[S-SS07] Fault Rheology and Earthquake Physics

[SSS07-P06] Possible effect of pre-existing cracks on tangent modulus of stress-strain curve of granite specimens under hydrothermal conditions

*Manami Kitamura¹, Miki Takahashi¹ (1.National Institute of Advanced Industrial Science and Technology )

Keywords:Granite, Crack, geothermal

To understantd mechanical properties of cracked rocks consisiting the hydrothermal resevoir and their dependence on the temperature are required for developing geothermal system safely and efficiently. We conducted a series of experiments on cracked granite specimens under hydrothermal conditions in order to clarify an effect of temperature and the effec of cracks on tangent modulus of elasticity (apparent Young’s modulus) of stress-strain curve. We prepared cylinders of cracked granite specimens (Oshima granite), using an appratus of an autoclave installed in Tohoku Univ. The parvasivly distributed cracks were formed by a rapid decompression processing: rapid reduction in temperature and pore pressure within 2 seconds from vapor/supercritical state (10 – 48 MPa, 550°C) to ambient condition (Kitamura & Takahashi, 2017AGU; Takagi et al., 2017GRC Trans.). The initial porosity of intact granites is 0.61 ± 0.17 % in average, and it for cracked granites ranges from 1.70 to 2.14 %. We conducted triaxial compression tests under sets of conditions of 104 MPa in confining pressure (P_c), 39 MPa in pore pressure (P_p), RT (room temperature) – 750°C in temperature (T), and 0.1 or 2.0 μm/sec in displacement rate (i.e., strain rate of ~10^-6 s^-1 or ~10^-5 s^-1, respectively) using a gas-medium, high-pressure, high-temperature triaxial apparatus at GSJ/AIST. The set of P_c and P_p simulates an environment at ~4 km depth. For all results of the experiments, the tangent modulus of stress-strain curves started from very small values followed by gradual increasing, indicating apertures of cracks oriented perpendicular to the maximum princial stress (s₁) were closing. Thereafter, a maximum tangent modulus, which is usually adopted as the Young’s modulus, was appered. Then, the tangent modulus began to decrease, indicating dilatancy occured until the strength reached to a peak stress followed by a failure. Thus the tangent modulus have a relation to crack porosity (or cartain characteristic of crack network). Here, we focused on the first stage of low tangent modulus regime (compaction regime). Especially, we monitored the tangent modulus at differential stress s_d =< 100 MPa to reveal the effect of cracks, where s_d is s₁ - P_c. Based from a a result to measure in-situ s_d at ~4 km depth for Kakkonda geothermal region, Iwate Pref. Japan (e.g., Muraoka et al., 1998), we estimated s_d = 100 MPa at 4 km depth. The tangent modulus was determined for each interval of s_d, 10 – 40 MPa, 30 - 60 MPa, 50 – 80 MPa, and 70 – 100 MPa. The tangent modulus increase with increasing s_d for both intact and cracked granites. Increment of the tangent modulus is greater for cracked specimens than for the intact ones under lower differential stress conditions of <100 MPa. These results indicate that most of cracks have been closed quicly even at low s_d. For one experiment under a condition at T = 350^oC, we tried to monitor a change in state of the cracks during the loading, using a technique of permeability measurment method, pore pressure oscillation method. The permeability was decreased from 2.7×10^-19 m² at the compaction stage and showed continuousl decarease until s_d reached to 300 MPa. The minimum value of the permeability was 8.4×10^-20 m², then started to gradually increase to 1.0×10^-18 m² just before s_d closed to the peak stress, caused by the dilatancy. Therefore, the tangent modulus could be stongly related to the state of cracks and strongly controlled by a magnitude of s_d.