Hydraulic fracturing technique has been applied in enhanced geothermal systems to enhance reservoir rock permeability. It is necessary to increase heat production from a geothermal reservoir. In order to implement economical and reasonable hydraulic fracturing simulations, an experimental study was conducted to investigate the crack propagation stage. In this experiment, we used mortar cylindrical specimen with a hole in the center as a preliminary experiment of hydraulic fracturing. The compressive strength and tensile strength of mortar cylinder specimen have been obtained by using uniaxial compression and Brazilian tests, respectively. Then, the minimum and maximum principal stress was used to apply differential pressure in the experiments. Based on the results, the crack propagation stage was not obvious. Therefore, the improvement of the experimental equipment should be done to make water injection concentrated at the center of the specimen. Furthermore, the effects of high pressure and temperature are not considered in this work and should be considered in the future work to obtain more reliable results.

1. Introduction

About 8% of the world's active volcanoes exist in Japan. If this heat is used for power generation, it becomes a semi-permanent domestic energy resource. In recent years, the extracting geothermal energy by using enhanced geothermal system (EGS) has attracted attention. This technique uses hydraulic fracturing to extract the heat stored in the hot dry rock (HDR) reservoir. Hydraulic fracturing technology is used to create artificial fractures by injecting cold water into the injection well (Hori, 2001).

Kosugi et al. (1980) conducted a hydraulic fracturing simulation experiment with a thick-walled cylindrical specimen and a crushing test with a cylindrical specimen in order to investigate the cracking conditions under sealing pressure. As a result, in hydraulic fracturing, it is clearly shown that the influence of confining pressure is larger than other factors (porosity, Poisson's ratio, etc.). In addition, Ishijima et al. (1981) observed hydraulic fracturing behavior and microfracture sound activity under confining pressure. It shows that the fracture pressure and the sealing pressure are in a linear relationship within the range of the sealing pressure of about 34 MPa or less. In addition, there are roughly two types of micro-breaking sound (AE) behavior that appears due to the increase in pore pressure.

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