水圧破砕における亀裂の発生と開口・閉合の挙動について -不連続面の少ない中国の花崗岩地域での水圧破砕実験(第3報)- [in Japanese] Characteristics of Initiation, Reopening and Closing Behavior of Hydraulic Fractured Crack -Hydraulic fracturing experiment in granitic rock with scarce joints in China(3rd Report)- [in Japanese]
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Through an extensive hydraulic fracturing experiment in granitic rock with low frequencies of joints, the initiation, reopening and closing behaviors have been carefully examined. zn this 3 rd report, the features which have not been dealt with in the previous two reports are discussed; the fracture direction and two deflection points in a shut-in curve.<BR>A numerical analysis based on the weakest-link theory reveals that, with increase of the maximum compression stress <I>SH</I>, the mean and the variability of hydro-fracturing tensile strength <I>T</I> increases, while the variability of fracture direction decreases.<BR>In this study, the vertical crack with a circular periphery tends to extend passing through the packers. When sufficient rate of flow is supplied for reopening the crack, the crack tip is fully open. After closing the water valve, the pressure is continuously decreasing, and the first deflection point <I>Pctc</I> is observed at the cracktip closing. With further pressure decrease, the crack surface is closing gradually from the outer to inner part. When the part of crack extended passing through the packers is closed, the second deflection point <I>Ps</I><SUB>2</SUB> is observed. Considering the crack-closing process, <I>Ps</I><SUB>2</SUB> should be larger than (3<I>Sh</I>-<I>SH</I>)/2 while <I>Pctc</I> is equal to the minimum compression stress <I>Sh</I>.<BR>In this experiment, different values of <I>Ps</I><SUB>2</SUB> have been observed at two depths while the values of Pctc (=<I>Sh</I>) are almost the same. According to the crack closing mechanism described above, smaller <I>Ps</I><SUB>2</SUB> suggests larger SH. At a depth with smaller <I>Ps</I><SUB>2</SUB>, the variability of crack direction is small and <I>SH</I> is large. This result suggests that the variability of crack direction and <I>Ps</I><SUB>2</SUB> possibly give useful information for stress estimation.
- Journal of MMIJ
Journal of MMIJ 113(2), p.107-114, 1997-02
The Mining and Materials Processing Institute of Japan