A New Strategy and Its Verification for In-situ Measurements of the Maximum and Minimum Stresses at Deep Depths by Hydraulic Fracturing

水力压裂法原则上适用于深层的原地应力测量。然而，正如最近所指出的，其现有的测试系统和程序涉及的基本问题，必须解决之前的原则付诸实践。最严重的问题与压裂系统的水力柔度有关。为了测量重新打开压力，必须使用具有足够小柔度的压裂系统。如果没有，则无法从假设垂直钻孔的重新打开压力估计最大水平应力，即使最大水平应力是应力测量中最关心的。这种限制使得难以将水力压裂应用于应力测量，因为常规系统的柔度太大。考虑到这些事实，我们最近提出了一种新的方法，这将使我们能够测量的最大和最小应力的水力压裂在一个深度大于几公里的现场测试。迄今为止，我们已经通过实验室和现场实验，使用从松代地面钻取的80 m深的钻孔，逐步测试和验证了该方法。最后，我们成功地将该方法应用于现场测试的水力压裂在811米的深度在一个垂直钻孔钻从地面在田泽湖。

Hydraulic fracturing is suitable in principle for in-situ stress measurements at deep depths. However, as has been pointed out recently, its current testing systems and procedure involve essential problems which have to be solved before putting the principle into practice. The most serious problem is associated with the hydraulic compliance of fracturing systems. For measuring the reopening pressure, it is necessary to use the fracturing system with sufficiently small compliance. If not, there is no way to estimate the maximum horizontal stress from the reopening pressure assuming a vertical borehole, even though the maximum horizontal stress is the greatest concern in the stress measurement. This limitation makes it difficult to apply hydraulic fracturing for the stress measurement, because the compliance of the conventional system is so large. Taking into account of these facts, we have proposed recently a new method which will allow us to measure both of the maximum and minimum stresses by in-situ tests of hydraulic fracturing at a depth greater than a few km. We have proceeded to test and verify the method step by step so far through laboratory and field experiments using a borehole of 80 m in depth drilled from the ground surface at Matsushiro. Finally, we succeeded to apply the method to an in-situ test of hydraulic fracturing at a depth of 811 m in a vertical borehole drilled from the ground surface at Tazawako.