Layer | Fill | Outline |
---|
Map layers
Theme | Visible | Selectable | Appearance | Zoom Range (now: 0) |
---|
Fill | Stroke |
---|---|
Collaborating Authors
ABSTRACT: Microfocus X-ray CT imaging was conducted on a granite core, 50 mm in diameter and 100 mm in length, containing a single fracture in the longitudinal direction. The three-dimensional geometry and the aperture distribution of the fracture were evaluated by analyzing the CT data. A very notable artifact called beam hardening was found in the obtained CT images because of the high X-ray absorption coefficient of the material compared with the source intensity of the X-rays. To reduce this phenomenon, two methods were applied when handling the CT data. One was to consider the slice direction when detecting the fracture route, and the other was to correct the CT data using a simulated image of the beam hardening of granite. Moreover, we developed a procedure to determine the fracture route automatically from the CT image. As a result, the spatially averaged aperture thickness and the contact ratio of the fracture asperities under unconfined conditions were estimated to be 0.39 mm and 2.0%, respectively. 1. INTRODUCTION Precise measurement of the geometric characteristics of rock fractures such as the elevation distribution of fracture walls, aperture distribution, and contacting asperities within the fracture, is essential because of their significant influence on the mechanical and hydrological behaviors of rock fracture. On a laboratory scale, the elevation distribution of fracture walls can be measured with micrometer accuracy using a system that combines a laser displacement sensor and a high-precision automatic positioning stage (e.g., [1]). However, aperture distribution and contacting conditions within the fracture are rather difficult to measure experimentally. In previous researches, various techniques have been proposed such as 1) the surface topography approach, in which the topography of a pair of fracture surfaces is measured separately by a laser beam profiler, and the aperture is computed indirectly as the distance between the two fracture surfaces [2,3]; 2) the injection approach, in which the specimen containing the fracture is cut into slices after some resin has been injected, and the aperture is measured as the thickness of the injected resin [4,5]; and 3) the casting approach, in which replicas of the fracture apertures are made by casting [6]. X-ray computed tomography (CT) is a useful technique for visualizing the inner structure of rock samples in a noninvasive and nondestructive manner. X-ray CT has been applied for visualization of the heterogeneous micro-structure of rocks and micro-crack propagation [7,8], visualization of fluid flow of sedimentary rocks [9], measurement of tracer diffusion into rock matrix [10], tracer migration in rock fractures [11], and visualization of fluid flow within deformed rocks [12]. It has also been used to measure fracture aperture and to detect contact areas (e.g., [11,13-15]). This study used a microfocus X-ray CT scanner to take CT images of a granite core containing a single fracture in the longitudinal direction. Based on the result of the CT imaging, we evaluated the three-dimensional geometry and the aperture thickness distribution of the fracture. A strong artifact called beam hardening was observed in the CT images.
- Geology > Rock Type > Igneous Rock > Granite (1.00)
- Geology > Geological Subdiscipline > Geomechanics (0.93)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Naturally-fractured reservoirs (0.69)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.54)
- Reservoir Description and Dynamics > Reservoir Characterization > Faults and fracture characterization (0.47)
Taking precise measurements of the geometric X-ray computed tomography (CT) is a useful technique characteristics of rock fractures, such as the elevation for visualizing the inner structure of rock samples in a distribution of fracture walls, the distribution of noninvasive and nondestructive manner. X-ray CT has apertures, and the contacting asperities within the been applied for the visualization of the heterogeneous fractures, is essential because of their significant micro-structure of rocks and micro-crack propagation [7, influence on the mechanical and hydrological behaviors 8], the visualization of the fluid flow of sedimentary of rock fractures. On a laboratory scale, the elevation rocks [9], the measurement of tracer diffusion into the distribution of fracture walls can be measured with rock matrix [10], the tracer migration into the rock micrometer accuracy using a system that combines a fractures [11], and the visualization of the fluid flow laser displacement sensor and a high-precision automatic within deformed rocks [12]. It has also been used to positioning stage (e.g., [1]).
- Asia > Japan (0.49)
- North America > United States > California (0.28)
- Geology > Geological Subdiscipline > Geomechanics (0.69)
- Geology > Rock Type > Igneous Rock > Granite (0.44)
Abstract In order to estimate the changes in fracture aperture under various long-term confining and thermal conditions, measurements of the fracture aperture are conducted using microfocus X-ray CT (µX-ray CT). Through the imaging data, the height of the fracture surface and the contact points are evaluated, and contact ratios for the fracture, the JRC and the aperture distribution are estimated. On the other hand, measurements are also conducted using a laser scan profile sensor, and some parameters are estimated. In comparing these parameters, the validity of the µX-ray CT data and an analysis of the data will be discussed. In addition, a fracture flow simulation will be conducted using the surface roughness and aperture data obtained by the µX-ray CT. 1. INTRODUCTION It is well known that for a rock fracture under various thermal and stress conditions, a reduction in permeability occurs due to chemical and mechanical compaction. The chemical and mechanical compaction may include a pressure solution and a reduction in permeability could be explained by this pressure solution [1]-[5]. Yasuhara, et al. [6] conducted flow-through experiments on a single fracture in granite under controlled temperature and stress conditions. Specifically, two different experiments, short-term and long-term, were performed to examine the influence of the loading time on the evolution of fracture permeability. The measured changes in fracture permeability showed reversible and irreversible behaviors under short-term and long-term conditions, respectively. These results indicated that the surface roughness and the structure of a fracture may be changed by pressure solution. However, no change in the fracture structure could be directly obtained. A change in the hydraulic conductivity was indirectly obtained. The joint surface roughness is basically measured with a non-contact laser scan profiler or a CCD camera, and then, high quality profiling data can be obtained. On the other hand, the fracture roughness and the aperture distributions of a single rock fracture under certain confining stress conditions have not been evaluated, although it is very important that a measurement system is developed to measure these parameters under confining stress conditions. Measuring the joint surface roughness and the aperture distribution is not enough to obtain the height of surface roughness, the contact conditions and the aperture distribution under confining stress conditions.
- Asia > Japan (0.30)
- North America > United States (0.28)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Igneous Rock > Granite (0.63)
Co-Registration of CT Images by SIFT Method and Observation of Temporal Changes in Granite Fracture Aperture Under Long-Term Loading
Yoshida, R. (Yamaguchi University) | Nakashima, S. (Yamaguchi University) | Yoshizu, Y. (Kansai Electric Power Co., Ltd) | Song, C. (Kyoto University) | Kishida, K. (Kyoto University)
ABSTRACT: Laboratory tests were conducted in this study in order to examine the changes in aperture of an open single fracture in a granite core over time, under confining pressures of 1-3 MPa and temperatures of 20°C and 60°C. During the experiment, the fracture aperture was monitored with X-ray CT. For a quantitative evaluation of the fracture aperture, the CT images were processed with image co-registration by the Scale-Invariant Feature Transform (SIFT) method and fracture surface extraction using the Canny edge detection algorithm. Based on the fracture surface geometry data extracted from the CT images, the aperture and contact ratio were calculated and compared. The effect of the confining pressure and temperature on the aperture and contact ratio was not clear from the results. However, the trend of the fluctuation in the aperture and contact ratio over time was found to be consistent with the trend of the fluctuation in permeability.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Igneous Rock > Granite (0.64)
ABSTRACT: This study deals with automatic image segmentation using a deep learning algorithm for X-ray CT imaging. Mudstone cores were extracted from a concrete dam foundation after curtain grouting and imaged with medical helical X-ray CT to examine the post-grouting effect. The CT images were automatically segmented into the rock part, grout part, and void part using a deep learning algorithm. The automatic segmentation method successfully identified the rock part and the grout part of the mudstone specimen with a simple planar fracture, although the range in CT values of the rock part and that of the grout part overlapped each other. It also identified the difference in cement milk mixtures. However, for the specimen having intricate fracture geometry, extracted from a sheared stratum, the narrow ungrouted cracks (less than 1.0 mm in width) were sometimes misidentified. This indicates that automatic segmentation requires more training data in the case of intricate fracture geometry. 1. Introduction X-ray CT is often utilized in geo-science fields as a means of observing geo-materials in a non-invasive manner. Through CT image processing and analysis, various types of useful information can be obtained on the internal structure of rock samples, such as the particle shape, void structure, fracture geometry, mineral composition, and so on. Researchers have visualized the heterogeneous micro-structure of rocks and micro-crack propagation (Verhelst et al. 1995, Sugawara 1997), the fluid flow of sedimentary rocks (Sato et al. 2002), the tracer diffusion and migration into the rock matrix and fractures (Nakashima et al. 2004; Sato et al. 2007), and the fluid flow within deformed rock (Hirono et al. 2003). X-ray CT has also been used to measure fracture apertures and to detect contact areas (Sato et al. 2003, 2007; Yoshino et al. 2003; Nakashima et al. 2010, 2017; Yoshida et al. 2018). In a CT image analysis, one of the most basic but challenging processes is material recognition and segmentation based on the voxel CT values. To treat thousands of slices of CT images, some automatic algorithm for image segmentation is needed.
- Asia > Japan (0.49)
- North America > United States > Texas (0.28)