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).
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.
A study was undertaken to investigate changes in the strength and failure pattern of otherwise standard rock core specimens containing regularly spaced discontinuities. The objective was to assess changes in the nature of rock failure resulting from these discontinuities. Test core specimens were prepared from sandstone having dimensions of 44 and 110 mm for diameter and height respectively. The core specimens were cut at right angles to the longitudinal axis of the core producing composite specimens having 2, 3, 4 or 5 regularly-spaced surfaces. As well as testing specimens having dry, clean fracture surfaces, the test program also considered different infill materials and hence friction values using dry and oiled-impregnated paper. The study found the strength of specimens decreased with the number of discontinuities. While the UCS strength of the intact rock specimen was 35.8 MPa, the strength of composite specimens having five segments was reduced by 50% to 17.9 MPa. This is despite the reduction in the slenderness ratio of each segment in the composite specimen that would usually result in an increase in strength. Less sensitive were changes in infill material having little discernible effect over for the limited range of friction surfaces investigated.