Determination of stress state in deep subsea formation by combination of hydraulic fracturing in situ test and core analysis: A case study in the IODP Expedition 319

摘要

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_(Hmax) and S_(hmin), are determined from critical borehole pressures, i.e., the reopening pressure P_r and the shut-in pressure P_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_r. For effective measurement of P_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_(Hmax) determination from P_r was developed. Furthermore, a new method for stress measurement not restricted by the system compliance and P_r is herein proposed. In this method, the magnitudes and orientations of S_(Hmax) and S_(hmin) are determined from (i) the cross-sectional shape of a core sample and (ii) P_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 ~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. Key Points Proposal of new stress measurement methods applicable to deep subsea formations Success in application to a 1.6 km borehole in seafloor at a water depth of 2 km Finding stress decoupling through a upper boundary of an accretionary prism