Vol.9 No.2 2016
Research paper : Development of rock deformation techniques under high-pressure and high-temperature conditions (Koji MASUDA)−104−Synthesiology - English edition Vol.9 No.2 (2016) piston or by injecting more pressure medium from the outside. A higher pressure can be achieved when a solid material is used as the pressure medium than when a fluid is used. When solid materials such as talc, NaCl, or pyrophyllite are used as the pressure medium, they are enclosed in the vessel and the pressure is increased by a piston to create a high-pressure and high-temperature environment. However, when a solid medium is used, the pressure values cannot be measured accurately and the deformation of the sample under pressure cannot be measured with precision, so this method is not suitable for rock deformation experiments. Therefore, a fluid (liquid or gas) is used as the pressure medium in most rock deformation experiments. In reproducing high-pressure and high-temperature conditions with a liquid pressure medium, the maximum achievable temperature is about 500 ºC, even if silicon oil, a liquid with special properties, is used as the pressure medium. As it will be explained in the next section, it is necessary to achieve a higher temperature than 500 °C in this research; therefore, we used an inert gas (argon gas) as the pressure medium. Gas is an ideal pressure medium for applying a uniform pressure (hydrostatic pressure). However, when gas is used, particular attention must be paid to possible leakage. Also, because gas has a large compression ratio (i.e., the ratio of the volume change to the pressure change is large), a pump system that can deliver large volumes of gas is necessary to obtain high pressure. Moreover, because the change in volume is large, special care must be taken when operating the device. The characteristics and proper handling of high-pressure gas must be understood, and all safety regulations must be followed in compliance with the law.A high-pressure experimental apparatus using gas as the pressure medium was developed later in Japan as compared with in other countries. The first gas-pressure testing device was designed and manufactured at Kyoto University around 2000. We obtained permission to use the technology developed at Kyoto University, and manufactured the second device in Japan. The maximum pressure (confining pressure) achievable by these devices was 200 megapascals (MPa). As shown in Fig. 7, not only can this system apply pressure (confining pressure) to rock samples inside the pressure vessel, it can also deliver fluid (liquid or gas) directly to the sample from the outside, circulate the fluid, and control the pressure (pore fluid pressure) to a maximum of 200 MPa. We were thus able to conduct deformation and friction experiments with cylindrical rock samples with a maximum diameter of 20 mm and a length of 40 mm under such conditions.To accurately measure the load applied to the sample inside the pressure vessel, we developed an internal load cell. Normally, to measure the load applied to the sample, the load applied to the piston is measured outside the pressure vessel. However, because friction due to the O-ring used as the seal between the piston and the pressure vessel affects the load measurement, it is better to measure the load applied to the Internal load cellLower pistonThermocouplesThermocouplesHeater caseInternal furnace (heater)Spacer (pyrophyllite)Spacer (alumina)SampleSpacer (tungsten carbide)Spacer (tungsten carbide)Spacer (silica wool)Spacer (alumina)Copper jacketUpper pistonThermocouplesTC1Pressure vesselT1T2TC2TC3Fig. 7 Schematic diagram of the pressure vesselLeft: Cross section of the sample assembly. The pressure (pore pressure) can be controlled and the fluid can be circulated by delivering fluid directly to the sample from the outside. The locations of the temperature measurements with the thermocouples are indicated by T1, T2, TC1, TC2, and TC3. T1 and T2 are the top and bottom surfaces of the cylindrical sample. TC1 and TC2 are located near the interior walls of the pressure vessel, and TC3 is the location of the internal load cell. Right: Diagram showing the structure of the pressure vessel.