Vol.9 No.2 2016
Research paper : Development of rock deformation techniques under high-pressure and high-temperature conditions (Koji MASUDA)−105−Synthesiology - English edition Vol.9 No.2 (2016) sample directly, inside the pressure vessel. The performance of our device is superior at controlling the axial load, confining pressure, and pore pressure compared with gas pressure testing devices developed overseas.By putting the rock samples in a high-pressure vessel (Fig. 8) and using a high-pressure gas as the pressure medium, it became possible to replicate environmental conditions at a depth of about 10 km. At 10 km below the ground surface, the pressure is about 200–300 MPa and the temperature is about 300 ºC. However, if we only reproduce the same pressure and temperature conditions as those underground, then we would have to wait about a thousand years to observe one cycle of the processes associated with a mega-earthquake. What technological developments are necessary to solve this problem?4.4 High-temperature technologyAccording to our working hypothesis, chemical reactions play a major role in the processes that dominate the long-term changes that occur deep underground. The rate at which chemical reactions occur is generally related to temperature, as expressed in Equation (1).Rate = Aexp (1)HRTHere, A is a constant, H is the activation energy, R is a gas constant, and T is absolute temperature. Therefore, it should be possible to speed up the reaction to a rate observable in the laboratory by raising the temperature to increase the reaction rate. In this way, it should be possible to investigate in the laboratory a phenomenon that progresses slowly deep underground. Because the results would be meaningless if the sample were to partially melt or if the dominant deformation process is changed when the temperature is raised, we determined the temperature range within which the same mechanism would be maintained before we set the temperature at which to conduct the experiment. In fact, to speed up the reaction necessitates the development of a technology capable of achieving higher temperatures than the actual high-temperature, high-pressure conditions found in nature, and we developed the necessary technology. Here, we created a technology capable of achieving a high temperature of about 800 ºC at pressures up to 200 MPa. Specifically, we designed a heater for installation in the pressure vessel.The temperature of samples inside a pressure vessel can be raised by means of exterior or interior heating. In exterior heating, the entire pressure vessel is heated, but restrictions are imposed by the time needed for raising and lowering the temperature, and the temperature cannot surpass that at which the material of which the pressure vessel is made loses its integrity. In interior heating, the heating mechanism is placed inside the pressure vessel, and the sample inside the pressure vessel can be raised to temperatures surpassing the temperature limitations of the exterior heating method. In principle, interior heating is simple; a heating coil needs to be designed. However, for technological reasons, it took two years to develop a suitable heater. The developed heater (left side of Fig. 9) consists of the two independent parts (two heating coils), and power is supplied from the outside separately to the top and bottom coils. To keep the temperature distribution of the sample uniform, the power supplied to the top and bottom coils is controlled by a Fig. 8 The pressure vessel, which contains the sample, to be installed in the materials testing devicePressure (confining pressure) is applied to the rock sample in the pressure vessel during the test to replicate the conditions underground.Fig. 9 The heater (electric heating element) developed at AISTA temperature higher than the actual underground temperature can be achieved under high pressure. Left: Heater. Two heating coils are wrapped around a ceramic tube. Right: Exterior appearance of the heater.