水圧破砕機構における応力状態, 孔井の傾斜および既存き裂の影響

異方性岩石における水圧破砕に関する実験的研究 (第2報)

抄録

The orientation of hydro-fracture is usually determined from stress condition on the basis of the theory of isotropy. However, the strength of anisotropic rocks in hydraulic fracturing shows orientation dependance from the results obtained in 1st report. The purpose of this report is to discuss how to account for rock anisotropy on the mechanism of hydraulic fracturing. Some laboratory hydraulic fracturing tests were conducted for the influences of stress condition, inclination of borehole and pre-existing fracture in Inada granite and Kofu andesite.
The permeation factors obtained from the relationship between the breakdown pressure and confining pressure are shown as the following equations.
Inada granite; P_b=0.57 (3_σh-σ_H) +T_HF
Kofu andesite; P_b=0.65 (3_σh-σ_H) +T_HF
Hydro-fractures in Inada granite were recognized to be created in the direction of the rift planes under the condition that the stress normal to the rift planes excessed the other stress by about 20 kg/cm2 (Figs. 7 and 11). This result suggests that the influence of rock anisotropy must not be negligible problem in stress analysis by using the data of hydro-fracture orientation.
It is suggested from the results of hydraulic fracturing tests in inclined borehole that the orientation of hydro-fracture is liable to be more influenced by borehole inclination than rock anisotropy and stress condition (Fig. 10).
The results of fracture intersection tests show that the hydro-fracture must not be created far from 8 mm across the pre-existing fracture under condition of this report.