To clarify the effects of the vertical stress along the borehole on the hydraulic fracture formation, we conducted hydraulic fracturing experiments using the fracturing fluids of the water and the super critical carbon dioxide. A borehole was bored perpendicular to the weak plane of the rock, and horizontal stresses of 5 and 3MPa and a vertical stress of 12MPa were applied. The obtained results were compared with the results obtained at a lower applied vertical stress of 1MPa.

The total length of the fractures measured at the specimen surfaces for the vertical stress of 12MPa tended to be smaller than that for a vertical stress of 1MPa. This suggests that the horizontally oriented pores and microcracks were compressed by the high vertical stress and that the infiltration of the fracturing fluids significantly affected fracture propagation.

1 Introduction

Japan is one of the most volcanically active countries in the world and has rich geothermal resources. Geothermal power generation does not result in discharges of greenhouse gases, such as carbon dioxide, to the same extent as other energy generation methods that combust fossil fuels. This form of energy generation is also stable, not being dependent on weather or season. Geothermal energy therefore constitutes an eco-power generation method that mitigates global warming. To promote geothermal research and development, Muraoka et al. proposed the Japan Beyond-Brittle Project (JBBP) (Asanuma et al. 2012). This international project is designed to demonstrate the feasibility of a new type of power generation method, using an artificial brittle fracture reservoir system in high temperature ductile zones.

Figure 1 shows the relation between shear strength and depth obtained by Byerlee's law and the plastic flow law for various strain rates (έ) at a high geothermal gradient of 130°C/km. This high geothermal gradient was observed at the Kakkonda geothermal field, Japan (Muraoka et al. 1998). The creep parameters obtained by Jaoul et al. (1984) were used to produce this figure. In brittle failure, shear strength increases with depth, since confining pressure (σn) increases with depth. Contrastively, in ductile failure, shear strength decreases with depth, since temperature increases with depth.

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