Characteristics of Initiation, Reopening and Closing Behavior of Hydraulic Fractured Crack /Hydraulic fracturing experiment in granitic rock with scarce joints in China (3rd Report)

摘要

Through an extensive hydraulic fracturing experiment in granitic rock with low frequencies of joints, the initiation, reopening and closing behaviors have been carefully examined. In 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.A numerical analysis based on the weakest-link theory reveals that, with increase of the maximum compression stress SH, the mean and the variability of hydro-fracturing tensile strength T increases, while the variability of fracture direction decreases.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 Pctc is observed at the crack-tip 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, Ps_2 is observed. Considering the crack-closing process, Ps_2 should be larger than (3Sh -SH)/2 while Pctc is equal to the minimum compression stress Sh.In this experiment, different values of Ps_2 have been observed at two depths while the values of Pctc (= Sh) are almost the same. According to the crack closing mechanism described above, smaller Ps_2 suggests larger SH. At a depth with smaller Ps_2, the variability of crack direction is small and SH is large. This result suggests that the variability of crack direction and Ps_2 possibly give useful information for stress estimation.