Japan Agency for Marine-Earth Science and Technology, Japan

In situ test of hydraulic fracturing (HF) provides the only way to observe in situ stress magnitudes directly. The maximum and minimum horizontal stresses, S (sub Hmax) and S (sub hmin) , are determined from critical borehole pressures, i.e., the reopening pressure P (sub r) and the shut-in pressure P (sub s) , etc, observed during the test. However, there is inevitably a discrepancy between actual and measured values of the critical pressures, and this discrepancy is very significant for P (sub r) . For effective measurement of P (sub r) , it is necessary for the fracturing system to have a sufficiently small compliance. A diagnostic procedure to evaluate whether the compliance of the employed fracturing system is appropriate for S (sub Hmax) determination from P (sub r) was developed. Furthermore, a new method for stress measurement not restricted by the system compliance and P (sub r) is herein proposed. In this method, the magnitudes and orientations of S (sub Hmax) and S (sub hmin) are determined from (i) the cross-sectional shape of a core sample and (ii) P (sub s) obtained by the HF test performed near the core depth. These ideas were applied for stress measurement in a central region of the Kumano fore-arc basin at a water depth of 2054 m using a 1.6 km riser hole drilled in the Integrated Ocean Drilling Program (IODP) Expedition 319. As a result, the stress decoupling through a boundary at 1285 m below seafloor was detected. The boundary separates new upper layers and old lower ones with an age gap of approximately 1.8 Ma, which is possibly the accretionary prism. The stress state in the lower layers is consistent with that observed in the outer edge of accretionary prism. Abstract Copyright (2013), . American Geophysical Union. All Rights Reserved.