日本地球惑星科学連合2018年大会

講演情報

[JJ] Eveningポスター発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG64] 脆性延性境界と超臨界地殻流体:島弧地殻エネルギー

2018年5月21日(月) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:土屋 範芳(東北大学大学院環境科学研究科環境科学専攻)、浅沼 宏(産業技術総合研究所・再生可能エネルギー研究センター)、小川 康雄(東京工業大学理学院火山流体研究センター)

[SCG64-P01] 超臨界条件下における亀裂を含む花崗岩の力学的・水理学的特性

*北村 真奈美1高橋 美紀1高木 健太2平野 伸夫2土屋 範芳2 (1.産業技術総合研究所、2.東北大学大学院環境科学研究科)

To extract geothermal energy effectively and safely from magma and/or adjacent hot rock, we need to tackle many issues which require new technology development. One of them we are targeting on is to develop a technology to mitigate induced-earthquakes. It is required to understand roles of factors on occurrences of the induced-earthquake (e.g., strength, crack density, and fluid-rock reaction) and their intercorrelations (e.g., Asanuma et al., 2012).

Our purpose of this series of experiments is to clarify a relationship between the rock strength and the fracture density under hydrothermally supercritical conditions. We conducted triaxial deformation test on intact granite rock strength under high-temperature (250 – 750°C), high-pressure (104 MPa) condition at a constant load velocity (0.1 μm/sec) using a gas-rig at GSJ, AIST. We used Oshima granite, which has initially <0.2 % of the porosity, 4.26±0.42 km/s in Vp at dry condition (dry). All experimental products showed brittle feature having several oblique fracture surfaces, but both value of peak stress and amount of stress drop became smaller at higher temperature. Additionally, Young’s modulus decreases with increasing the temperature from 57.4 GPa at 250°C to 32.3 GPa at 750°C . At 400 °C, the stress drop accelerated the deformation with ~98 times faster velocity than that at load-point. In contrast, at 650 °C and 750 °C, the velocity during stress drop kept the same order of the load-point velocity. Therefore, the deformation feature may start to be of ductile when the temperature exceeds 650 °C.
Highly dense cracked granite specimens were formed by a rapid decompression test (RDT) using an autoclave settled at Tohoku University (Takagi et al., 2017GRC Trans.). We reduced fluid pressure within 2 seconds from vapor/supercritical state (10 – 48 MPa, 550 °C) to ambient pressure. X-ray CT scanning on the specimens after RDT showed that numerous microcracks were generated in them. The RDT imposed the porosity increasing towards 3.75 % and Vp decreasing towards 1.46±0.22 km/s (dry). We conducted the permeability measurement on fractured granite after RDT with the porosity of 1.06% under a combined condition of 400 °C in temperature and 104 MPa in the confining pressure. Permeability measured by pore pressure oscillation method decreases from 4.3×10-18 m2 at pore fluid pressure (Pp) of 70 MPa to 9.0×10-19 m2 at Pp of 38 MPa. In the meeting, we will present results of triaxial deformation test on such cracked granites and show the relationship between strength and crack density under supercritical conditions.