The Agency of Industrial Science and Technology is developing geothermal MWD system which detects thrust force, torque, etc. while drilling a geothermal energy well. The MWD system has a sensor-sub that offers sensors, and a sonde that processes sensor signal and sends information to the ground with mud-pulse. In order to detach the sonde as occasion demands, signal transmission between the sensor-sub and the sonde is carried through a non-contact connector. Mutual electromagnetic induction type connectors are made for the non-contact signal transmission. For high temperature use up to 250°C, the connectors consist of polyamide-imide coated copper coils and mild steel cores. To facilitate uniting the connector, the coil moves freely in the axial direction, and the gap of lmm is made in the radius direction. Effects of relative displacement due to vibration, electromagnetic induction noise, pressure, temperature and mud on the transmission efficiency are examined. In case of using facing coil type connector with coil length of 16mm, change of transmission efficiency is negligibly small while relative displacement being less than 4mm. Pressure, temperature and the mud slightly effect on the signal transmission efficiency. On the other hand, the connector is strongly affected by the electromagnetic induction noise.
The chemical compositions of gas samples from the steaming ground of the Komatsu Jigoku in the Kuju geothermal area, Kyushu, Japan, were measured. Using a gas geothermometer based on H2, CO2, CH4 and H2S contents (D'Amore and Panichi, 1980), the underground temperatures of the Komatsu Jigoku were calculated. From the relationships between the obtained temperatures and the CO2/H2 and CH4/H2 ratios, two gas geothermometers excluding H2S contents are developed. The equations are presented as follows, TCO2/H2 (°C) = 352.45 - 53.36 log (CO2/H2)TCH4/H2 (°C) = 228.95 - 24.06 (CH4/H2). These gas geothermometers were applied to gas samples from the production wells of the Hatchobaru and the Otake geothermal areas, Kyushu, Japan. The obtained temperatures (TCO2/H2 and TCH4/H2) agree with those (Tquartz) estimated by silica content in the geothermal waters within 20°C in a temperature range between 200°C and 250°C. Besides, the bubble gases from hot springs having different water temperatures (97°C and 25°C) gave reasonable temperatures (195°C to 209°C) in the Otake production area. Thus, it can be concluded that these simple gas geothermometers are useful for estimating the temperatures of geothermal systems (<250°C). Furthermore, CO2/H2 geothermometer is found to be successfully applicable to the temperature beyond 250°C. On the other hand, the values of TCH4/H2 become relatively lower than those of TCO2/H2 and Tquartz above 250°C. The reason of lowering is not clear at present. Further test of the CH4/H2 geothermometer at other geothermal systems is required to investigate the reason.
A hydraulic fracture was made in the laboratory parallel to the rift plane in a block of Inada granite. Cylindrical specimens containing a part of the fracture were then taken from the block by boring the block perpendicular to the fracture surfaces. To determine the initial aperture distribution of the fracture, height distributions of the two surfaces of the fracture were measured from a common reference surface along matched linear paths of about 41mm in length with a profilometer which had a stylus tip of 0.025mm in radius. The common surface was determined by fitting one half of the specimen into the other half and holding both halves in a jig consisting of two V-blocks of steel. Spectral and statistical analyses were carried out for the roughness of the linear profiles and the initial aperture distributions of the fracture. Main results obtained in this study are summarized as follows: 1) Many positions along the profile were measured as having a negative initial aperture. This was considered to be caused by opening of secondary cracks existing behind the main surfaces due to the release of the specimen weight. Accordingly, the initial aperture in this study was taken to be that measured when half the weight of the specimen was applied to the surfaces. 2) The roughness of the linear profile was fractal in the range of this study. The fractal dimension determined from power spectral density ranged from 1.36 to 1.48 (mean. 1.43). However, the initial aperture distribution created by the two fractal surfaces was not fractal because the power spectral density was flattened for the wave length greater than the average grain size of the granite. 3) The RMS (root mean square) of the roughness of the linear profile ranged from 0.43mm to 0.89mm (mean. 0.61mm). The mean value and the RMS of the initial aperture ranged from 0.13mm to 0.25mm (mean. 0.17mm) and from 0.12mm to 0.27mm (mean. 0.19mm), respectively. 4) The RMS of the initial aperture increased with that of the roughness of the linear profile and the mean value of the initial aperture increased with the fractal dimension of the roughness of the linear profile. 5) The frequency characteristics of the central curve of the initial aperture were almost the same as those of the roughness of the linear profile. Tortuosity, which was defined as the squared ratio of actual length of the path to the nomminal length, was estimated to be 1.25 on the average.
The influence of thermal stress on the so-called shut-in pressure is investigated both theoretically and experimentally, where the shut-in pressure is generally used as an indicator of in-situ compressive stress normal to a crack plane which is induced or activated through hydraulic fracturing. It is considered here that pre-existing high temperature around a borehole is disturbed due to circulation of cold water before a packer system is set at a depth in the borehole. The present results show that the shut-in pressure decreases with increasing the circulating time due to the effects of the thermal stress induced by the disturbance of the temperature. The difference between the shut-in pressure and the in-situ compressive stress normal to the crack plane is almost proportional to the circulating time to the power of 0.6. Hence, a value of the in-situ compressive stress normal to the crack plane can be estimated from the variation of the shut-in pressure with the circulating time as follows. At first, the shut-in pressure is plotted as a function of the circulating time to the power of 0.6 is plotted. The plot fits to a straight line. The line is extrapolated to the time when the circulating time is zero. Then, a value of pressure determined by the extrapolation gives the value of the in-situ compressive stress normal to the crack plane.
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