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

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.

水力压裂（HF）原位测试是直接观测原位应力大小的唯一方法。最大水平应力$S_{Hmax}$和最小水平应力$S_{hmin}$是由测试过程中观测到的临界钻孔压力，即重新开启压力$P_r$和关井压力$P_s$等确定的。然而，临界压力的实际值和测量值之间不可避免地存在差异，而且这种差异对于$P_r$来说非常显著。为了有效地测量$P_r$，压裂系统必须具有足够小的柔量。我们开发了一种诊断程序，用于评估所采用的压裂系统的柔量是否适合根据$P_r$确定$S_{Hmax}$。此外，本文还提出了一种不受系统柔量和$P_r$限制的应力测量新方法。在这种方法中，$S_{Hmax}$和$S_{hmin}$的大小和方向是由（i）岩芯样本的横截面形状和（ii）在岩芯深度附近进行的水力压裂测试所获得的$P_s$确定的。这些方法被应用于在水深2054米的熊野弧前盆地中心区域进行应力测量，使用的是综合大洋钻探计划（IODP）319航次中钻探的一个1.6千米的隔水导管井。结果，检测到了海底以下1285米处通过一个边界的应力解耦。该边界将年龄差距约为180万年的新上层和旧下层分开，这可能是增生楔。下层的应力状态与在增生楔外缘观察到的应力状态一致。