ABSTRACT
Pp is pore pressure inside the fracture before opening. Nevertheless the data of field tests so far indicate
that the measured reopening pressure lies close to the shut-in pressure, i.e. Pr = Ps, are far more numerous than can reasonably be expected (e.g., Evans et al. 1989; Lee & Haimson 1989). Considering here that the interpretation of Eq. (2) for the shut-in pressure Ps is supported by various experimental and theoretical works, we should recognize that the measured reopening pressure does not actually coincide with the interpretation of Eq. (1) but is basically the same as Ps. If it is true, we could estimate with hydraulic fracturing only the minimum horizontal stress Sh but not the maximum horizontal stress SH . We examined such a paradox in our previous works based on the theoretical and experimental approaches (Ito et al. 1999, 2005, 2006). We found that (i) independently of the stress state, the induced vertical fractures actually open at
a borehole pressure of Pr0 to be smaller than Sh in other words Ps, and (ii) due to the effect of hydraulic compliance of fracturing systems, C, the measured reopening pressure becomes larger than Pr0 and close to Ps. However, if the system compliance C can be reduced small enough, then the measured reopening pressure approaches Pr0. The true reopening pressure Pr0 has relation theoretically to SH and Sh, which is given by not Eq. (1) but the following equation.
