Hidekichi Yokoyama

Damper application of magnetic fluid for a vibration isolating table

Abstract An application of magnetic fluid to damp a loading table of a vibration isolator is attempted. A piston type damper with no solid contact between an inner and outer cylinder is designed by utilizing a levitation force for a nonmagnetic body in magnetic fluid. The performance of the isolator is evaluated through a series of experiments and it is concluded that the isolator cuts off the ground noise effectively with suppressing its resonant vibration.

Acoustic emission measurement of geothermal reservoir cracks in Takinoue (Kakkonda) field, Japan

Abstract Crack extension caused by a shut-down operation in production wells (build-up test) in Takinoue (Kakkonda) geothermal power plant, Iwate Prefecture, Japan, has been successfully measured by an acceleration-sensitive long-distance acoustic emission measurement technique. The distribution and activity of subsurface cracks in a geothermal reservoir can be determined by the combined procedure of the pressure build-up test and AE measurements. The feasibility and importance of the measurements are demonstrated by AE data and their interpretation.

In-Situ AE Measurement of Hydraulic Fracturing at Geothermal Fields

A triaxial acceleration-sense down-hole AE measurement system was developed and successfully applied to the monitoring of both man-made and natural hydraulic fracture in a geothermal field. The acceleration-sense instrumentation allows long-distance measurement of subsurface crack extension. AE events during hydraulic fracturing in Nigorikawa geothermal field could be detected at a distance of 1600m from the AE source. AE events during a closure operation of a well-head valve in the Kakkonda geothermal field could also be detected by the system. In this paper, the feasibility of the simple measurement system for in-situ monitoring of subsurface crack extension in a geothermal field is demonstrated with reference to the fracture mechanics model of a geothermal reservoir.

Long-distance acoustic emission monitoring of hydraulically induced subsurface cracks in Nigorikawa geothermal field, Japan

Abstract Acoustic emissions (AE) during a massive hydraulic fracturing experiment in a geothermal production well in Nigorikawa field, Hokkaido, Japan, were measured using an acceleration sensitive downhole AE sonde installed in a 30 m well. Crack extensions at a depth of 1500 m were detected and characterized by long-distance AE measurements. The mechanism by which productivity was increased in the geothermal well through hydraulic fracturing has been represented by a model based on AE data and geological considerations.