leaching. The influence of the above chemical reaction on the mechanical properties of lignite is small, despite the formation of DOC to produce biogenic methane.
Lignite seams of the Tenpoku coal field in Hokkaido, Japan, are considered to be used for biomethane production by subsurface cultivation and gasification (SCG, Aramaki et al., 2015; Tamamura et al., 2016), with the formation of dissolved organic carbon (DOC) from lignite by induced oxidation using hydrogen peroxide (H2O2) being the first stage of this method. Subsequent stages feature methanogen cultivation to produce biomethane, using the DOC as a substrate, and the last stage corresponds to gas recovery. The formation of DOC for methanogen cultivation via chemical reactions can change the mechanical properties of lignite, which is the subject of this research.