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

Abstract

[1] In situ test of hydraulic fracturing (HF) provides the only way to observe in situ stress magnitudes directly. The maximum and minimum horizontal stresses, SHmax and Shmin, are determined from critical borehole pressures, i.e., the reopening pressure Pr and the shut-in pressure Ps, 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 Pr. For effective measurement of Pr, 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 SHmax determination from Pr was developed. Furthermore, a new method for stress measurement not restricted by the system compliance and Pr is herein proposed. In this method, the magnitudes and orientations of SHmax and Shmin are determined from (i) the cross-sectional shape of a core sample and (ii) Ps 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.

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ORCID for Lisa C. McNeill: orcid.org/0000-0002-8689-5882

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