Problem for the maximum stress estimation by hydrofracturing method and its potential solution

For the stress measurement by hydrofracturing, the observed borehole pressures of P_r and P_s, i.e. the reopening and shut-in pressures, should be related independently to the maximum stress S_H and the minimum stress S_h, and the independent relationships give the principle for the two values of S_H and S_h to be determined from the two observations of P_r and P_s. However, the data of field tests so far indicate that the measured reopening pressure lies close to the shut-in pressure, i.e. P_r = P_s, are far more numerous than can reasonably be expected. To examine such a paradox, we carried out field tests of hydrofracturing, and then we confirmed that borehole pressure is fairly influenced by the hydraulic compliance of test equipments, C, as expected from our theoretical model which has been developed so far. The model suggests that if the C will be reduced small enough, then the measured reopening pressure P_r ^(a) approaches the true reopening pressure, P_r0, i.e. the borehole pressure at which the fracture truly begins to open from its mouth at the borehole wall. Thus we could measure P_r0 approximately from P_r ^(a) by using stiff hydrofracturing system. The detected P_r0 allows us to estimate S_H on condition that the magnitude of S_h is known from the shut-in pressure.