Mamoru Abe

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.

Shear Displacement Behavior of Rocks in Shear Fatigue Process. Experimental study on cyclic fatigue process of rocks under direct shear test (2nd Report).

In this study, the behavior of upper and lower (permanent) shear displacements was investigated with cyclic shear loading tests for two kinds of rocks. The behavior of shear displacement obtained in this study may be classified into two processes. In one process, shear displacement is induced mainly by the closure of pores and cracks, the deformation within grains and sliding. In the other one, it is induced mainly by the growth of cracks. In case of neglecting the effect of normal stress, the slope of the equation expressing the relationship between the minimum rate of upper shear displacement and the number of cyclic loading on log-log graph is -1.02 for Kimachi sandstone and -0.92 for Tohoku marble. It is -1.09 for Kimachi sandstone and -1.00 for Tohoku marble when the rate of lower shear displacement is minimum. Upper shear displacement where the rate of upper shear displacement increases rapidly corresponds to shear displacement at the peak shear stress in the convenient direct shear test. Similarly, lower shear displacement where the rate of lower shear displacement increases rapidly corresponds to the permanent shear displacement at the peak shear stress.

Measurements of velocity and attenuation of longitudinal waves in hot water‐saturated rocks under simulated hydrothermal environment

We have developed a measuring system and techniques for testing longitudinal wave-velocity and attenuation in rock specimens under simulated hydrothermal environment. High temperature piezoelectric transducers were employed for the pulse transmission measurement in an autoclave. The system was carefully calibrated in a range of 5t250” C and 0.1-20 MPa. Accuracy for the evaluation of velocity and attenuation is less than 30 m/s and 2 dB/cm. Six kinds of rock samples were tested. Velocities in all. samples gradually decrease with increasing temperature. Velocity hysteresis between heating and cooling processes was significantly recognized in crystalline rocks, such as granite and marble. No hysteresis was observed for tuff and andesite. Notable change of attenuation with temperature was not observed except for the marble, in which significant thermal cracking was observed.