Figure 3 - uploaded by Hirofumi Muraoka
Content may be subject to copyright.
Source publication
Three-dimensional imaging of the Kakkonda granite from the borehole WD-1a was performed to clarify meso-and microscopic structures of pores and fractures in the geothermal reservoir by medical and industrial X-ray computerized tomography (CT) scanners. The granite was recovered from WD-1a drilled at the Kakkonda geothermal field, NE Japan, by the N...
Context in source publication
Context 1
... third-generation Hitachi Medical Corporation CT-W2000 CT scanner was used for mesoscopic observation. This scanner is equipped with an X-ray emission source and 768 elements of X-ray detector (Fig. 3). The performance of this scanner is 0.75 mm for spatial resolution, 1mm for minimum slice thickness, 130 kV for maximum voltage of the X-ray tube, and 160 mm for minimum imaging ...
Citations
... The resolution of CT images is significantly influenced by the sample size as well as by the density of specific minerals and crack elements. Very thin cracks in granite samples can be enhanced by a potassium iodide solution (Ohtani et al., 2000) or by other chemical solutions with similar functions. It is important to try to reduce the sample size for obtaining high-quality CT images on the premise that the sample size is suitable for obtaining reasonable testing results. ...
This article reviews laboratory experimental studies on hydraulic fracturing under triaxial and true triaxial stress conditions in crystalline rock for geothermal purposes, and places particular focus on the stimulation of Enhanced Geothermal Systems. First, parameters that influence hydraulic fracture initiation and propagation and breakdown pressure are reviewed and discussed. The parameters including micro-structure, fluid viscosity, injection rate, and fluid infiltration, and stress conditions are identified as the key controlling factors in hydraulic fracture growth in hard rock. Second, innovative injection schemes, such as cyclic and fatigue hydraulic fracturing, are reviewed because they show advantages both in fracture network creation in granite and in mitigating and controlling induced seismicity via fluid injection. Third, this review includes fracture-inspection techniques, non-destructive methods of acoustic emission (AE) monitoring and X-ray computed tomography (CT), and microscopic observations used for quantifying the efficiency of injection protocols. In addition to AE parameters, such as AE event rate and source location, we emphasize the importance of in-depth AE analysis on the failure mode and radiated seismic energy. X-ray CT and microscopic observation enable fractures in the rock volume to be quantified, and thereby lead to a better understanding the mechanism behind hydraulic fracturing. Combined measurements of AE and CT yield insights into the complex process of hydraulic fracture and permeability enhancement. The discussion section is enriched with diagrams that connect the injection rate and the resulting fluid infiltration zone and fracture process zone, granite-specific hydraulic fracturing behavior, and practical upscaling elements for potential field applications in geothermal fields.
... Kumar et al. (1997) and Dijk et al. (1999) used NMRI to measure the aperture distribution. Johns et al. (1993), Keller (1998), Ohtani et al. (2000) and Stephanie et al. (2001) used the X-ray computerized tomography (CT) technique for aperture measurement. The X-ray CT and NMRI methods have low spatial resolution, high cost and difficulties with calibration, measurement and analysis. ...
The geometry of the rock joint is a governing factor for joint mechanical and hydraulic behaviour. A new method for evaluating
the aperture distribution, based on measurement of joint surfaces and three dimensional characteristics of each surface, is
developed. This method allows one to determine and visualize the aperture distribution under different normal stresses and
shear displacements, which is difficult to observe experimentally. A new laser scanner system is designed and developed for
joint surface measurements. Special attention is paid to both surfaces’ data gained by measurements and processing, such as
x-y coordinate table modification, data referencing, and matching between upper and lower surfaces. The surfaces of an artificial
joint in granite are measured, processed, analyzed and three dimensional approaches are carried out for surface characterization.
Parameters such as “asperity’s heights”, “slope angles”, and “aspects” distribution at micro scale, local concentration of
elements and their spatial localization at local scale are determined by Geographic Information System (GIS). These parameters
are used for joint surfaces matching and its real behavior quantitative analysis. The upper surface is brought down to make
contact with the lower surface and the distance between the two surfaces is evaluated from the joint mean experimental aperture,
which is obtained from normal and shear tests. Changes of aperture distribution at different normal stresses and various shear
displacements are visualized and interpreted. Increasing normal load causes negative changes in aperture frequency distribution
which indicates high joint matching. However, increasing shear displacement causes a rapid increase in the aperture and positive
changes in the aperture frequency distribution, which could be due to un-matching, surface anisotropy and spatial localization
of contact points with proceeding shear.
